Nicolas Barrier
email: nicolas.barrier@univ-amu.fr

Anne A Petrenko

Yann Ourmières

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

Keywords: Northern Current, Gulf of Lion, Intrusions, Julio, Upwelling, Cross-shelf exchanges, Wind-setup, Easterlies, Mistral, Tramontane

published or not. The documents may come

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

Nicolas Barrier, Anne Petrenko, Yann Ourmieres

Introduction

Coastal areas are a key environment for the marine ecosystem, since they receive large amounts of nutrients through river outflows that in turn favour phytoplankton activity [START_REF] Cruzado | The ERA-Interim reanalysis: configuration and performance of the data assimilation system[END_REF]. They also play a significant role in the global biogeochemical cycles of carbon, nitrogen and phosphorus (Mantoura et al, 1991;[START_REF] Liu | Exploring continental margin carbon fluxes on a global scale[END_REF]. But coastal areas are also subjected to high demographic pressure and consequently to great risks of pollution. In this context, a better understanding of the coastal circulation is critical since it controls the dispersion of anthropogenic and river-discharged pollutants [START_REF] Huthnance | Seasonal, interannual, and mesoscale variability of the Black Sea upper layer circulation derived from altimeter data[END_REF], but also the advection of nutrients and larvae [START_REF] Largier | Considerations in Estimating Larval Dispersal Distances from Oceanographic Data[END_REF]. This is especially true for the Gulf of Lion, which is a wide and shallow continental shelf in the northwestern Mediterranean Sea (Fig. 1), featuring a heavily urbanised shoreline.

Several forcings influence the coastal circulation in the Gulf of Lion. The Mistral and the Tramontane (hereafter referred to as northwesterlies) are cold and dry continental gustly winds that occur anytime in the year. They are highly constrained by the orography (Rhône river valley for the Mistral, Pyrenees and Massif Central for the Tramontane) and may therefore be associated with strong wind curls when they reach the gulf. In winter, through cooling and evaporation, they may trigger dense water formation and cascading on the continental shelf [START_REF] Ulses | Dense shelf water cascading in the northwestern mediterranean during the cold winter 2005: Quantification of the export through the gulf of lion and the catalan margin[END_REF]. While, in summer, they are associated with the development of upwellings [START_REF] Millot | Wind induced upwellings in the Gulf of Lions[END_REF]. East-southeast winds (hereafter referred to as easterlies) carry clouds and rain over the Gulf of Lion and are associated with downwellings. They are less frequent than northwesterlies and are especially strong from autumn to spring [START_REF] Millot | Circulation in the Western Mediterranean Sea[END_REF]. In addition to wind forcing, the Gulf of Lion is subjected to the freshwater inputs of the Rhône River. Its plume can extend over large areas when a strong volume discharge is combined with a shallow thermocline (Fraysse et al, 2014). The associated changes in the density field can impact the shelf circulation [START_REF] Estournel | The Rhone River Plume in Unsteady Conditions: Numerical and Experimental Results[END_REF][START_REF] Reffray | Secondary flows induced by wind forcing in the Rhône region of freshwater influence[END_REF][START_REF] André | Modelled variability of the sea surface circulation in the North-western Mediterranean Sea and in the Gulf of Lions[END_REF].

41.5 • N 42 • N 42.5 • N 43 • N 43.5 • N 44 • N 3 • E 4 • E 5 • E 6 • E 7 • E Port Vendres S ète
The coastal circulation in the Gulf of Lion is also strongly influenced by the large-scale ocean circulation of the Mediterranean. The Northern Current, also known as the "Liguro-Provencal Current", is the northern branch of the cyclonic gyre circulation of the northwestern Mediterranean Sea [START_REF] Millot | Circulation in the Western Mediterranean Sea[END_REF]. It originates before the Ligurian Sea from the merging of the Western and Eastern Corsican Currents (Astraldi et al, 1990). This baroclinic current is, at first order, in geostrophic balance and flows westward along the continental shelf.

During winter and certain upwelling conditions, the Northern Current separates the fresh and nutrient-rich coastal waters from the salty and oligotrophic offshore waters. But, as evidenced in observations and numerical modelling, the Northern Current occasionally penetrates over the Gulf of Lion, causing drastic changes in the biogeochemistry and in turn in the primary production on the shelf (Ross et al, in revision.).

Intrusions of a slope current on an adjacent shelf have already been evidenced elsewhere in the world ocean, such as the surface intrusions of the Gulf Stream [START_REF] Oey | Shoreward intrusion of upper Gulf Stream water onto the US southeastern continental shelf[END_REF]Gawarkiewicz et al, 1992) and of the Kuroshio (Chen et al, 1996;[START_REF] Tang | Continental slope flow northeast of Taiwan[END_REF][START_REF] Wu | Intra-seasonal variation in the velocity field of the northeastern South China Sea[END_REF]Caruso et al, 2006). [START_REF] Oey | Shoreward intrusion of upper Gulf Stream water onto the US southeastern continental shelf[END_REF] and Chen et al (1996) suggest that these intrusions are likely forced by wind-induced onshore Ekman transport. [START_REF] Wu | Intra-seasonal variation in the velocity field of the northeastern South China Sea[END_REF] and Caruso et al (2006) suggest that they are forced by strong wind-stress curls. Other causes are also proposed, such as the meanderings of the main current [START_REF] Oey | Shoreward intrusion of upper Gulf Stream water onto the US southeastern continental shelf[END_REF] or the abrupt changes in bottom topography (Chen et al, 1996).

The Gulf-Stream and the Kurushio currents are poleward currents that are strongly influenced by the β-effect, contrary to the Northern Current that is more zonally oriented. Other intrusions, more similar to those occurring on the Gulf of Lion, have been evidenced on the northwestern shelf of the Black Sea [START_REF] Oguz | Observations on the Rim Current structure, CIW formation and transport in the western Black Sea. Deep Sea Research Part I[END_REF][START_REF] Huthnance | Seasonal, interannual, and mesoscale variability of the Black Sea upper layer circulation derived from altimeter data[END_REF] or on the Papua Gulf in New Guinea [START_REF] Wolanski | Water circulation in the Gulf of Papua[END_REF]. However, the physical mechanisms of these intrusions have not been investigated yet.

Intrusions of the Northern Current on the eastern part of the Gulf of Lion have been investigated by Gatti et al (2006) and Gatti (2008), using a combination of in-situ observations and numerical modelling. The authors suggest that some intrusions are forced by easterlies, through either Ekman transport or a shoreward displacement of the Northern Current. They also propose that northwesterlies may favour intrusions through the associated positive windstress curl. This curl would provide a source of vorticity to the Northern Current that may drive intrusions. She also suggests that intrusions on the eastern part of the Gulf of Lion are also likely to occur after the relaxation of upwelling-favourable winds, as shown by [START_REF] Millot | The effect of Mistral wind on the Ligurian current near Provence[END_REF]. Using analytical model and numerical simulations, [START_REF] Echevin | Interaction of a coastal current with a gulf: Application to the shelf circulation of the Gulf of Lions in the Mediterranean Sea[END_REF] assessed the interaction between a coastal current, represented as a baroclinic Kelvin wave, and a shelf break. They suggest that Ekman transport associated with southeasterlies induce a downwelling, which in turn generates a westward coastal current that transports Northern Current waters onto the shelf.

Therefore, although the intrusions of the Northern Current on the eastern part of the Gulf of Lion have been shown to be wind-driven, the associated physical mechanisms remain unclear. Furthermore, the potential influence of the ocean stratification is also not well understood. For instance, [START_REF] Millot | The effect of Mistral wind on the Ligurian current near Provence[END_REF] suggest that intrusions of the Northern Current occur when the ocean is stratified, while [START_REF] Petrenko | Variability of circulation features in the gulf of lion NW Mediterranean Sea. Importance of inertial currents[END_REF] has shown that intrusions may be observed independently of stratification.

The aim of the present study is to address these uncertainties and therefore to gain more understanding on the physical mechanisms behind the intrusions of slope current on continental shelves. Using a combination of ADCP current observations, tide-gauge data and regional high resolution numerical modelling, the physical mechanisms that link the wind forcings and the intrusions on the eastern part of the gulf are investigated for the 2012-2013 period. The paper is organised as follows. In section 2, the observations and numerical model are described. In section 3, the methodology used in the detection of simulated and observed intrusions is presented. Section 4 is dedicated to the analysis of the physical mechanisms that drive the intrusions. Conclusions and discussions are provided in section 5.

2 Data description 2.1 Julio mooring Gatti (2008) has suggested that the Julio 1 site, located on the 100 m isobath at 5.255 • E-43.135 • N (magenta point in Fig. 1), is a judicious location for the observation of intrusions occurring on the eastern side of the Gulf of Lion. As such, it has been proposed as a site in the framework of the MOOSE 2 observing system (http://www.moose-network.fr/). A bottom-moored ADCP (RDI

η ibc = - p a -p a ρ 0 g
and is subtracted from the raw daily timeseries. p a is the atmospheric sealevel pressure and p a its long-term mean (computed on the 1979-2014 period), ρ 0 the reference density of water (here, 1000 kg.m -3 ) and g the gravity. The sea-level pressure used here is issued from the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (Dee et al, 2011).

High-resolution numerical modelling

The physical mechanisms that drive the intrusions of the Northern Current on the eastern part of the Gulf of Lion are investigated using the high resolution model simulations of [START_REF] Guihou | A case study of the mesoscale dynamics in the North-Western Mediterranean Sea: a combined datamodel approach[END_REF], which are derived from the Glazur64 configuration of [START_REF] Ourmières | Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products[END_REF]. Glazur64 is based on the "Nucleus for European Modelling of the Ocean" modelling framework (NEMO, [START_REF] Madec | NEMO ocean engine[END_REF] and is implemented on a 1/64 [START_REF] Schaeffer | Influence of high-resolution wind forcing on hydrodynamic modeling of the Gulf of Lions[END_REF].

The reader interested in the details of the Glazur64 configuration are referred to [START_REF] Guihou | A case study of the mesoscale dynamics in the North-Western Mediterranean Sea: a combined datamodel approach[END_REF]. left panel). However, Glazur64 strongly overestimates the zonal component of the current at the Julio site (Fig. 2, right panel): the simulated zonal velocity is indeed twice as large as the observed one and shows a large weakening at depths, while the observed profile is more homogeneous. This discrepancy is presumably due to a misposition of the simulated Northern Current, which may be too close to the coast hence encroaching on the Julio location. However, this model bias is not critical for the comparison of observed and simulated intrusions. Indeed, more relevant indexes to detect the presence of intrusions will be presented in the following section

Detection of Northern Current's intrusions

The intrusions of the Northern Current on the eastern part of the Gulf of Lion have been detected using the Julio ADCP mooring observations as follows.

First, the current component that flows perpendicular to the Julio section (magenta line at angle θ = 49.4 • from north in Fig. 1) has been computed using the measured zonal and meridional velocities:

U jul = V cos θ -U sin θ
with V and U the meridional and zonal velocities at the Julio mooring. By convention, U jul is counted positive for a northwestward flowing current. Then, U jul has been vertically averaged, hence leading to a half-hourly time-series, on which daily averaging has been performed. Finally, the resulting daily timeseries has been standardised: the temporal mean, computed over the overlapping time-period between the observations and model simulation, has been removed and the resulting anomalies have been divided by their standard deviation. The resulting standardized index, which will be referred to as the "observed" Julio index, is shown in Fig. 3 (black line). In the following, we will consider that intrusions occur, in the in-situ data, when the observed Julio index is greater than 1, i.e. when the depth-averaged across-section velocity exceeds its mean by 1 standard deviation.

The same methodology is applied on the Glazur64 model simulation at the grid point the closest to Julio. As expected from figure 2, the temporal mean of the simulated U jul velocity is overestimated when compared with 2 0 1 2 -0 3 -0 1 2 0 1 2 -0 4 -0 1 2 0 1 2 -0 5 -0 1 2 0 1 2 -0 6 -0 1 2 0 1 2 -0 7 -0 1 2 0 1 2 -0 8 -0 1 2 0 1 2 -0 9 -0 1 2 0 1 2 -1 0 -0 1 2 0 1 3 -1 0 -0 1 2 0 1 3 -1 1 -0 1 2 0 1 3 -1 2 -0 1 -3 - the observed one (8.4 cm.s -1 and 4.3 cm.s -1 , respectively). However, their standard deviations are very similar (10.3 cm.s -1 and 9.1 cm.s -1 for the observations and model simulation, respectively). The simulated standardized Julio index (red line in Fig. 3) shows a correlation of 0.59 with the observed index. This correlation is significant at the 95% level of confidence, according to a Student t-test in which the number of degrees of freedom has been corrected from the 1-day lag autocorrelation of both time series (Bretherton et al, 1999).

As shown in Fig. 3, there is a good agreement between the occurrence of the simulated and observed intrusions, in spite of the overestimated zonal velocity simulated at Julio. The model simulation can therefore be considered as robust enough to investigate the physical mechanisms that are responsible for the intrusions.

Nevertheless, there are some examples when the two indexes are out of phase. For example, five intrusion events are observed but are not reproduced in Glazur64 (black arrows on Fig. 3). On 2012-02-14, the observed index has a value of 2.2, while the simulated one has a value of 0.0. On 2012-03-07 the observed index is 1.5 and the simulated one is 0.7, while on 2012-03-17, the observed index is 1.56 and the simulated one is -0.1. On 2013-12-24, the observed Julio index reaches 1.7 and remains above 1 the next day, while the simulated index has a value of -0.4. On 2013-12-29, the observed Julio index reaches 1.6 and increases up to 2.1 the next day, while the simulated Julio index has a value of -1.5. Conversely, one strong intrusion event is simulated on 2012-04-27 by the model but is not observed (red arrow on Fig. 3). At this time, the simulated Julio index reaches 2 and remains above 1 the following 4 days, with a maximum value of 2.8 on 2012-04-28. The observed index also shows an increase but remains below the detection threshold. These discrepancies might be due to an underestimation by the numerical model of the higher levels of instability of the Northern Current during winter, or to a misrepresentation of wind forcings by the Aladin model. However, the lack of data observations prevents us from providing robust conclusions. Therefore, we decided to concentrate our efforts on the understanding of the forcing mechanisms of the intrusions which are both observed and simulated.

In order to assess the robustness of the Julio index in the detection of intrusive events, another methodology is added in Glazur64. Daily volume transport across the Julio section is computed by using the "Physical Analysis of the Gridded Ocean" (PAGO) suite of programs [START_REF] Deshayes | CMIP5 Model Intercomparison of Freshwater Budget and Circulation in the North Atlantic[END_REF] 3 .

PAGO permits the computation of transport indexes along predefined sections with limited interpolation by connecting two section endpoints as a continuous sequence of grid faces following a great circle pathway. Current velocities along the section do not undergo any interpolation, hence allowing a better precision on the volume transport calculation. The calculated volume transport is standardised in the same way as the Julio index. This simulated volume index (blue line in Fig. 3) is highly correlated with the simulated Julio index (correlation of 0.92, significant at the 95% level of confidence), and both simulated indexes detect the same intrusion events. Therefore, the simulated Julio index provides a good indicator of the simulated volume transport between the coast, and the comparison between the in-situ and simulated Julio indexes provide a good indicator for concomitant intrusion events.

However, we can mention the rare cases of intrusions that are confined very close to the coast. These intrusions are characterized by a simulated volume index that is above 1 and by Julio indexes (observed and simulated) that are less than 1 (blue arrows of Fig. 3). This is for instance the case for the intrusion event that starts on 2012-07-23, during which the simulated volume transport index is above 1 for 3 days, while the simulated Julio index reaches a maximum of 0.9, hence below the detection threshold.

Physical mechanisms of the intrusions

In this section, the physical mechanisms associated with the intrusions are investigated. We have focused our attention only on strong intrusion events (grey shadings in Fig. 3, see also Table 1), during which all three intrusion indexes are larger than 1 at least one day, hence corresponding to variations superior to their standard deviations. Moreover, we chose to merge intrusions when only 1 or 2 days separate them. It is the case of intrusions I2 and I3 that both include two intrusions when one looks at at the details of Fig. 3.

The physical mechanisms associated with these intrusion events are described in the following. Fraysse et al (2014). This freshwater plume is then advected westward past 4 • E by the intruding current (Fig. 6, panels d and e). Furthermore, as discussed by [START_REF] Echevin | Interaction of a coastal current with a gulf: Application to the shelf circulation of the Gulf of Lions in the Mediterranean Sea[END_REF], the downwelling of warm (in summer) and freshwater may also contribute significantly to the westward coastal current. 2 0 1 2 -0 3 -0 1 2 0 1 2 -0 4 -0 1 2 0 1 2 -0 5 -0 1 2 0 1 2 -0 6 -0 1 2 0 1 2 -0 7 -0 1 2 0 1 2 -0 8 -0 1 2 0 1 2 -0 9 -0 1 2 0 1 2 -1 0 -0 1 2 0 1 3 -1 0 -0 1 2 0 1 3 -1 1 -0 1 2 0 1 3 -1 2 -0 highlight the intrusion events that are investigated in section 4.

2 0 1 3 -1 0 -1 7 2 0 1 3 -1 0 -1 8 2 0 1 3 -1 0 -1 9 2 0 1 3 -1 0 -2 0 2 0 1 3 -1 0 -2 1 2 0 1 3 -1 0 -2 2 2 0 1 3 -1 0 -2 3 -3 -2 -1 0 1 2 3

Intrusions index

Julio index (obs.) Julio index (model) Volume index (model)

10 m.s -1 a)
Aladin 10 m wind at Julio

50 cm.s -1 42 • N 42.5 • N 43 • N 43.5 • N b) 2013-10-18 c) 2013-10-19 42 • N 42.5 • N 43 • N 43.5 • N 3 • E 4 • E 5 • E 6 • E d)
2013-10-20

3 • E 4 • E 5 • E 6 • E e)
2 0 1 2 -0 3 -0 1 2 0 1 2 -0 4 -0 1 2 0 1 2 -0 5 -0 1 2 0 1 2 -0 6 -0 1 2 0 1 2 -0 7 -0 1 2 0 1 2 -0 8 -0 1 2 0 1 2 -0 9 -0 1 2 0 1 2 -1 0 -0 1 2 0 1 3 -1 0 -0 1 2 0 1 3 -1 1 -0 1 2 0 1 3 -1 2 -0 1

8).

Using satellite sea-surface temperature, [START_REF] Millot | The effect of Mistral wind on the Ligurian current near Provence[END_REF] have shown that such wind patterns are favourable to upwellings along the Gulf of Lion coasts and that, when the wind relaxes, the frontal zone between the cold upwelled waters and the warm waters originating from the Northern Current tends to move northwestward and to penetrate over the shelf. As shown in The sea-surface height pattern associated with the intrusions induced by northwesterlies (Fig. 9) is very different from the one associated with the intrusions driven by easterlies (Fig. 4). First, the sea-surface height on the Gulf of Lion coast is not much impacted. This is further confirmed by the observa- Aladin 10 m wind at Julio

50 cm.s -1 42 • N 42.5 • N 43 • N 43.5 • N b) 2012-08-31 c) 2012-09-01 42 • N 42.5 • N 43 • N 43.5 • N 3 • E 4 • E 5 • E 6 • E d)
2012-09-02 tions at the Sonel stations during intrusions I2 and I3, which show a decrease and negative anomalies rather than the increase and positive anomalies of the other intrusions (Fig. 5). Furthermore, one can observe that the sea-surface height pattern on 2012-09-03 (Fig. 9) shows negative values centred at ap- northwestward force that is likely to be responsible for the displacement of the Northern Current on the Gulf of Lion shelf. This evolution of the Northern Current after northwesterly wind bursts can be related to what happens on the continental shelf off California, as described by Gan and Allen (2002). During northwesterly upwelling favourable winds, negative zonal pressure gradients develop west of the Cape Croisette (located south of Marseille, at the north-eastern end of the Julio section, Fig. 1), balancing the nonlinear advective effects, while east of the cape, positive zonal pressure gradients geostrophically balance the onshore flow at depth. When the northwesterly winds drop, the upwelling relaxes and the Coriolis force no longer counterbalances the westward pressure force that ultimately dominates the momentum balance, hence accelerating the current westward. But west of capes, the eastward pressure force is still balanced by the nonlinear advective effects and hence does not contribute to accelerate the current eastward. The net effect is therefore a westward flowing current. Contrary to the easterlies, which may trigger intrusions any time of the year, northwesterlies do not always yield intrusions. For example, in 2012-03-05, the simulated wind-stress is very close to the one simulated on 2012-08-31 (compare Fig. 10 with Fig. 9). But this wind burst is not associated with an upwelling nor with an intrusion on the following days (Fig. 10).. The temperature pattern following the wind burst shows colder temperatures on the western Gulf of Lion coasts and rather homogenous temperatures in the open sea (figure not shown). Simulated daily temperature anomalies at Julio, computed by removing the smoothed seasonal cycle shown in Fig. 7, show reduced variance between December and April both at the surface and at 20 m depth (data not shown). This tends to confirm the absence of upwellings during the unstratified winter season. A similar behaviour has been evidenced on the North Carolina shelf by [START_REF] Lentz | The Influence of Stratification on the Wind-Driven Cross-Shelf Circulation over the North Carolina Shelf[END_REF], who suggests that, during unstratified conditions, the wind-driven cross-shelf circulation is weaker because the Ekman depth is greater than the water depth.

3 • E 4 • E 5 • E 6 • E e)

Combination of both wind patterns

Intrusions I1, I4 and I7 are due to a combination of northwesterlies and easterlies. For I1, the intrusion is triggered by a strong northwesterly wind event on 2012-05-16. Following the mechanisms described in section 4.2, the current starts intruding on the shelf on 2012-05-18. This intrusion is then reinforced by strong easterlies that blow on 2012-05-19 and 2012-05-20, which induce a strong increase in sea-surface height on the western part of the Gulf of Lion and accelerate the intrusive current as discussed in section 4.1. Similar patterns occur for the other two intrusions. In short, intrusion I4 is triggered by northwesterlies blowing on 2012-10-15 and is reinforced by easterlies blowing between 2012-10-18 and 2012-10-20. Intrusion I7 is triggered by the northwesterlies of 2013-11-14 and reinforced by the easterlies blowing on 2013-11-17 and 2013-11-18.

Discussion and conclusions

In this paper, the physical mechanisms associated with the intrusions of the Northern Current on the eastern side of the Gulf of Lion have been investigated using a combination of in-situ observations (Julio ADCP mooring and Sonel tide-gauge data) and high resolution numerical modelling. We have shown that easterlies and nortwesterlies are likely to favour such intrusions, but through physical mechanisms that are very different in nature. Easterlies generate intrusions through a wind-setup mechanism and an increase in sea-surface height along the coast, which induces a geostrophic alongshore northwestward coastal current. Because of the barotropic nature of this mechanism, easterlies are favourable to intrusions independently of the stratification and thus any time of the year. On the other hand, intrusions in response to northwesterlies principally occur during stratified conditions and are associated with the relaxation of the upwelling that occurs between Marseille and Toulon (Fig. 1). When the upwelling develops, the Coriolis force associated with the onshore flow balances a northwestward pressure gradient force. But when the winds drop and the upwelling relaxes, the Coriolis force weakens and the pressure force dominates the momentum balance. Consequently, the current is advected northwestward and reaches the Julio location approximately one day after the wind relaxation. This is very close to the mechanism that takes place in the upwelling system off northern California, as discussed for instance in Gan and Allen (2002).

The difference in the physical mechanisms associated with these two wind patterns may be evidenced using the Julio mooring observations and the Aladin wind fields by using lead-lag covariance analysis. 3-hourly wind-anomalies are first computed by removing the 2012-2013 mean. The lead-lag covariances between these wind-anomalies and the 3-hourly Julio index (obtained by averaging the half-hourly across-section velocity and by standardising the resulting time-series) are then computed for each grid point. These covariance maps measure how much the Julio index and the wind anomalies vary together as a function of the time lag between the two series. They have the same units as the wind-anomalies and the visual inspection of the covariance maps shows that the strength of the covariance increases from 0-lag to 9 hours lag (wind anomalies lead), at which point it shows a pattern similar to an easterly wind pattern (Fig. 11, top panel). The covariance then decreases in the Gulf of Lion until lag 39 hours, where it reaches a minimum. Then, the covariance increases until lag 81 hours (approximately 3 days), when it reaches a secondary maximum of weaker amplitude than the first one, looking like a northwesterly-like wind pattern (Fig. 11, bottom panel). This covariance analysis is consistent with the results described in the above, namely a fast response of the Northern Current intrusions to easterlies and a delayed response to northwesterlies.

A remaining question, however, is whether easterlies or northwesterlies are necessary or sufficient conditions for intrusions of the Northern Current on the eastern part of the Gulf of Lion. Fig. 12a shows the simulated wind-stress at the Julio location and the orientation of strong wind events (defined as periods when the wind-stress amplitude exceeds its mean by one standarddeviation), while Fig. 12b shows the simulated volume index over the entire 2012-2013 period. One can notice that, as discussed in section 4.1, strong easterly wind events are always associated with a sharp increase in volume transport across the Julio section, and can therefore be considered as sufficient conditions for intrusions. However, as discussed in section 4.2, northwesterlies are associated with intrusions mainly in summer, when the ocean is stratified (cf. Fig 7). Hence northwesterlies are not sufficient conditions for intrusions.

But some intrusion events may also occur when the wind forcing is weak, as for instance in November 2012. These events correspond to meanders of the Northern Current encroaching on the shelf, which are presumably due to instabilities of the Northern Current, as discussed in Gatti (2008). Therefore, neither type of wind is a necessary condition, since intrusions can occur without strong winds.

Nevertheless, the results of the present study are promising since they potentially allow to anticipate cross-shore transports and potential intrusions on the shelf of current-carried plankton or pollution. For instance, Berline et al Note that the color and arrow scales are different between the two panels. 2 0 1 2 -0 1 -0 1 2 0 1 2 -0 2 -0 1 2 0 1 2 -0 3 -0 1 2 0 1 2 -0 4 -0 1 2 0 1 2 -0 5 -0 1 2 0 1 2 -0 6 -0 1 2 0 1 2 -0 7 -0 1 2 0 1 2 -0 8 -0 1 2 0 1 2 -0 9 -0 1 2 0 1 2 -1 0 -0 1 2 0 1 2 -1 1 -0 1 2 0 1 2 -1 2 -0 1 2 0 1 3 -0 1 -0 1 2 0 1 3 -0 2 -0 1 2 0 1 3 -0 3 -0 1 2 0 1 3 -0 4 -0 1 2 0 1 3 -0 5 -0 1 2 0 1 3 -0 6 -0 1 2 0 1 3 -0 7 -0 1 2 0 1 3 -0 8 -0 1 2 0 1 3 -0 9 -0 1 2 0 1 3 -1 0 -0 1 2 0 1 3 -1 1 -0 1 2 0 1 3 -1 2 -0 1 (2013) suggest, using the Glazur64 model configuration and a Lagrangian particle tracking software, that jellyfishes are more abundant on the Ligurian Sea coast when the Northern Current is close to the shore. Given the results described in the present study, one can suggest that increased transport of jellyfish on the Gulf of Lion coast may be anticipated in the case of northwesterly winds under stratified conditions, since the intrusions occur one day after the wind relaxes. However, for the intrusions associated with easterlies, since they occur in phase with the wind forcing, the anticipation of jellyfish stranding is likely to depend on the skill of the weather forecast to predict such wind events. Improving our understanding of the physical processes controlling the environmental conditions of coastal regions have significant socio-economical implications, especially regarding fisheries and marine pollution.

Fig. 1

 1 Fig. 1 Bathymetry of the Glazur64 configuration. The locations of the Julio mooring and section are indicated by a magenta point and a magenta line, respectively.

2. 4 Fig. 2

 42 Fig. 2 Time-averaged of the observed (black) and simulated (red) velocity profiles at the Julio site, computed over the overlapping time period between the observations and model simulation (from February to the end of October 2012, and from October to December 2013).

Fig. 3

 3 Fig. 3 Observed (black line) and simulated (red and blue lines) standardised intrusion indexes. Only the overlapping time period between the observations and model simulation (from February to the end of October 2012, and from October to December 2013) is shown.Black arrows indicate intrusion events that are observed but not simulated, the red arrow indicates the simulated intrusion event which is not observed and blue arrows highlight simulated intrusions that are captured by the volume index but not by the Julio one. The vertical dotted line separates the two time series. The horizontal dotted line corresponds to the detection threshold. The grey shadings highlight the intrusion events that are investigated in section 4.

4. 1

 1 Intrusions due to easterliesIntrusions I5 and I6 occur during easterly wind conditions. As discussed in the introduction, easterlies carry rain and clouds over the Gulf of Lion. In the model simulation, intrusions due to easterlies are associated with a very strong increase in sea-surface height over the Gulf of Lion, as shown for instance in Fig.4for intrusion I6. Such an increase is also evidenced in the Sonel tide-gauge observations: sea-surface height at Fos sur Mer and Sète shows an increase of approximately 20 cm during intrusions 5 and 6 (Fig.5). This increase is in agreement with the wind setup mechanism proposed by Csanady(1982), which characterizes a balance between the wind-stress and the horizontal pressure gradients associated with the sea-level slope. The increase in across-shore pressure gradient induces a geostrophic westward alongshore current until 4 • E and southwestward west of 4 • E.As discussed in Fraysse et al(2014), easterlies are also associated with downwelling conditions along the coast. In the model simulation, downwelling indeed occurs, as confirmed by the presence at 20 m depth of diluted freshwater (S ≈ 37.2) originating from the Rhône river (Fig.6, panel c), consistently with

Fig. 7

 7 Fig. 7 shows the smoothed, daily climatology of the simulated temperature at Julio, which represents well what occurs in the region (data not shown). It is

Fig. 4

 4 Fig. 4 Top panel: simulated wind-stress (arrows) and wind-stress curl (color shadings) during intrusion I6. Bottom panel: simulated sea-surface height (color shadings) and 20 m current velocity (arrows) during intrusion I6. The locations of the Sonel tide-gauge stations and of the Julio mooring are shown in cyan and magenta, respectively. The 200-m isobath is shown with a bold black line.

Fig. 5

 5 Fig. 5 Daily sea-surface height anomalies from the Sonel dataset corrected from the inverted barometer effect (see section 2.2 for details). Anomalies have been computed by removing the 2012-2013 mean. The vertical dotted line separates the two time series. The grey shadings

Fig. 6

 6 Fig. 6 Top panel: Aladin wind at Julio (arrows) and intrusion indexes (lines). Bottom panels: simulated salinity (color shadings) and ocean currents (arrows) at 20 m during intrusion I6. The 200-m isobath is shown with a bold black line.

Fig. 7

 7 Fig. 7 Hövmoller diagram of the smoothed daily climatology of the simulated temperature at the Julio location (annual and semi-annual harmonics retained, see text for details). The vertical dotted line separates the two time series. The grey hatchings indicate the intrusion events that are investigated in section 4.

Fig. 8

 8 Fig. 8 for intrusion I2, this behaviour is reproduced by the Glazur64 model simulation. When the wind blows, the ocean currents at 20 m beyond the 200 m isobath are southwestward, reflecting the ocean response to wind-stress via Ekman transport (Fig. 8, panel b). Upwellings start developing along all linear coasts and shows maximum vertical velocities on the shelf between Marseille and Toulon, collocated with the strongly positive wind-stress curl (data not shown). The upwelling reaches its maximum amplitude on 2012-09-01 (Fig. 8, panel c), when the wind relaxes. The Northern Current reaches the Julio location on 2012-09-02 (Fig. 8, panel d), hence one day after the wind relaxation.

Fig. 8

 8 Fig. 8 Top panel: Aladin wind at Julio (arrows) and intrusion indexes (lines). Bottom panels: simulated temperature (color shadings) and ocean currents (black arrows) at 20 m during intrusion I2. The 200-m isobath is shown with a bold black line.

  proximately 6 • E, 42.5 • N, which are tilted northwestward, and positive values on the shelf along the Var coast. The negative values near 6 • E and 43 • N are collocated and move with the cold temperature pattern of Fig. 8 (Fig. 8, panels c, d, e). This sea-surface height pattern is associated with positive zonal gradients near 5.25 • E and with negative zonal gradients near 5 • E. Interestingly, these gradients provide a good indicator of the pressure gradients at 20 m (data not shown). The positive pressure gradients on the east induce a

Fig. 9

 9 Fig. 9 Top panel: simulated wind-stress (arrows) and wind-stress curl (color shadings) during intrusion I2. Bottom panel: simulated sea-surface height (color shadings) and 20 m current velocity (arrows) during intrusion I2. The locations of the Sonel tide-gauge stations and of the Julio mooring are shown in cyan and magenta, respectively. The 200-m isobath is shown with a bold black line.

Fig. 10

 10 Fig. 10 Top panel: simulated wind-stress (arrows) and wind-stress curl (color shadings) on 2012-03-05. Bottom panel: simulated sea-surface height (color shadings) and 20 m current velocity (arrows) on 2012-03-07. The locations of the Sonel tide-gauge stations and of the Julio mooring are shown in cyan and magenta, respectively. The 200-m isobath is shown with a bold black line.

Fig. 11

 11 Fig. 11 Covariance at lag 9 hours (top panel) and 81 hours (bottom panel) between the 3-hourly standardised Julio index and the Aladin wind anomalies (wind anomalies lead).

Fig. 12

 12 Fig. 12 Top panel: Wind-stress amplitude at the Julio location as simulated by the Glazur64 model. Red dots indicate the direction of strong wind events (defined as periods when the wind-stress amplitude exceeds the mean by one standard-deviation). Souther-lies=0, Westerlies=90, Northerlies=180, Easterlies=270. Bottom panel: simulated volume index over the entire 2012-2013 period. The dashed line depicts the intrusion detection criteria.

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Table 1

 1 List of intrusions that are investigated in section 4. NW stands for northwesterlies and E for easterlies.

	Number	Start Date	End Date	Wind Forcing
	I1	2012-05-18 2012-05-21	NW+E
	I2	2012-08-28 2012-09-07	NW
	I3	2012-09-15 2012-09-23	NW
	I4	2012-10-18 2012-10-22	NW+E
	I5	2013-10-03 2013-10-05	E
	I6	2013-10-19 2013-10-28	E
	I7	2013-11-14 2013-11-27	NW+E

Acknowledgements The authors thank Gilles Rougier and Denis Malengros for their technical assistance with the Julio mooring. The Glazur64 simulations were performed using GENCI-IDRIS resources (Grant 2014011707). The analysis and plots of this paper were performed with both Python and the NCAR Command Language (version 6.2.1, 2011, Boulder, Colorado, UCAR/NCAR/CISL/VETS, http://dx.doi.org/10.5065/D6WD3XH5). The authors also acknowledge Julie Gatti, whose thesis strongly influenced this work.