Christoph Bauer

Romain Duwald

Geraldine Maria Labrador

Pau Simon Pascal

Moneva Lorente

Johann Bosson

Jérôme Lacour
email: jerome.lacour@unige.ch

Jean-David Rochaix
email: jean-david.rochaix@unige.ch

Moneva Pau

Lorente

Specific labeling of mitochondria of Chlamydomonas with cationic helicene fluorophores

Keywords: Electronic supplementary information (ESI) available: Synthetic protocols, analyses, optical properties

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Introduction

Fluorophores have been widely used for visualizing specific subcellular structures with high spatial and temporal resolution in living cells. 1 Both fluorescent proteins and chemical dyes coupled to proteins have provided robust tools for studying the localization and dynamics of a large set of proteins. 2 In addition, specific dyes have been developed for the labeling of specific organelles. 3 As an example, the Mitotracker dyes are targeted specifically to mitochondria and can be used to follow mitochondrial fusion and fission events. Some of these Mitotracker dyes require an electrochemical potential across the mitochondrial envelope to enter these organelles while others are taken up in the absence of such a potential. Thus these dyes provide important information on the energetic state of mitochondria. They have been used extensively in animal and plant cells but less in algal cells. Although Mitotracker dyes have been used successfully to monitor mitochondrial fusion in cells of the green unicellular alga Chlamydomonas reinhardtii, 4 in our hands they gave rise to variable results. Other dyes specific for mitochondria in Chlamydomonas include rhodamine 123, which acts as a sensitive potentiometric dye in this alga. 5 Here we have screened several recently synthesized cationic dyes, which are potential candidates for specifically staining cellular organelles -and late endosomes and mitochondria in particular. 6 This study concerns the use of these fluorophores, and others, for labelling mitochondria in algal cells.

Results and discussion

We tested twelve cationic helicenes and one triangulene compound (Fig. 1). 7 Diaza [4]helicenes 1-7 were prepared in two to five steps from 1,3-dimethoxybenzene. 8 Azaoxa 8-10 and diaza 11-12 [6]helicenes were prepared in six steps from 2methoxynaphthalene using a single common advanced intermediate in a highly convergent process. 9 Diazaoxa triangulene 13 was also prepared from 1,3-dimethoxybenzene in four steps. 6a Some of the helicenes can be isolated as single enantiomers in gramscale but, for this study, these compounds were used as racemates. 10 Details are provided in the Materials and methods section. They display absorption and fluorescence within the transparency window of biological media 8c,11 and have proven to act as efficient probes for bio-imaging, 6b,12 binding selectively DNA 13 or behaving as chiroptical switches. 14 Moreover, their significant Stokes shifts, varying from 1090 to 1580 cm -1 (for 11 and 5, respectively), allow one to perform bio-imaging experiments with high signal to noise ratio (Fig. 2). For labeling subcellular structures of Chlamydomonas cells, we used a Leica confocal SP 5 microscope with tunable fluorescence excitation and emission wavelengths. Cells of Chlamydomonas in exponential growth phase were incubated for 10-20 min with these compounds. The absorption and fluorescence emission spectra of some of these helicenes are shown in Fig. Several of the compounds tested gave rise to specific fluorescence signals (Table 1). Chlorophyll autofluorescence was measured with excitation at 470 nm and fluorescence emission from 650 to 700 nm. Of the thirteen compounds tested, five gave rise to a specific signal (1, 2, 4, 5 and 10). Compounds 5 and 10 gave the strongest response (Table 1). In particular, we noticed that extension of the ester-C8 chain of 5 to C18 in compound 6 led to a drastic decrease of staining. This is in sharp contrast to what has been observed for the labelling of mitochondria with HeLa, U87MG and PC3 cell lines. 6b We chose compounds 4 and 5 (Fig. 1) for further experiments and used them at a concentration of 1 μM. Under these conditions, the fluorescence signal can be readily detected, the cellular structure is well preserved and cell survival is not compromised. To identify the subcellular structures labeled by these fluorophores, we used several reporter lines expressing proteins of known cellular location fused to Yellow Fluorescent Protein (YFP). 15 These lines include proteins of the cytoplasm, nucleus, endoplasmic reticulum, mitochondria and chloroplast. YFP fluorescence was detected using excitation at 510 nm and fluorescence emission between 520 nm and 545 nm. Comparison of the fluorescence signals obtained with YFP fused to the mitochondrial protein CAH4 (carbonic anhydrase) and helicenes 4 and 5 revealed overlapping fluorescence patterns indicating that these compounds label specifically mitochondria (Fig. 3). Comparison with the fluorescence obtained with the Mitotracker dye Deep Red further confirmed the mitochondrial location of 5 (Fig. 4). The similarities of fluorescence patterns of 5 with those of mitochondrial protein CAH4 were striking and seemed even better correlated than those obtained with the Mitotracker dye. To further test the specificity of mitochondrial labeling with 5, we compared its fluorescence patterns with those obtained with two YFP-tagged chloroplast proteins, the stromal protein Rubisco and Tic20, a component of the protein translocon of the inner chloroplast membrane. As seen in Fig. 5, the fluorescence patterns obtained with these proteins were very different from those of 5 indicating that this compound does not bind or enter chloroplasts. To examine whether the entrance of 5 into mitochondria requires an electrochemical potential, cells were treated with 5 μM FCCP to collapse this gradient. Under these conditions, no fluorescence was detected indicating that this fluorophore enters mitochondria only in the presence of an electrochemical gradient (Fig. 6A). Treatment of the cells with myxothiazol, an inhibitor of the bc1 complex and hence of the respiratory electron transfer chain did not significantly affect the fluorescence of 5 as compared to untreated cells (Fig. 6B). This result was expected since, under these conditions, the ATP synthase acts as an ATPase and generates an electrochemical gradient by pumping protons into the mitochondrial matrix.

Conclusions

We have shown that a small number of cationic helicene and triangulene compounds can be used for specifically labeling mitochondria from algal cells. These compounds are remarkably different from those targeting mitochondria in HeLA, PC3 or U87MG cancer cell lines. 6b It is most likely that they can be utilized for the same purpose in other organisms. Helicene 5 readily penetrates algal cell and gives rise to clear fluorescence patterns when it is used at concentrations in the μM range. Under these conditions cell structures are well preserved and cell survival is not compromised. Thus, helicene compounds provide new useful tools for examining the mitochondrial network and its dynamics including fission and fusion events. Another interesting property of some of these helicenes is their chemiluminescence. 16 In fact, the luminescence of some of these compounds can be triggered by oxidations or reductions and they could hence be potentially used to monitor the cellular redox environment. However, we were unable to detect any (bio)luminescence with the compounds tested so far.

Materials and methods

The Chlamydomonas strains used in this study were wild type 137C, and several derivatives of cmj30 expressing proteins in the nucleus, mitochondria, chloroplast, endoplasmic reticulum, cytoplasm tagged with YFP 15 and obtained from Dr Martin Jonikas and Silvia Ramundo. Cells were grown in TAP medium with an irradiance of 60 μmol m -2 s -1 at 25 °C.

Luminescence

Fluorescence quantum yields in acetonitrile of cationic dyes (relative to cresyl violet (MeOH, 54%)) were previously reported in the literature: 1 (φ = 13%), 4 (φ = 37%), 8 (φ = 22%) and 11 (φ = 31%). 8c,11

Fluorophores

Fluorophores 1-7 were obtained in no more than four synthetic steps from tris(2,6-dimethoxy)triarylcarbenium 14. The [4]helicene scaffold was built by successive nucleophilic aromatic substitutions of MeO groups by primary amine nucleophiles (R = nPr, nOct) at 90 °C in DMF for 1 h. 8b,10b Access to ester derivatives 3-7 required a multi-step sequence. First, the regioselective Vilsmeier-Haack formylation of 1 followed by a Pinnick-Kraus oxidation led to the formation of a carboxylate derivative. 8c Conversion to the corresponding acyl chloride and addition of excess of alcohols yielded esters 3-7 in high yields (Scheme 1). For the preparation of triangulene 13, triarylcarbenium 14 was treated with hydrazine and then n-octylamine to afford non-symmetrical [4]helicene 15. 17 Heating this compound in molten pyridinium chloride led to 13 in 85% yield. Azaoxa [6]helicenes 8-10 were obtained by mixing precursor 16 with an excess of primary amine in acetonitrile. 9 Higher temperature and microwave irradiation allowed the access to diaza [6]helicenes 11 and 12.

Fig. 1

 1 Fig. 1 Structure of cationic helicene and triangulene fluorophores 1-13.

  2; compounds 1, 5, 10, 11 and 13 were selected as representative examples. For the detection of fluorescence signals, we chose an excitation wavelength at 550 nm and measured fluorescence emission from 580 to 620 nm to minimize interference with the endogenous chlorophyll fluorescence.

Fig. 2

 2 Fig. 2 Absorption (A) and fluorescence (B) spectra of 1 (λabs = 616 nm, λem = 667 nm Stokes shift: 1240 cm -1 , 51 nm), 5 (λabs = 593 nm, λem = 653 nm, Stokes shift: 1580 cm -1 , 60 nm), 10 (λabs = 562 nm, λem = 613 nm, Stokes shift: 1480 cm -1 , 51 nm), 11 (λabs = 614 nm, λem = 658 nm, Stokes shift: 1090 cm -1 , 44 nm) and 13 (λabs = 549 nm, λem = 594 nm, Stokes shift: 1380 cm -1 , 45 nm).

Fig. 3

 3 Fig. 3 Helicene 5 labels mitochondria. Cells of Chlamydomonas containing Mit-YFP (YFP-mitochondria) were treated with 1 μM Helicene 5 and immobilized on glass slides treated with poly l-lysine. They were examined by phase contrast (left column, grey) and fluorescence microscopy on a Leica confocal SP 5 microscope with the following settings: Helicene 5 fluorescence excitation at 550 nm and fluorescence emission from 580-620 nm (labelled Hel 5, middle column, magenta); YFP-mitochondria, fluorescence excitation at 510 nm and fluorescence emission from 520-545 nm (labelled YFP-mito, right column, green). Bars 2 μm.

Fig. 4

 4 Fig. 4 Fluorescence patterns of Helicene 5 and Mitotracker Deep Red are similar. Cells of Chlamydomonas containing YFPmitochondria (labelled YFP-mito, upper right, green) were treated with 1 μM Helicene 5 (labelled Hel 5, upper left, magenta) and 1 μM Mitotracker Deep Red (labelled DR-mito, lower left, cyan) and immobilized on glass slides treated with poly l-lysine. Fluorescence settings were as in Fig. 3 and for Mitotracker Deep Red: fluorescence excitation at 620 nm and fluorescence emission from 655 to 680 nm. Bar 2 μm.

Fig. 5

 5 Fig. 5 Cell fluorescence patterns of Helicene 5 are different from those of chloroplast proteins. A. Comparison of fluorescence of Helicene 5 (labelled Hel 5, middle, magenta) with the small subunit of Rubisco fused to YFP (labelled YFP-Rub, right, green) (chloroplast stromal protein). B. Comparison of fluorescence of Helicene 5 (labelled Hel 5, middle column, magenta) with Tic20-YFP (labelled YFP-Tic20, right column, green) (chloroplast inner envelope protein). Bars 2 μm.

Fig. 6

 6 Fig. 6 Fluorescence of Helicene 5 of Chlamydomonas cells requires an electrochemical potential across the mitochondrial membrane. A. Chlamydomonas cells containing MitYFP were treated with 5 μM FCCP (labelled + FCCP, lower row) or without (upper row, labelled -FCCP). Whereas mitochondrial YFP staining (labelled YFP-mito, right column, green) stays intact, the Helicene 5 staining (labelled Hel 5, middle column, magenta) disappears upon FCCP treatment (lower row, middle). B. Cells were treated with 5 μM myxothiazol (lower row, labelled + MXT) or without (upper row, labelled -MXT). Mitochondrial YFP staining (labelled YFP-mito, right column, green) as well as Helicene 5 staining (labelled Hel 5, middle column, magenta) are undisturbed by myxothiazol treatment. Bars 2 μm.

Table 1

 1 Excitation emission conditions

	Target	Excitation wavelength	Emission wavelength window
	Fluorophores 1-13	550 nm	580-620 nm
	YFP	510 nm	520-545 nm
	Mitotracker deep red	620 nm	655-680 nm

Acknowledgements

We would like to acknowledge the Mass Spectrometry platform at the University of Geneva for the mass spectroscopy analysis. We thank the University of Geneva and the Swiss National Science Foundation for funding.

Scheme 1 Synthesis of cationic dyes. [a] RNH2 (25 equiv.), DMF, 90 °C, 1 h. [b] POCl3 (24 equiv.), DMF, 90 °C, 8 h. [c] NaH2PO4 (1 equiv.), H2O2, MeCN, 60 °C, 1 h. [d]

Confocal microscopy and image analysis

For labeling, cells were mixed with their respective dyes diluted in cell culture medium and incubated for 10 to 20 minutes. Before imaging cells were immobilized on poly-l lysine coated slides. Imaging was then performed with a Leica SP5 confocal microscope equipped with a tunable white light laser (WLL) allowing for free choice of excitation wavelength (within a window ranging from 470 to 670 nm). AOBS (acusto-optical tunable filter) and spectral detection allowed additionally to freely choose appropriate detection windows. See Table 1 for excitation emission conditions used. Fluorescence emission was detected using a 63× Plan-Apo oil immersion objective (NA 1.4) and newest generation hybrid (HyD) detectors with the pinhole closed to one airy unit. Cells were zoomed to obtain an effective pixel size of 50 to 80 nm fulfil ling Nyquist criterion. For visualization purposes, helicene fluorophore emission is shown in purple, YFP emission in green and mitotracker deep red emission in cyan.

Conflicts of interest

There are no conflicts to declare.


Nikita V Sandalyuk
email: nikitasandaliuk@gmail.com

Anthony Bosse

Tatyana V Belonenko

The 3D structure of Mesoscale Eddies in the Lofoten Basin of the Norwegian Sea: A composite analysis from altimetry and in situ data

3D structure of mesoscale eddies in the Lofoten Basin were revealed by combining the altimetry data and CTD profiles from the multiple platforms • Significant eddy-induced anomalies are concentrated within the zero vorticity radius and vertically to the depth of ~900-1000 m

• Eddy-induced transport is generally westward emerging from the Norwegian Atlantic Slope Current

Introduction

The poleward flow of the Atlantic Water (AW) through the Nordic Seas represents the key component of the meridional overturning circulation [START_REF] Spall | Non-Local Topographic Influences on Deep Convection: An Idealized Model for the Nordic Seas[END_REF][START_REF] Lozier | A sea change in our view of overturning in the subpolar North Atlantic[END_REF][START_REF] Chafik | Volume, heat, and freshwater divergences in the subpolar North Atlantic suggest the Nordic Seas as key to the state of the meridional overturning circulation[END_REF]. AW is subjected to important heat losses, convection, and densification on its way to the Arctic (Rossby et al., 2009b;Bosse et al., 2018). It enters the Nordic Seas with the Norwegian Atlantic Current through the straits between Iceland, Faroe Islands, and Shetland Islands. The Norwegian Atlantic Current is constituted by two main branches: the Norwegian Atlantic Front Current (NwAFC) and the Norwegian Atlantic Slope Current (NwASC) which bound the Lofoten Basin (LB) from the eastern and the western sides respectively (Fig. 1). The NwASC is about two times stronger than the NwAFC along ©2020 American Geophysical Union. All rights reserved.

the Svinøy section at 63N (e.g. [START_REF] Mork | A quantitative description of the Norwegian Atlantic Current by combining altimetry and hydrography[END_REF]. Both branches have a prominent seasonal cycle that has been linked to wind forcing [START_REF] Jakobsen | Near-surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: Variability from the mesoscale to interannual[END_REF][START_REF] Mork | A quantitative description of the Norwegian Atlantic Current by combining altimetry and hydrography[END_REF][START_REF] Skagseth | Characteristics of the Norwegian Coastal Current during Years with High Recruitment of Norwegian Spring Spawning Herring ©2020 American Geophysical Union. All rights reserved. (Clupea harengus L.)[END_REF]Bosse & Fer, 2019a). Gascard and Mork (2008) used Lagrangian observations from ARGO and RAFOS floats and describe NwASC at the depth of 300-1000 m as "a turbulent, broad (100 km) and slow current (~6 cm s -1 mean velocity) progressing to the north".

The LB is a 3300 m deep topographic depression bounded by the Vøring Plateau in the south, the Mohn's Ridge in the west and the Eurasian continental shelf in the east. It is one of the most energetic regions regarding eddy kinetic energy in the Nordic Seas [START_REF] Poulain | Near-surface circulation of the Nordic seas as measured by Largangian drifters[END_REF], characterized by the presence of local maxima of sea surface height (SSH) and strong flow variability.

Figure 1. The Lofoten Basin region with the bathymetry [START_REF] Amante | ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis[END_REF] and schematic path of the Norwegian Atlantic Slope Current (NwASC) and the Norwegian Atlantic Front Current (NwAFC). The red spiral indicates the area where the Lofoten Vortex (LV) is generally observed.

A large-scale barotropic cyclonic circulation is formed in the LB due to the conservation of potential vorticity [START_REF] Belonenko | Circulation of waters in the Lofoten Basin of the Norwegian Sea[END_REF][START_REF] Voet | The mid-depth circulation of the Nordic Seas derived from profiling float observations[END_REF]. [START_REF] Volkov | Puzzling over the dynamics of the Lofoten Basina sub-Arctic hot spot of ocean variability[END_REF] detected a time-averaged cyclonic wavelike propagation of the mesoscale sea level anomalies around the center of the LB. They identified dipole and quadrupole wavelike modes of propagation and demonstrated that these modes are responsible for the observed amplification of SSH variability in the center of the LB. Moreover, the cyclonic circulation in deep layers keeps a large amount of AW inside the LB area and significantly increases its residence time in the basin (Fig. 2).

Mesoscale eddies trap water and transport momentum, volume, heat, salt and biogeochemical constituents from their generation regions to dissipation sites (Chelton et. al., 2011a). One of the key features of the mesoscale dynamics in the LB is the generation of eddies from the NwASC by baroclinic instability and their westward propagation which results in heat and salt transport to the central part of the basin [START_REF] Köhl | Generation and stability of a quasi-permanent vortex in the Lofoten Basin[END_REF]Rossby et al., 2009a;[START_REF] Spall | Non-Local Topographic Influences on Deep Convection: An Idealized Model for the Nordic Seas[END_REF][START_REF] Andersson | Variability of the Norwegian Atlantic Current and Associated Eddy Field from Surface Drifters[END_REF][START_REF] Volkov | Formation and Variability of the Lofoten Basin Vortex in a High-Resolution Ocean Model[END_REF]. The shedding of eddies also contributes to the gradual cooling of NwASC on its way to the Arctic Ocean. Such heat loss, in turn, may have a significant influence on the sea ice cover in the Barents Sea [START_REF] Isachsen | Observed and Modeled Surface Eddy Heat Fluxes in the Eastern Nordic Seas[END_REF][START_REF] Sandø | The Importance of Heat Transports and Local Air-Sea Heat Fluxes for the Barents Sea Climate Variability[END_REF][START_REF] Årthun | Quantifying the Influence of Atlantic Heat on Barents Sea Ice Variability and Retreat[END_REF]. In addition, mesoscale eddies transport nutrients and modulate biological activity by providing local rich feeding habitat for the biota [START_REF] Falkowski | Role of eddy pumping in enhancing primary production in the ocean[END_REF][START_REF] Gaube | Satellite observations of chlorophyll, phytoplankton biomass, and Ekman pumping in nonlinear mesoscale eddies[END_REF]. Therefore, a detailed study of mesoscale eddies in the LB is crucial to the understanding of dynamic, climatic and biological processes in the region. Mesoscale features of the LB have previously been investigated in a number of studies based on surface and subsurface drifters (e.g. [START_REF] Poulain | Near-surface circulation of the Nordic seas as measured by Largangian drifters[END_REF][START_REF] Jakobsen | Near-surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: Variability from the mesoscale to interannual[END_REF]Gascard & Mork, 2008;Søiland et al., 2008;Rossby et al., 2009aRossby et al., , 2009b;;[START_REF] Voet | The mid-depth circulation of the Nordic Seas derived from profiling float observations[END_REF][START_REF] Koszalka | Surface circulation in the Nordic seas from clustered drifters[END_REF], numerical modeling (e.g. [START_REF] Köhl | Generation and stability of a quasi-permanent vortex in the Lofoten Basin[END_REF][START_REF] Spall | Non-Local Topographic Influences on Deep Convection: An Idealized Model for the Nordic Seas[END_REF][START_REF] Bashmachnikov | Pattern of vertical velocity in the Lofoten vortex (the Norwegian Sea)[END_REF][START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF], altimetry (e.g. [START_REF] Volkov | Puzzling over the dynamics of the Lofoten Basina sub-Arctic hot spot of ocean variability[END_REF][START_REF] Volkov | Formation and Variability of the Lofoten Basin Vortex in a High-Resolution Ocean Model[END_REF]Raj et al., 2016;Raj & Halo, 2016) and in situ observations (e.g. Soiland et al., 2013Soiland et al., , 2016;;Yu et al., 201;[START_REF] Belonenko | Thermohaline structure of the Lofoten vortex in the Norwegian sea based on field research and hydrodynamic modeling[END_REF]Bosse et al., 2019). The majority of these studies focused on the quasi-permanent anticyclonic Lofoten Vortex (LV) located in the deepest part of the LB.

Russian oceanographers were the first to describe the LV as a long-lived circulation feature [START_REF] Alexeev | Structure and circulation of water masses in the area of an anticyclonic vortex in the northeastern part of the Norwegian Sea[END_REF]Ivanov & Korablev, 1995a, 1995b). LV represents a lens of warm and saline AW in the layer of 300-1000 m depth with a horizontal scale of 100 km. The LV translates in a cyclonic motion relative to the center of the basin at a speed of 3-4 cm/s [START_REF] Søiland | On the structure of the Lofoten Basin Eddy[END_REF]Bosse et al., 2019). Intense maximum orbital velocities in the vortex reach 50-70 cm/s [START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF]Bosse et al., 2019;[START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF][START_REF] Travkin | Seasonal variability of mesoscale eddies of the Lofoten Basin using satellite and model data[END_REF]. There is still no unified opinion about the mechanisms responsible for its exceptional stability. It is clear that deep convection in winter is a necessary condition for the existence of the LV since it creates favorable conditions for its annual regeneration [START_REF] Bloshkina | Convective structures in the Norwegian and Greenland Seas based on simulation results with high spatial resolution[END_REF][START_REF] Fedorov | Winter convection in the Lofoten Basin according to ARGO buoys and hydrodynamic modeling[END_REF]Bosse et al., 2019). In recent years a number of studies also suggested that a primary source of energy sustaining LV could be provided by anticyclones shed from the NwASC and propagating into the central basin [START_REF] Köhl | Generation and stability of a quasi-permanent vortex in the Lofoten Basin[END_REF]Rossby et. al., 2009;[START_REF] Søiland | On the structure of the Lofoten Basin Eddy[END_REF][START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF][START_REF] Travkin | Seasonal variability of mesoscale eddies of the Lofoten Basin using satellite and model data[END_REF]Bosse et al., 2019). Raj et al. (2016) presented a comprehensive observation-based quantitative analysis of mesoscale eddies in the Lofoten Basin using a combination of gridded altimeter, Argo floats and surface drifter data. As a result, the authors documented the horizontal structure and kinematic properties of LB eddies, without excluding the LV from their analysis. However, the vertical thermohaline structure of the mesoscale eddies and their contribution to the heat and salt transports in the LB region remains unexplored.

The main objective of this work is to assess the vertical thermohaline structure of LB eddies (excluding the LV) and their impact on water masses and transports in the LB. We here focus on the general mesoscale activity of the LB, excluding from our study the quasi-permanent anticyclonic LV which has already been thoroughly analyzed (see references hereinabove).

Moreover, the vertical extension of the LV core can reach the depth of 1200 m [START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF][START_REF] Søiland | On the long-term stability of the Lofoten Basin Eddy[END_REF] which significantly exceeds the average lower boundary of the core for the mesoscale eddies in the study region. To achieve our goal, we apply the method of colocalization of altimetry data and CTD profiles from multiple platforms (research vessels, autonomous profiling floats, and gliders), which allow us to obtain composite structures of cyclonic (CE) and anticyclonic (AE) eddies. The spatial distribution of thermohaline characteristics of mesoscale eddies in the basin could be used to estimate the heat and salt transported by these vortices at the basin-scale.

The paper is organized as follows: section 2 provides details about the data and methods used in this study; section 3 focuses on the obtained radial and three-dimensional structures of the composite AE and CE, as well as eddy-induced heat and salt transports. Summary and conclusion are formulated in the section 4.

Data and Methods

Assessment of the spatial thermohaline structure of mesoscale eddies is a challenging task. The main problem is the difficulty of obtaining a sufficient number of hydrological profiles within the borders of single eddy. However, thanks to the development of the ARGO program, currently including almost 4000 profiling floats worldwide (Argo, 2020), and the development of automatic eddy identification algorithms to identify and track mesoscale eddies in SLA fields, a new methodology of colocalization the altimetry and in situ data has recently been developed. The method was presented in [START_REF] Willis | Combining altimeter and subsurface float data to estimate the timeaveraged circulation in the upper ocean[END_REF] and [START_REF] Chaigneau | Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: A composite analysis from altimetry and Argo profiling floats[END_REF] and has been extensively used in a number of studies covering different regions of the World Ocean (e.g. [START_REF] Yang | Mesoscale eddies in the northwestern subtropical Pacific Ocean: Statistical characteristics and three-dimensional structures[END_REF][START_REF] Dong | Mesoscale eddies in the Northwestern Pacific Ocean: Three-dimensional eddy structures and heat/salt transports[END_REF][START_REF] He | A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports[END_REF][START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF][START_REF] Kubryakov | Thermohaline structure, transport and evolution of the Black Sea eddies from hydrological and satellite data[END_REF][START_REF] Keppler | Observed characteristics and vertical structure of mesoscale eddies in the southwest tropical Pacific[END_REF].

Mesoscale Eddy Trajectory Atlas Product

To identify the position of mesoscale eddies and track their propagation, we used the "Mesoscale Eddy Trajectory Atlas Product'' based on the algorithm developed by Chelton et. al. (2011a) and applied to sea level anomaly (SLA) from merged multimission satellite measurements. The algorithm was modified as described in [START_REF] Schlax | The "Growing Method" of Eddy Identification and Tracking in Two and Three Dimensions[END_REF] based on the approach described by [START_REF] Williams | Visualization and analysis of eddies in a global ocean simulation[END_REF]. The "growing" method of eddy identification defines the anticyclonic eddy interior by finding a local maximum in SSH and all neighboring pixels whose SSH values lie above a sequence of decreasing thresholds. This "growth" of the eddy interior is continued until some criteria for this coherent structure are violated. Cyclonic eddies are defined in the same way by finding the local minimum of SSH [START_REF] Schlax | The "Growing Method" of Eddy Identification and Tracking in Two and Three Dimensions[END_REF].

The position of each eddy center is defined as the SSH-based eddy centroid formed by the outermost closed contour of SSH. The coordinates of the centroid (𝑥 𝑐 , 𝑦 𝑐 )are defined to be Here x(i) is the longitude (the i index) and y(j) is the latitude (the j index) of the pixels with its specified SSH value defined as h(i, j), and the point (𝑖, 𝑗) ∈ 𝐸, where E is the connected set of pixels defining the eddy [START_REF] Schlax | The "Growing Method" of Eddy Identification and Tracking in Two and Three Dimensions[END_REF].

The minimal size of the detected eddies is limited by the spatial resolution of the gridded SLA fields. Raj et. al., 2016;[START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF][START_REF] Gordeeva | Statistical analysis of long-lived mesoscale eddies in the Lofoten basin from satellite altimetry[END_REF]. For more details about the algorithm please refer to [START_REF] Williams | Visualization and analysis of eddies in a global ocean simulation[END_REF] and [START_REF] Schlax | The "Growing Method" of Eddy Identification and Tracking in Two and Three Dimensions[END_REF].

The dataset is now routinely generated and quality-controlled by the Data Unification and Altimeter Combination System (DUACS) and distributed by AVISO+. We use version 2.0 of the dataset delivered in September 2018 with the new input data and improved eddy identification and tracking procedure. The dataset and the documentation with a detailed description of the modified algorithm are available at https://www.aviso.altimetry.fr. The final version of the dataset contains the location of detected eddies on each day for the whole altimetry period (1993-ongoing), polarity (cyclonic/anticyclonic), rotational speed, amplitude, radius (scale) and associated metadata.

Hydrographical data set

In order to construct the three-dimensional composite structure of eddies, we used vertical temperature and salinity profiles acquired from the data set "Hydrography of the Nordic Seas, 2000-2017: A merged product" (Bosse & Fer 2018). The dataset is freely available at https://doi.org/10.21335/NMDC-1131411242. This merged data set contains hydrographical observations from different sources for the 2000-2017 period, between 61-80N and 17W-23E. Shipborne CTD profiles were downloaded from the ICES database and combined with Norwegian Marine Data Center for the Svinøy, Gimsøy, Barents Sea Opening, and Bear Island sections, as well as dedicated cruises in the Lofoten Basin [START_REF] Fer | Ocean currents, hydrography and microstructure data from PROVOLO cruises[END_REF].

Profiles collected by Argo profiling floats were retrieved from the Coriolis data center (Argo, 2020). Finally, 10 glider missions carried out in the Lofoten Basin by the University of Bergen under NACO (North Atlantic Current Observatory) and PROVOLO (Water mass transformation processes and vortex dynamics in the Lofoten Basin of the Norwegian Sea) projects were added to the dataset (Fer & Bosse, 2017;[START_REF] Bosse | Seaglider missions in the Norwegian Sea during the PROVOLO project[END_REF]. Prior to merging, the data from different sources were consistently interpolated on a 5 db pressure grid from 0 to 3000 db. Duplicates from various sources were removed and a merging of profiles at mesoscale was used to homogenize the sampling rate of the different platforms. Out of 77,625 initial profiles, the final merged data set contains 57,753 state-binned profiles. Details on the data processing can be found in Bosse et. al. (2018).

Figure 2 shows the objective maps of the annual mean climatology of temperature and salinity at 300 m depth derived from this data set. The objective maps demonstrate that the warmest and saltiest waters are observed in the eastern and southeastern parts of the study area along the Norwegian shelf where the mean conservative temperature exceeds 8° C and absolute salinity reaches 35.2 g/kg. At the scale of the LB, a temperature and salinity gradient can be observed in the southeast/northwest direction.

Figure 2b demonstrates a spatial distribution of the CTD profiles acquired from the hydrographical data set for the LB region. The maximum number of CTD casts were carried out in the LV area. The high density of profiles in the LV area is not surprising as the quasipermanent anticyclonic vortex in the LB represents a natural laboratory for studying vortex dynamics in the ocean and has been the focus of extensive dedicated surveys over the years by research vessels and more recently by autonomous gliders [START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF]Bosse et al., 2019).

Since the aim of our work is mesoscale eddies of the LB region (except the quasi-permanent Lofoten vortex), we exclude LV area from the initial data sets. The general area of LV defined as 1°-5°E, 69°-71°N (Soiland & Rossby, 2013;[START_REF] Belonenko | Circulation of waters in the Lofoten Basin of the Norwegian Sea[END_REF][START_REF] Belonenko | Thermohaline structure of the Lofoten vortex in the Norwegian sea based on field research and hydrodynamic modeling[END_REF][START_REF] Raj | The Lofoten Vortex of the Nordic Seas[END_REF][START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF]Bosse et al., 2019).

Figure 2b shows that the whole study area is well covered with observations. More than 50% of 30x30km bins contain 200-300 profiles and less than 5% of bins have less than 50 profiles, therefore we can conclude that the LB region is well covered with CTD profiles. The total number of the profiles for the LB region is 17,985 without including the LV area (18,905 with the LV area) which corresponds to the ~31% of the initial data set.

Colocalization method

Each selected profile was associated with the nearest eddy if it satisfied the two following conditions: (1) The profile and the eddy have the same observation day; (2) The profile is located within a distance less than the eddy radius R derived from the "Mesoscale Eddy Trajectory Atlas Product". Figure 3a displays the geographical distribution of the acquired profiles in the study area. The distance between the location of the profile and the eddy center was normalized by the radius R. Normalized distance 0 corresponds to the eddy center while normalized distance 1 corresponds to the eddy maximum velocity average within the contour defining the eddy. The satellite-derived eddy radius R is equivalent to a dynamical radius characterized by zero vorticity (R = √2R_m for Gaussian eddies with R_m the radius defined by maximum orbital velocity). This approach is based on the assumption made by [START_REF] Zhang | Universal structure of mesoscale eddies in the ocean[END_REF] that mesoscale eddies, regardless of their amplitude, polarity or scale have the same structure, therefore the radial distribution of the eddy's thermohaline structure can be considered as universal. Subsequent works that followed that technique confirmed the feasibility of this approach (e.g. [START_REF] He | A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports[END_REF][START_REF] Kubryakov | Thermohaline structure, transport and evolution of the Black Sea eddies from hydrological and satellite data[END_REF][START_REF] Keppler | Observed characteristics and vertical structure of mesoscale eddies in the southwest tropical Pacific[END_REF].

However, we increase the size of the composite eddy to 1.5R (equivalent to about 2.1R_m), as the presence of eddies also influences its vicinity. Mesoscale eddies often have elongated shapes since they are prevalent in the areas where dynamic effect of the background currents is dominant. As a result, eddies experience a significant influence from the background flows at distances that can exceed its radius. When the vortex interacts with external currents, initial perturbations are formed at the vortex boundary. These disturbances can grow and simultaneously expand due to the current shear. Such influence can be tracked within distance of 1.5 and even 2R_m [START_REF] He | A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports[END_REF].

Analyzed profiles were classified into two groups depending on the vorticity sign: AE and CE. The number of profiles located within the distance less than R away from center of AE and CE is 703 and 456, respectively. The total number of profiles located within the distance of 1.5R from the eddy center is 1504 for AE and 1131 for CE. Their position relative to the eddy center is given in the fig. 3b and3c. The total number of unique eddies with the detected profiles within the distance of 1.5R from the eddy center is 292 AEs and 258 CEs. In order to better understand the vertical thermohaline structure of the mesoscale eddies and estimate zonal transport induced by CEs and AEs, the temperature (T') and salinity (S') anomalies were calculated by subtracting monthly local mean climatological T/S profiles from T/S profiles associated with the eddies. The mean climatology was computed from the hydrographical data set within 2.5 R around each eddy observation. To construct threedimensional composite eddy structure retained T'/S' profiles were finally interpolated onto a regular 0.1x0.1R grid at each depth level (5 db) using Barnes objective analysis with the smoothing length scale of 0.5 for both radial and vertical axis [START_REF] Barnes | Mesoscale objective map analysis using weighted time-series observations[END_REF][START_REF] Keppler | Observed characteristics and vertical structure of mesoscale eddies in the southwest tropical Pacific[END_REF].

Additional quality criteria were applied to the selected hydrographic profiles. Profiles with the first record below 20 m and less than 10 unique records were removed [START_REF] Chaigneau | Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: A composite analysis from altimetry and Argo profiling floats[END_REF]. Moreover, profiles with values greater than three standard deviations from the monthly climatology were considered as outliers and eliminated as well. Eddy profiles in areas shallower than 500m were also discarded.

Another question can arise due to the noise of the sampling. All profiles come from different eddies with various shapes and asymmetry. We obtain 3D composite structures by averaging a significant number of profiles. This procedure eventually reduces the statistical noise of the composite structures. As pointed out in previous studies (e.g., [START_REF] Willis | Combining altimeter and subsurface float data to estimate the timeaveraged circulation in the upper ocean[END_REF][START_REF] Chaigneau | Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: A composite analysis from altimetry and Argo profiling floats[END_REF][START_REF] Keppler | Observed characteristics and vertical structure of mesoscale eddies in the southwest tropical Pacific[END_REF] Taking into account the spatial resolution of SLA gridded data and difference in size between latitude and longitude for the study region, we divide LB area into 1° × 0.35° grid cells (about 40km squares at 69.5N). For every cell, we tracked the number of eddies crossing the cell boundaries (i.e., western/eastern boundary for zonal transport and northern/southern boundary for meridional transport).

Transport (D) is calculated as 𝐷 = 𝑉𝑁

𝑀 , where V is the volume of water transported by a single composite eddy, N is the number of detected eddies that crossed the cell boundary and M is the time of the analysis period. Westward and northward transports are defined as positive. For every cell the zonal (resp., meridional) volume transport was calculated as an average between transports of the western and eastern (resp., northern and southern) borders of the cell.

Results

Radial Structures

In order to analyze and compare the vertical thermohaline structure of AEs and CEs in the LB region, we made radial composite sections of temperature, salinity and density anomalies (Figure 4). Significant eddy-induced T′/S′ anomalies (> 0.1° and > 0.01 g/kg for AE and < 0.1° and < 0.01 g/kg for CE, corresponding to about 10% of the maximum T'/S' associated with eddies) are concentrated within one radius R and the depth of about 900m, associated with the displacement of the AW pycnocline. We also note that the composite eddies of both polarities significantly influence the thermohaline characteristics at the horizontal distance up to at least 1.5R.

The positive temperature anomaly in the composite AE is concentrated within the depth range of 150-900 m. The eddy core is located in the layer of 550-700 m with a maximum of about +1.0 ℃ at 600 m (Figure 4a). The distribution of the salinity anomaly in the composite AE shows a similar pattern with a maximum of +0.05 g/kg at around 600 m (Figure 4b). The radial section of density anomalies for the composite AE mostly follows the temperature anomaly distribution with a minimum at ~600m, density being mostly controlled by temperature in this region. Using the Thermodynamic Equation of Seawater (TEOS-10) we estimated that the temperature anomalies contribute more than 90% to the density anomaly of eddies of both polarities. The thermohaline structure of the composite CE shows a similar pattern yet with negative anomalies for temperature and salinity, and positive for density. The core of the composite CE is located within the 300-600m layer with a negative peak at the 500m. The minimum temperature anomaly equals -1.26 °C (Fig. 4d) and the minimum value of salinity anomaly is -0.06 g/kg (Fig. 4e). However, in comparison with the composite AE, the composite CE has a "dipole-like" structure in the salinity distribution (Fig. 4e). The positive anomaly of salinity is concentrated in the upper layer from the surface to 200 m with a maximum value of 0.07 g/kg.

It can be explained by the deflection of isohaline in CEs. However, a similar structure is hardly noticeable in the temperature and density radial cross-sections. Such "dipole-like" structures manifest themselves in the composite salinity distribution, but with insignificant influence on density (values of density in the upper layer are close to zero, see Fig. 4f).

Figures 5 and6 show the three-dimensional structure of the AE and CE composites in the LB respectively and horizontal distributions of the temperature and salinity anomalies (T′, S′) at selected depth levels.

The core of the composite AE has a gradual increase of T′ and S′ anomaly with depth (Fig. 5): from 0.4 °C and 0.03 g/kg at 200 m to 0.9°C and 0.05 g/kg at 600 m. The maximum T′ and S′ in the composite AE is 1.0°C and 0.07 g/kg at 650 m and about zero anomaly near the surface and at around 1000 m. The temperature anomalies > 0.1°C are detectable down to depths of ~ 950 m in the composite AE. From the depth of 600 m, the eddy signature gradually decreases with depth in both salinity and temperature. Figure 6 demonstrates 3D distributions of temperature and salinity anomalies for the composite CE. The minimum T′ and S′ in the composite CE is -1.6°C and 0.30 g/kg at 500 m and maximum T′ and S′ is 0.62°C and 0.013 g/kg in the surface layer. Despite significant differences, both composite eddies have some common features. Note that the composite AE and CE both have an asymmetric spatial distribution of T′/S′ with extrema (maximum for AE and minimum for CE) shifted at ~0.5 R relative to the eddy axis in the southeast direction (Fig. 5 and6). In other words, AEs in the LB are warmer and saltier in the southwest than the northeast side, similarly, CEs are colder and less salty in their southeast side. It might be a specific feature of mesoscale eddies in the LB region due to the east/west temperature and salinity gradients at the basin-scale.

The asymmetry of temperature and salinity in eddies is characteristic of mesoscale eddies in many ocean basins due to the non-linear eddy dynamic [START_REF] Pegliasco | Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems[END_REF][START_REF] He | A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports[END_REF][START_REF] Ma | Atmospheric responses to oceanic eddies in the Kuroshio Extension region[END_REF][START_REF] Simons | Linking kinematic characteristics and high concentrations of small pelagic fish in a coastal mesoscale eddy[END_REF]. Mesoscale eddies have a strong influence on the hydrological field during their propagation. Generated by the instability of currents, they trap and keep thermohaline and biogeochemical properties of their parent flow and redistribute them throughout their pathway. Non-linear eddies trap fluid inside their core, therefore it might be surprising that the extrema in T′/S′ distributions are not exactly found at the geometric eddy centers. However, the asymmetry in the eddies was found as an ubiquitous feature [START_REF] Chelton | The influence of nonlinear mesoscale eddies on near-surface oceanic chlorophyll[END_REF]Kubryakov et al., 2016). [START_REF] Yuan | Eddy-induced sea surface temperature gradients in Eastern Boundary Current Systems[END_REF] analyzing eddy-induced sea surface temperature gradients in Eastern Boundary Current Systems indicated that temperature anomalies in the eddies are subjected to many processes, including upwelling, flow instabilities, flow-topography interactions, and mesoscale variability. Temperature and salinity in eddies are closely related to underlying ocean submesoscale processes. As the background fields of temperature and salinity have a strong east-west gradient in the Lofoten Basin (see Fig. 2), we can expect that the extrema of eddy-induced T′/S′ would be shifted in roughly the same direction.

Figure 7 shows the mean vertical profiles of temperature, salinity and density anomalies obtained by averaging all profiles associated with AEs and CEs and located within the distance of 0.5 R from the eddy center. The mean AE and CE signature is mostly symmetric and of opposite sign in terms of temperature and salinity anomalies (positive for AEs and negative for CEs). The temperature anomalies larger than 0.1°C for the AE (resp., smaller than -0.1°C for the CE) are detectable above 900 m depth (Fig. 7a). Again, we can see the distinct characteristics in the morphology previously observed in the radial sections: the largest temperature anomalies for the AE with typical values approximately 0.9° C at ~ 650 m, while for the CE the strongest temperature anomalies (< -1°C) are found at ~ 500 m (Fig. 456). The salinity anomalies for CEs have a positive sign in the upper 200 m weakening with depth, changing sign at ~200 m and reaching minimum at ~600 m (-0.04 g/kg). A double structure with a core of more saline water in the upper layer and of fresher water deeper than 200 m is hence observed (Fig. 7b). The largest mean salinity anomalies in the composite AE are found at ~600 m depth (0.05 g/kg). The mean salinity anomalies for both composite AE and CE are close to zero at 1000 m depth. The vertically averaged temperature and salinity anomalies are 0.52 °C and 0.02 g/kg for the AE and -0.62 °C and -0.01 g/kg for the CE. The profiles of mean potential density anomalies are mostly consistent with temperature anomalies (Fig. 7c) for both AE and CE which again demonstrates that CEs density anomalies are mostly driven by temperature anomalies. However, the upper layer of CE is characterized by a positive density anomaly driven by salinity. A possible explanation for the different salinity anomalies near the surface of the AE and CE could be related to outcropping isopycnals near the CEs. These would create a frontal region that, unlike for the AEs that are characterized by an insignificant density anomaly near the surface, could act as a barrier by trapping the surface waters and therefore sustaining the salinity anomalies.

Eddy-induced volume, heat and salt transports

We use three-dimensional composites to estimate mass, heat and salt anomalies transported by AEs and CEs as well as the eddy contribution relative to heat and salt fluxes in the LB. The mean AEs radius R is 55.2 ± 19.4 km and 55.0 ± 19.2 km for CE with an average vertical extent of H ~930 m for both AE and CE, so the mean eddy volume V equals (8.9 ± 1.1)*10 12 m 3 for AE and (8.8 ± 1.07)*10 12 m 3 for CE (Table 1). According to the "Mesoscale Eddy Trajectory Atlas Product", the average propagation speed of mesoscale eddies c in the Lofoten Basin equals 0.04 ± 0.01 m/s for eddies of both polarities. Taking into account those mean eddy parameters the volume transport Vt for a single composite AE and CE can be calculated as:

𝑉 𝑡 = 𝑉𝑐 0.5𝑅 (1)
The total heat (HC) and salt (𝑆𝐶) contents transported by the composite AE and CE were calculated as follows [START_REF] Chaigneau | Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: A composite analysis from altimetry and Argo profiling floats[END_REF][START_REF] Dong | Mesoscale eddies in the Northwestern Pacific Ocean: Three-dimensional eddy structures and heat/salt transports[END_REF]:

𝐻𝐶 = 𝜌 𝐶 𝑝 ∫ 𝑇 ′ 𝑑𝑥 𝑑𝑦 𝑑𝑧 (1) 𝑆𝐶 = 𝜌 ∫ 𝑆 ′ 𝑑𝑥 𝑑𝑦 𝑑𝑧 (2)
where Cp = 4,200 J kg -1 K -1 and ρ = 1,025 kg m -3 are specific heat capacity of the seawater and mean upper ocean density respectively; T' is the eddy-induced temperature anomaly, S' is the eddy-induced salinity anomaly. The integration limits are defined as H and R. Notice that HC and SC transports are generally positive for AE and negative for CE (Table 1). As a result of larger temperature anomalies, the composite CE has 1.4-1.5 times more impact on the thermohaline fields in the LB than the composite AE (Table 1). Heat transport (10 13 W) 1.9 ± 0.7 -2.5 ± 0.9 Salt transport (10 5 kg/s) 1.5 ± 0.5 -1.2 ± 0.4

Time-mean Eddy-induced Volume Transport

The mean eddy induced zonal and meridional transports in the LB are estimated in 1° × 0.35° bins (excluding the LV area) using the method described in section 3 (Figure 8). The eddy-induced zonal transport (Fig. 8a) has a general westward direction with an average magnitude of 0.46 Sv/deg² for the whole study region. The zonal eddy-induced transport is significantly strengthened in the northeastern part of the study area with maximum values of 3.95 Sv/deg². It is weakened in the southeastern part of the basin, where the values of the zonal transport are close to zero. The eastward transport appear in the western area of the LB controlled by the NwAFC dynamics (Bosse & Fer, 2019a). Here, the intensity of eastward transport reaches 1.26 Sv/deg².

The patterns of the geographical distribution of eddy induced meridional transport is more complex than zonal transport (Figure 8b). There are several significant characteristics worth noting. The northward transport takes place predominantly in the southern and eastern parts of the study region along the NwASC flows. It should also be noted that there is an area in the north-east of the LB within the longitude band 9°-14°E where southward transport also prevails.

By estimating fluxes of heat and salt across the contour enclosing the central part of the LB, we can evaluate the amount of heat and salt which propagates toward the LB center and thereby potentially affecting the LV, as eddy induced heat/salt transport by mesoscale eddies and eddy mergers were identified as important mechanisms for the LV stability and longevity. We defined this border as the contour of the 2800m isobath (Fig. 8). Based on data from the "Mesoscale Eddy Trajectory Atlas Product" the total number of mesoscale eddies crossing the The origin and mechanisms of CEs generation have not clearly been identified yet, whereas NwASC has been identified as an important source for AEs generation (Rossby et. al., 2009a;[START_REF] Isachsen | Observed and Modeled Surface Eddy Heat Fluxes in the Eastern Nordic Seas[END_REF]Raj et al., 2016;Trodahl & Isachsen, 2019;[START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF].

Considering the daily average number of AEs observed inside the 2800m isobath (0.8) and a turnover rate given by the inverse of the mean AE lifetime (34.3 days), the heat transported by composite AEs into the LB center would represent an average value of 38 W m -2 , compensating for about half of the annual net heat loss to the atmosphere in the area reported by previous studies [START_REF] Isachsen | Observed and Modeled Surface Eddy Heat Fluxes in the Eastern Nordic Seas[END_REF][START_REF] Richards | Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas[END_REF][START_REF] Yu | The Lofoten Basin eddy: Three years of evolution as observed by Seagliders[END_REF]Bosse et al., 2019).

Note that this number might be underestimated, as eddies smaller than about 30 km radius are not well detected by satellite, but could still be major players of the heat convergence toward the Lofoten Basin (e.g. [START_REF] Richards | Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas[END_REF][START_REF] Isachsen | Observed and Modeled Surface Eddy Heat Fluxes in the Eastern Nordic Seas[END_REF][START_REF] Lundrigan | Mean and eddy-driven heat advection in the ocean region adjacent to the Greenland-Scotland Ridge derived from satellite altimetry[END_REF] reported from moored instruments in the Lofoten Basin mesoscale eddies with a mean radius of 17.4±9.0km corresponding to about 25 km zero vorticity radius.

Zonal and meridional eddy volume transport

The eddy-induced zonal and the meridional transports in the area enclosed by the boundary of 2800 m isobath is mainly westward along the northern and eastern boundaries of the LV area and eastward along the south-western boundary. A general northward transport is observed at the southern and eastern boundaries and southward transport at the northern and western boundaries of the LV region. This pattern of the eddy propagation agrees well with the counterclockwise eddy spiraling around the LB center [START_REF] Volkov | Puzzling over the dynamics of the Lofoten Basina sub-Arctic hot spot of ocean variability[END_REF][START_REF] Volkov | Formation and Variability of the Lofoten Basin Vortex in a High-Resolution Ocean Model[END_REF] and a recent study highlighting the connection of AW from south across the outer rim of the Vøring Plateau [START_REF] Dugstad | Vertical structure and seasonal variability of the inflow to the Lofoten Basin inferred from high-resolution Lagrangian simulations[END_REF]. Figure 9 shows the meridionally (resp., zonally) integrated zonal (resp., meridional) volume transport induced by the mesoscale eddies in the LB region. The largest westward transport is observed in the longitude bands 9°-12°E (Figure 9a) and over the whole LB the eddy-induced volume transport is westward except in the westernmost part of the region (longitude band 5-1W). It is consistent with results of several studies highlighting greatest eddy activity in the eastern part of the basin due to instability of the NwASC [START_REF] Köhl | Generation and stability of a quasi-permanent vortex in the Lofoten Basin[END_REF]Rossby et. al., 2009a;[START_REF] Isachsen | Observed and Modeled Surface Eddy Heat Fluxes in the Eastern Nordic Seas[END_REF][START_REF] Søiland | On the structure of the Lofoten Basin Eddy[END_REF]Raj et al., 2016;Trodahl & Isachsen, 2019;[START_REF] Zinchenko | Analysis of Mesoscale eddies in the Lofoten Basin based on satellite altimetry[END_REF]. In general, the zonal component of transports induced by AEs exceeds that induced by CEs which is not surprising as the number of individual AEs identified in the LB exceeds the number of individual CEs. Note that the total eddy-induced zonal volume transports is westward except for the western part of the study region (longitude band 5°-1°W) where mesoscale eddies can be generated by instability of the NwAFC.

The distribution of the zonally integrated meridional volume transport is more complex (Figure 9b) and can be divided into three distinct areas with northward transport south of 69° With the present methodology, we are not able to subtract potential errors due to spatiotemporal gaps in the altimetric tracks sampling, as well as the influence of the seasonal and interannual variability of the basin properties, and internal waves.

The study of [START_REF] Lundrigan | Mean and eddy-driven heat advection in the ocean region adjacent to the Greenland-Scotland Ridge derived from satellite altimetry[END_REF] showed that the distribution of the eddy radii in the LB ranges between 19 km and 25 km. Since the minimum radius from the Mesoscale Eddy Trajectory Atlas Product is 14 km, we also take into account these eddies in our calculations. Moreover, the difference in the distribution of eddy sizes can be also caused by differences in the methods of the eddy detection.

As pointed out by [START_REF] He | A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports[END_REF] the heat and salt transports calculated from the composite eddies and altimetry data do not consider diapycnal mixing with ambient waters.

These problems can be further investigated by using oceanic model simulations. As mentioned in [START_REF] Keppler | Observed characteristics and vertical structure of mesoscale eddies in the southwest tropical Pacific[END_REF], this type of study can be of great use to track the eddy signature in shipborne CTD profiles or profiles made by ocean gliders. The obtained three-dimensional structures can be also used as a base for the future estimation of the impact of mesoscale eddies on the biogeochemical processes and environment in the LB region, as well as for validation of high-resolution climate and regional models.

Figure. 2

 2 Figure. 2. (a) Objective maps of temperature (color shading) and salinity (contours) climatology at the 300 m depth calculated from the data set "Hydrography of the Nordic Seas, 2000-2017: A merged product" (Bosse & Fer 2018). (b) Spatial distribution of the available hydrographic profiles in the Lofoten Basin (the number profiles in 1°× 0.35° bin for the period 2000-2017). The green box indicates the area where the Lofoten Vortex (LV) is generally observed.

Figure 3 .

 3 Figure 3. (a) Geographical position of the hydrographical profiles acquired inside anticyclonic (red dots) and cyclonic (blue dots) eddies for the period 2000-2017 in the LB region and bathymetric contours; (b,c) The distribution of the obtained profiles in the normalized eddycoordinate system associated with anticyclonic and cyclonic eddies.

  the method of in situ measurements colocalization with eddies in time and space is subjected to potential errors associated with positioning precision of hydrographic profiles and eddy centers detection. The position of eddy centroid is calculated by analyzing daily interpolated SLA maps which have certain limitations and mapping errors due to the spatio-temporal gaps in altimetry tracks sampling. See Chelton et al. (2011a) for detailed information on the AVISO data processing and implemented solutions for reducing the errors. Since the data set we use contains merged profiles from the different sources, the positioning error can vary depending on different types of profiles. For their study region, Chaigneau et al. (2011) estimated the average total error of positioning Argo floats of ~1 km, while under certain extreme conditions it can reach ~5 km. It should be noted, that estimated positioning error is considerably lower of an average radius of the mesoscale eddies in the LB. Despite certain shortcomings of the method, the large number of profiles increase the statistical significance of the signals associated with the eddies. Following the approach of Chaigneau et al. (2011) we performed the non-parametric Mann-Whitney U-test (p < 0.05) which justifies the statistical significance of obtained profiles. The test confirms that the T′/ S′ anomalies computed inside eddies are significantly different from the T′/S′ anomalies outside eddies.FollowingChagneau et al. (2011), we estimated the eddy-induced volume, heat and salt anomalies transported by AEs and CEs and their contribution to the heat and salt fluxes in the LB region. To estimate the spatial distribution of zonal and meridional eddy induced transport we follow the approach of[START_REF] Dong | Mesoscale eddies in the Northwestern Pacific Ocean: Three-dimensional eddy structures and heat/salt transports[END_REF], who proposed a new method using eddy trajectories instead of propagation velocity (e.g.[START_REF] Dong | Global heat and salt transports by eddy movement[END_REF].

Figure 4 .

 4 Figure 4. Radial cross-sections of temperature (a,d), salinity (b,e) and density (c,f) anomalies in the composite anticyclonic (top) and cyclonic (bottom) eddies.

Figure 5 .

 5 Figure 5. Three-dimensional temperature (a) and salinity (b) structure of the composite AE and horizontal distribution of temperature and salinity anomalies at the 300, 600, 700 and 800 m.

Figure 6 .

 6 Figure 6. Three-dimensional temperature (a) and salinity (b) structure of the composite CE and horizontal distribution of temperature and salinity anomalies at the 100, 300, 500 and 700 m.

Figure 7 .

 7 Figure 7. Mean vertical profiles of temperature (a), salinity (b) and density (c) anomalies averaged within 0.5 R for the composite AE and CE.

  2800 m isobath contour in/out is 131/104 for the AEs and 105/87 for the CEs. This corresponds to a net transport of 27 AEs and 18 CEs within that contour during the period 2000-2017. The evaluations of the total heat and salt transports induced by AEs and CEs inside the area bounded by the 2800m isobath are presented in Table2. The advection of heat or salt into the LV area has a slightly positive balance since the inflow of the heat or salt transported by eddies exceeds the outflow. It seems that most of the heat and salt transports induced by AEs is compensated by CEs, but other mechanisms are important for the fate of these eddy-induced transports, e.g. turbulent transfers to the atmosphere or the ocean interior. Based on the study[START_REF] Segtnan | Heat and freshwater budgets of the Nordic seas computed from atmospheric reanalysis and ocean observations[END_REF], our estimated transport across the contour of 2800 m isobath of 1 TW is about 3.3% of the residual heat transport over the whole area of the eastern Nordic Seas.

Figure 8 .

 8 Figure 8. (a) Zonal and (b) meridional eddy volume transport calculated for every 1°× 0.35° bin in the Lofoten Basin region (except the Lofoten Vortex area). The positive value refers to the westward (northward) transport, the negative value refers to the eastward (southward) transport. The green contour indicates the 2800m isobath enclosing the central part of the LB.

N

  and north of 70.8° N respectively, and southward eddy transport between 70.2° N and 70.5° N. The overall northward transport induced by the LB eddies reaches the maximum value around 68° N. It must be noted that north of 72°N the meridional transport for AE is northward while CE transport is southward. The patterns of eddy-induced volume transport are closely related to the distribution and propagation of eddies in the LB region. The predominance of the volume transport induced by westward propagating eddies throughout the LB region confirms the crucial role of mesoscale eddies forming in the western part of the basin along the NwASC flow. The westward zonal transport can also be larger due to the comparatively larger eddies forming near the NwASC than those forming in the eastern part along the NwAFC in particular(Raj & Halo, 2016). From the distribution of the eddy-induced zonal volume transport (Figure8a) we can assume that the zonal westward eddy-induced transport contributes significantly to the general transport of heat and salt in the study region. The eastward transport is likely associated with eddy shed from the NwAFC, however, it can also be influenced by westward propagating eddies recirculating back to the east following the topographic contours.

Figure 9 .

 9 Figure 9. (a) Meridionally integrated zonal volume transport and (b) zonally integrated meridional volume transport in the Lofoten Basin region (except the Lofoten Vortex area). The area bounded by the dashed line corresponds to the LV region.

4.

  Summary and conclusionBy using the updated version of the eddy dataset distributed by AVISO+ and applying a method of colocalization of in situ observations, we obtained detailed three-dimensional structures of composite AE and CE in the Lofoten Basin. The general structure of cyclonic and anticyclonic eddies reveals significant thermohaline anomalies concentrated within a distance of one radius (defined by zero vorticity contour) and extending vertically to the depth of ~900-1000 m. A key difference between CEs and AEs was found in terms of salinity and density anomalies in the upper layer.The temperature and salinity anomalies in the AEs reach 1.0° C and 0.05 g/kg respectively at 600 m and concentrate within the depth range of 150-900 m. Radial sections of density anomalies for the composite eddies closely follow the distribution of temperature anomalies with density anomalies in composite AE of -0.15 kg/m 3 at ~500 m. In general, the thermohaline structure of eddies in the LB is more influenced by temperature contributing to more than 90% to the density anomalies.The deep thermohaline structure of the CEs shows a similar pattern yet with negative values for temperature and salinity anomalies and positive for density. The core of the composite CE is located within the layer 300-600 m with a negative peak at 500 m. The minimum temperature anomaly equals -1.2 °C and salinity anomalies reach -0.06 g/kg. In contrast with the composite AE, the composite CE exhibits a "dipole-like" structure in the vertical salinity distribution with positive anomalies in the upper layer resulting in a positive density anomaly reaching the surface with values of 0.1 kg/m 3 . A possible explanation for this phenomenon is an outcropping of isopycnals in the CEs which could act as a barrier by trapping the surface waters and therefore sustaining the salinity anomalies.A prominent feature of mesoscale eddies was revealed in spatial distributions of thermohaline anomalies inside obtained composite structures. AEs in the LB are warmer and saltier in their southwest compared to their northeast side. Similarly, CEs are colder and less salty in their southeast side. This particular feature of mesoscale eddies in the LB likely reflects features of the large-scale temperature and salinity gradients which are strongly affected by the warm and salty flow of the Norwegian Atlantic Slope Current. However, the observed pattern of the east-west gradient in the composite AE is not consistent with the spatial distribution of the temperature anomalies obtained within the borders of AEs. The temperature anomalies vary from the east (~0.5°C) to the west (~1°C) in the opposite direction as the background AW temperature.Mean zonal eddy-induced transport is generally westward, significantly increasing and reaching a local maximum in the northeastern part of the study region. The predominance of the westward eddy-induced volume transport over the whole LB confirms the key role played by mesoscale eddies in the lateral heat and salt transfers from the NwASC toward the basin interior and their significant role in the general oceanic circulation of the LB.Raj et. al. (2016) used a similar technique of colocation the Argo profiles and altimetry data to study the vertical structure of eddies in the LB region. In their work, they identified 55 and 45 profiles inside AEs and CEs respectively in the LB, but in contrast with our study, they used a much smaller data set of hydrographical profiles (only Argo floats) and didn't calculate climatological anomalies for the obtained profiles. They pointed out that the penetration depth of the AW is generally deepest inside AE reaching 1200 m for the individual profiles and salinity profiles inside the AEs showing deeper and well-mixed saline AW. Considering that they didn't exclude profiles inside the permanent and deep Lofoten Vortex, this can be a reasonable explanation for this large value. These results are generally in good agreement with our work.

Table 1

 1

	. Characteristics of a composite AE and CE
		AE	CE
	The average annual number
	of individual eddies in the LB	26	23
	region
	Volume (10 12 m 3 )	8.9 ± 1.1	8.8 ± 1.1
	HC (10 19 J)	2.2 ± 0.8	-3.2 ± 1.1
	SC (10 11 kg)	1.8 ± 0.6	-1.6 ± 0.5
	Volume transport (Sv)	4.1 ± 0.7	4.0 ± 0.7

Table 2 .

 2 Annual heat and salt eddy-induced transports across the contour of 2800 m isobath

	AEs	CEs

Acknowledgements

This research was supported by the Russian Science Foundation .

A.B. received funding from the Research Council of Norway, through the project "Water mass transformation processes and vortex dynamics in the Lofoten Basin of the Norwegian Sea (PROVOLO)" (project No. 250784). We thank Ilker Fer from University of Bergen, Norway for initiating the PROVOLO project that produced the dataset "Hydrography of the Nordic Seas, 2000-2017: A merged product". The dataset is publically available at https://doi.org/10.21335/NMDC-1131411242. We thank Aviso User Service for the dataset "Mesoscale Eddy Trajectory Atlas Product". The dataset is available at https://www.aviso.altimetry.fr/en/data/products/value-added-products/global-mesoscaleeddy-trajectory-product.html.

The bathymetry dataset ETOPO 1 is available at http://dx.doi.org/10.7289/V5C8276M.


K Saheb Ettabaâ
email: karim.sahebettabaa@riadi.rnu.tn

I R Farah

M B Ahmed

B Solaiman
email: basel.solaiman@enst-bretagne.fr

Mining Spatio-Temporal Metadata for Satellite Images Interpretation

Keywords: interpretation of remotely sensed images, hierarchical blackboard architecture, multi-agent system. I

Mining the growing data issued from the interpretation of remotely sensed images to obtain the necessary information for land cover change studies becomes more difficult and makes the data volume problem particularly acute. Mitigating this problem requires using data efficiently as metadata for mining and selecting appropriate data for change studies.

In this paper, we propose an integrate hierarchical approach based on the use of a blackboard architecture and multi-agent system and having a reasoning ability to find the best strategy to extract and create metadata about extracted objects. This architecture models relation-ship between objects and primitives extracted from images as metadata and use a transition diagram to handle temporal dependencies and perform the detection of temporal changes of objects.

We validate our approach on a set of multi-temporal Spot images, to model the evolution of detected object

INTRODUCTION

Remotely sensed images have proved to be of great interest for earth resource assessment and environment monitoring. Automatic or semi-automatic analysis of these images becomes an important task in computer vision. Therefore important efforts have been made to increase the reliability of interpretation results of remotely sensed images. One of the most promising approaches is to exploit the metadata about the scene and the extracted objects in it.

The extraction of information from scene has been investigated in the domain of interpretation of digital images. Therefore the need for interpretation is obvious, as a task to automatically determine individual pixels or collection of pixels representing objects. Basic techniques for image interpretation are either pixel classification methods or structure based matching techniques. The major application in remote sensing lies in the automatic extraction of topographic features like roads, building, trees, etc. The main challenge is providing appropriate models for the objects founded in the scene and making efficient use of data obtained through different sensors or in different time periods.

Increasing amounts of large data and information sets extracted from images require new analysis techniques. The domain of data mining investigates new paradigrns and methods adapted for scalability, flcxibility and problem abstraction for large data sets.

A scene is a dynamic canvas through which human and natural systems interact. Understanding the many factors influencing land cover change of a scene has been the focus of scientific study across multiple disciplines. Direct measurements are not sufficient to provide an understanding of the forces driving change. Linking spatial and temporal observations to empirical models provides a comprehensive approach to understand land-cover change [START_REF] Decker | A real-time control architecture for an approximate processing blackboard system[END_REF]. One promising class of models designed to simulate and interpret is multiagent system based on blackboard architecture which used as a collection of intelligent agents gathered around a hierarchical blackboard, looking for information written on it, thinking about the current state of the solution, and writing their generated conclusions on it. Each agent combines contextual information with a knowledge source to contribute toward the solution in opportunistic way. The blackboard acts as a shared memory, visible to all agents and permitting communication inter-agents instead of using point-to-point communication [START_REF] Decker | A real-time control architecture for an approximate processing blackboard system[END_REF].

The sequence of interpretation was split up in two complementary processes, called top-down and bottom-up. In the top-down process, hypothesis, represented as hypothesis instance nodes, are generated from the possible occurrence of any expected object. Each one of the hypotheses corresponds to some concept of the conceptual network. The bottom-up process tries to validate the hypothesis in order to generate the symbolic description of the proposed scene. During the interpretation, the generic semantic network generates an associated network of instances based on the network hypothesis. Therefore, each object extract will be considered as a set of metadata.

We propose to work on a sequence of multi-temporal images, modeling the evolution of objects detected and comparing their expected states to a model simulation. The remainder of this paper is organized as follows: in a first time, we present the blackboard architecture developer over the last decade. Then, we introduce the semantic networks. After that, we present the proposed architecture.

II. METADATA

Increasing amounts of large data and information sets require new analysis techniques. The domain of data mining investigates new paradigms and methods adapted for scalability, flexibility and problem abstraction for large data sets.

Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource. Metadata is often called data about data or information about information. The term metadata is used differently in different communities. Some use it to refer to machine understandable information, while others use it only for records that describe resources.

There are three main types of metadata:

-Descriptive metadata describes a resource for purposes such as discovery and identificndation. -Structural metadata indicates how compou objects are put together. -Administrative metadata provides information to help manage a resource, such as when and how it was created, file type and other technical information, and who can access it. An important reason for creating descriptive metadata is to facilitate discovery of relevant information. In addition to resource discovery, metadata can help organize resources, facilitate interoperability and legacy resource integration, provide digital identification, and support archiving and preservation.

A.

Structuring Metadata Metadata schemes (also called schema) are sets of metadata elements designed for a specific purpose, such as describing a particular type of information resource. The definition or meaning of the elements themselves is known as the semantics of the scheme. The values given to metadata elements are the content.

Metadata schemes generally specify names of elements and their semantics. Optionally, they may specify content rules for how content must be formulated (for example, how to identify an object), representation rules for content (for example, capitalization rules), and allowable content values (for example, terms must be used from a specified controlled vocabulary).

Many current metadata schemes use SGML (Standard Generalized Mark-up Language) or XML (Extensible Markup Language). XML, developed by the World Wide Web Consortium (W3C), is an extended form of HTML that allows for locally defined tag sets and the easy exchange of structured information.

SGML is a superset of both HTML and XML and allows for the richest mark-up of a document. Useful XML tools are becoming widely available as XML plays an increasingly crucial role in the exchange of a variety of data on the Web. The attributes are grouped into eight categories:

-General, containing information about the object as a whole. -Lifecycle, containing metadata about the objects evolution. -Relation, identifying related objects.

The Description Schemes are based on XML, and can be expressed in textual form suitable for editing, searching, filtering, and human readability; or in a binary form for storage, transmission, and streaming delivery. Since the full description of a multimedia object can be quite complex, the standard provides for a Summary Description Scheme geared to browsing and navigation.

B.

Gathering Meta-data The process of gathering metadata is flot trivial. Determination of all useful metadata can be expensive, so it is absolutely necessary to find an effective way to gather meta data. This can be achieved by the following strategies: --Predefinition of a suitable order of gathering processes, by avoiding repeated calculations and by utilizing processes which supplement each other.

--Dividing the gathering processes into automatic, semiautomatic calculations and user interactions with different levels of user support. --Taking account of time for meta data extraction for appropriate parameterizations of slow processes and or for avoiding long extractions. -

III. DISTRIBUTED BLACKBOARD ARCHITECTURE FOR MULTI- TEMPORAL SCENE INTERPRETATION

The goal of our work is to propose a generic scene interpretation system. The main characteristic of such system is its reasoning ability in order to find the best strategy to extract visual information needed for this goal. To do this, it needs two kinds of knowledge which are descriptive and operational knowledge [START_REF] Dalle | Agents de Médiation pour la Conception de Systèmes de Traitement d'images[END_REF]. The descriptive knowledge used to describe the scene and the objects within. The operational knowledge used to describe how to combine different information to deduce new facts [START_REF] Ealet | Une architecture multi-agent pour la detection, la reconnaissance et l'identification des cibles[END_REF].

While the segmentation process, the system extract a large scale of information from multi-temporal images and the main difficulty is to select and process only the relevant information, to avoid problems like combinatorial explosion and very long processing time.

During the last two decades, several works concerning the interpretation of different types of scenes using the knowledge based approach have been successful. Among these works, we can cite the Sigma, Kids, Icare systems which based on an expert system, the Aerosol and Alain Boucher's system which based on multi-agent system [START_REF] Dong | A Behavioral Analysis and Verification Approach to Pattern-Based Design Composition[END_REF], the Vision, MessieI and Skids system which based on blackboard architecture [START_REF] Silva | The Reflective Blackboard Architectural Pattern for Developing Large Scale Multi-Agent Systems[END_REF].

Unfortunately, none of them propose a generic architecture for a scene interpretation. In fact, the knowledge representation and the reasoning strategies in most of these systems are often application. This is a big drawback because that means new development have to be made for each new application [START_REF] Farah | A generic multi-Agent system for analysing spatial-temporal geographic information[END_REF]. But on the other hand, an important, fact emerges is the use of blackboard architecture [START_REF] Vincent | Learning as a means to refine a knowledge based system[END_REF] and semantic networks to model the evolution of dynamic objects.

IV. SEMANTIC NETWORK

A semantic network contains two different types of knowledge: declarative and procedural knowledge. Declarative knowledge consists of concepts and links, while procedural knowledge contains methods for determination of attributes of concepts as well as for valuation of concepts and relations. The declarative knowledge can be described by a marked and directed graph, where the nodes are attributed. Therefore, graph-theory can be applied for the control and analysis process within a semantic network [START_REF] Niemann | Ernest: A semantic network system for pattern understanding[END_REF].

Semantic networks consist of nodes and links, and are defined as directional acyclic graphs. Specifically, in our system, nodes represent the objects expected in the scene, whilst links describe the relations between the objects. In this context, the initial description of the scene contents, including nodes and links, is called conceptual semantic network. We define three different kinds of nodes: the class nodes represent classes of objects, the compound nodes represent objects detected in the scene and end nodes represent sets of attributes and characteristics for each object.

The sequence of interpretation can be split up in two complementary processes, called top-down and bottom-up. In the top-down process, hypothesis, represented as hypothesis instance nodes, are generated from the possible occurrence of any expected object. Each one of the hypotheses corresponds to some concept of the conceptual network. The bottom-up approach tries to validate the hypothesis. In this way, the interpretation process generates the symbolic description of the proposed scene [START_REF] Tonjes | Knowledge-Based Interpretation of Remote Sensing Images Using Semantic Nets[END_REF].

A. Top-down process

The top-down process has the task to separate a region into sub-regions and to build hypothesis for the expected objects. The task is realized recursive from the upper nodes in the semantic network to the lower nodes. For this purpose any proposed agent can be integrated, which creates hypothesis for the sub-regions by means of four view points which are geometric, radiometric, spatial context and temporal. During the top-down-analysis the restrictions of the concepts are checked and hypothesis, which are not conform are deleted.

The depicted process chain is shown in figure1. The top-down step generates a hypothesis network; the bottom-upstep generates an instance network. Each semantic object specialist has its own detection strategy. In general way, a semantic object specialist starts by asking a low-level feature extraction according to one of the semantic, object view point (geometry, radiometry and spatial context) and uses the others view points to validate the selected features. Therefore after the validation step, the scene could be described as a collection of different types of objects and each object as a collection of samples with different description and parameters as in figure2.

B.

Bottom-up process The bottom-up process has the following tasks:

Extraction of object attributes and measurement of single objects. Measurement of the new group, generated by the bottom-up-step. If the top-down-analysis reaches the leaf nodes, the analysis turns from model based interpretation to data based interpretation (bottom-up). The bottom-up process can also be extern programs, designed by the user. The top-down path can generate different hypothesis for one region. The bottom-upstep has to decide for an explicit interpretation for a region (figure 1).

V. THE PROPOSED ARCHITECTURE

This system has distributed hierarchical blackboard architecture (figure2) and based on a set of knowledge sources communicate through a multi-level data structure. The knowledge sources are structured in a three level hierarchy: Strategy, tasks and specialists [START_REF] Farah | Analysing Spatial-Temporal geographic information based on blackboard architecture and multi-agent system[END_REF].

Our system is composed of three modules which are the user control module, the dynamic reasoning module and the execution control module. Each level represents a different view of the goal resolution space. The hypothesis is the basic data unit of the blackboard and represents a partial solution. The agents are invoked by the control mechanism in response to a particular change on the blackboard [START_REF] Decker | A real-time control architecture for an approximate processing blackboard system[END_REF].

The user control module is in charge of supervision and interaction with the operator. The blackboard structure of this module contains a symbolic description of the scene. The dynamic reasoning module is the center of intelligence of the intelligent control system. Strategies are received from the user control module. The blackboard structure of this module consists on three abstraction level hierarchies: Strategy, tasks and specialists [START_REF] Corkill | Collaborating Software: Blackboard and Multi-Agent Systems & the Future[END_REF], [START_REF] Carver | A Revisionist View of Blackboard Systems[END_REF]. This module interacts with a knowledge base, object model base, scene metadata and a geographical information system. The object base model contains descriptions of semantic objects that have been preextracted from the images. Each object is defined through a set of features extracted from the image and a classification scheme that assigns semantics to regions of an image. As a result, different kinds of objects can be extracted from the same region of an image using different feature sets. The attributes of the object base model include the metadata used for extracting the objects, stored in a XML metadata file and uses keywords with a keyword type for each metadata object, in order to provide users a way to effectively find metadata of objects that verify some criteria. In order to improve the process, an XML template defining all the fields to be filled is used (figure3).

The execution control module is responsible for the execution of actions specified by the dynamic reasoning module. On each level the agent control is responsible of the management of the structure and for scheduling of the agents. Each level receives events from other levels or from other modules. On each level, a control agent identifies the event type and processes it. Types of events can be modifications of the blackboard data or signals of agent termination [START_REF] Penny | Blackboard Systems: The Blackboard Model of Problemsolving and the Evolution of Blackboard Architectures[END_REF] [START_REF] Lassri | Atome : a blackboard architecture with temporal and hypothetical reasoning[END_REF]. The evolution and change of system agents is supported by the architecture. The architecture of the multi-agent systems is able to cope with evolution of the system. New agents are able to enter the systems as knowledge source or control, whereas old agents may leave the systems [3] [15]. Agents are also adaptable to different environment. In this situation, agents are reusable in different systems and environments [START_REF] Farah | A generic multi-Agent system for analysing spatial-temporal geographic information[END_REF].

Applications like change detection and environmental monitoring require the analysis of images from different acquisition times. By comparing the current image with the latest interpretation derived from the preceding image, land use changes and new objects can be detected. The temporal approach proposed employs a transition graph to describe the temporal dependencies between the classes of interest. The transition graph models the expected transition of objects in the scene.

Temporal changes can be formulated in a so called state transition graph where the nodes represent the temporal states and the edges model the state transitions. To integrate the transition graph in a semantic net the states are represented by concept nodes which are connected by a new relation: the temporal relation. As states can either be stable or transient, the corresponding state transitions differ in their transition time which can be also specified in the temporal relation. The start and end node of temporal relations may be identical forming a loop to represent that the state stays unchanged over time.

During the interpretation process [START_REF] Barnes | Image-Driven Data Mining for Image Content Segmentation, Classification, and Attribution[END_REF], the state transition diagram is used by a new inference rule. Analysis starts with the first image of the given sequence marked with time t1. If a state of the state transition diagram can be instantiated completely, the temporal knowledge is used to hypothesize one or more possible successors of this state for the next image in the chronological order (time t2). The system selects all successor states that can be reached within the elapsed time t2-t1 according to the transition times defined in the temporal relations.

VI. VALIDATION

In order to validate our approach, we are carrying out the pre-processing of SPOT4 images. The basic system implemented by a multi-agent system was each agent associated to an operator located in the operator base and provided by the dynamic reasoning module. As an input, the satellite images (Figure4) used in our application is provided from different sensor like XS1, XS2, XS3 and XS4. Agents use unsupervised operators for classification provided by the operator base and evaluated by a Multi-agent Engine which combine facts and rules provided from knowledge base [17] [18]. The output classified images (figure5) represent five classes (water zone, humid zone, bare soil, vegetation and dense vegetation). These classes allow the extraction of principal objects by means of radiometric, geometric and spatial features. This information was given as an input to the agent change detection in order to compute the similarity between objects and detect the change occurred in our zone of interest (figure 6). In our example, we cope with the transition occurred in the parcel zone and specially the transition from vegetation class to other classes. This transition was expanded with a state transition diagram to handle temporal dependencies to model relevant changes in the data (figure 7).

VII. CONCLUSION

We propose an integrated hierarchical approach based on the use of a hierarchical blackboard architecture and multi-agent system in order to increase the degree and the quality of interpretation. The proposal hierarchical architecture is motivated in order to reduce the information complexity and complex interpretation tasks which its main characteristic is its reasoning ability in order to find the best strategy to extract visual information needed for this goal. This configuration results from the auto-organization of our system and composed by agents who represent operators and having essentially a cooperative and a social attitude. This system organizes itself and stabilizes according to the reactions coming from its environment. The semantic net; employed in this architecture, models relation-ships between objects and primitives extracted from images as metadata. The system was expanded with a state transition diagram to handle temporal dependencies. The state transition diagram is used to perform the detection of temporal changes of objects.

Figure 1 .

 1 Figure 1. Semantic scene description top-down and botton-up process

Figure 4 .Figure 5

 45 Figure 4. Spot4, XS3 images Im1 : 10/02/2001 Im2 : 03/05/2001 Im3 : 26/07/2001

Figure 7 .

 7 Figure 7. Graph transition modeling the parcel transition