domain: sdu phys sde

text:
Esther Portela Rodriguez

Stephen R Rintoul

Laura Herraiz-Borreguero

Fabien Roquet

Sophie Bestley

Esmee Van Wijk

Takeshi Tamura

Clive R Mcmahon

Christophe Guinet

Robert Harcourt

Esther Portela
email: eportelanh@gmail.com

Mark A Hindell

Controls on Dense Shelf Water formation in four East Antarctic polynyas

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Coastal polynyas are regions of open water or low sea ice concentration where the ocean surface is directly exposed to the cold atmosphere. Persistent wind-driven sea ice export and winter heat loss in coastal polynyas results in continuous sea ice formation and brine rejection. In some polynyas, the buoyancy loss due to cooling and salinification is sufficient to drive deep convection and formation of Dense Shelf Water (DSW).

The cold and salty DSW that cascades down the Antarctic continental slope is the precursor of Antarctic Bottom Water (AABW), the densest water mass in the ocean. The down-slope transport of DSW and AABW formation play a crucial role in Earth's climate system, global deep-ocean circulation [START_REF] Orsi | Circulation, mixing, and production of Antarctic Bottom Water[END_REF][START_REF] Jacobs | Bottom water production and its links with the thermohaline circulation[END_REF], and biogeochemical cycles [START_REF] Shapiro | Dense water cascading off the continental shelf[END_REF]. They connect the surface waters, imprinted with the signal of exchange with the atmosphere, with the deep ocean and transport dense water and tracers towards lower latitudes, ventilating the ocean abyss [START_REF] Orsi | On the total input of Antarctic waters to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements[END_REF][START_REF] Foster | Frontal zone mixing and Antarctic Bottom water formation in the southern Weddell Sea[END_REF].

The two major sites of AABW formation around Antarctica are the Weddell Sea and Ross Sea [START_REF] Orsi | On the total input of Antarctic waters to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements[END_REF]. These regions host highly productive coastal polynyas on wide continental shelves with large neighboring ice shelves and cross-shelf depressions that have the capacity to accumulate and export large volumes of DSW [START_REF] Gordon | Energetic plumes over the western Ross Sea continental slope[END_REF].

In contrast, the Adélie Land source region is located on a narrower continental shelf with more limited storage capacity in East Antarctica, where the persistent Mertz polynya activity drives a relatively fresh Bottom Water supply (Gordon & Tchernia, 1972;[START_REF] Rintoul | On the Origin and Influence of Adélie Land Bottom Water[END_REF][START_REF] Williams | Formation and export of dense shelf water from the Adélie depression, East Antarctica[END_REF]. More recently, the Cape Darnley-Prydz Bay region has been identified as another region exporting DSW to form AABW [START_REF] Ohshima | Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya[END_REF][START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF], 2016;[START_REF] Williams | The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay[END_REF][START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]. While these studies highlight the role of coastal polynyas in the formation of these dense-water overflows, DSW is noticeably absent in other coastal polynyas [START_REF] Ribeiro | Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica[END_REF][START_REF] Silvano | Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF].

Previous studies have assessed the influence of cryospheric, oceanographic, and physiographic factors [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF][START_REF] Baines | Observations and Modelling of Antarctic Downslope Flows: A Review. OCEAN, ICE, AND ATMOSPHERE: INTERACTIONS AT THE ANTARCTIC CONTINENTAL MARGIN[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF] on DSW formation and export. While the primary mechanism for DSW formation is high sea-ice production and consequent salt rejection in coastal polynyas, physiographic factors (e.g. polynya size and depth, topography, and shelf geography), regional circulation, and the water masses present on the shelf likely modulate the production of DSW [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF][START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF][START_REF] Liu | On the Modified Circumpolar Deep Water Upwelling Over the Four Ladies Bank in Prydz Bay, East Antarctica[END_REF]. However, little is known about these interactions and their final effect on DSW formation.

The absence of DSW in some polynyas has been attributed to the inflow of warm modified Circumpolar Deep Water (mCDW) onto the shelves leading to high basal melt rates of the adjoining glaciers [START_REF] Ribeiro | Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF]. The resulting freshwater outflow increases the stratification of the water column, hinders vertical convection and can thereby suppress the formation of DSW on the continental shelf [START_REF] Silvano | Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water[END_REF]. The increased stratification, in turn, allows mCDW intrusions to reach the calving front of ice shelves. The presence of DSW in a given polynya can influence melt of nearby ice shelves in two ways. On the one hand, DSW has the potential to cause basal melt if it can access the grounding line, which is likely due to its high density. Due to the pressure dependence of the freezing point, the DSW is warmer than the in situ freezing point and therefore melts the underside of the ice shelf [START_REF] Silvano | Ocean-ice shelf interaction in East Antarctica[END_REF][START_REF] Jacobs | Melting of ice shelves and the mass balance of Antarctica[END_REF]. In addition, because DSW is denser than mCDW, the presence of DSW can also prevent warmer mCDW from reaching the grounding line [START_REF] Jacobs | Melting of ice shelves and the mass balance of Antarctica[END_REF], resulting in lower basal melt.

A better understanding of the factors regulating DSW formation in coastal polynyas will provide insight into the sensitivity of AABW formation and basal melt of ice shelves in a changing climate. The few decades of available DSW overflow records show a reduction of DSW export in some areas around Antarctica [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF]. This apparent weakening of DSW formation on the Antarctic continental shelves may partly explain the observed changes in AABW (e.g. warming, freshening and volume reduction [START_REF] Fox-Kemper | Ocean, Cryosphere and Sea Level Change. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Antarctica[END_REF][START_REF] Purkey | Global contraction of Antarctic Bottom Water between the 1980s and 2000s[END_REF][START_REF] Kobayashi | Rapid volume reduction in Antarctic Bottom Water off the Adélie/George V Land coast observed by deep floats[END_REF]Van Wijk & Rintoul, 2014)). This link between DSW formation and the change in AABW characteristics highlights the variability of these complex ice-ocean systems and their vulnerability in the face of rapid climate change. Here, we use observations from four coastal polynyas to investigate the factors that favour or restrict formation of DSW. Our approach includes (i) investigation of water-mass transformation over winter, (ii) assessment of the role of preconditioning and (iii) computation of a salt budget to estimate the relative contribution of brine rejection, entrainment and salt advection in driving observed monthly salinity changes. By comparing polynyas that do and do not produce DSW, we aim to gain improved understanding of the physical processes that regulate DSW formation, now and in the future.

We analysed four East Antartic coastal polynyas: Mackenzie (MP), Barrier (BP), Shackleton (SP) and Vincennes Bay (VBP) (Figure 1). These polynyas were chosen because they have good temporal and spatial observational sampling by animal-borne sensors (section 2.1) during autumn-winter and their features cover a range of sea ice formation rates, geographic settings, and water mass distributions. As shown below, in the years they were best sampled, Mackenzie was an active source of DSW, Barrier was a weak source of relatively light DSW, and Shackleton and Vincennes Bay produced little or no DSW.

Data

The vertical temperature-salinity profiles used in this study were collected by southern elephant seals (Mirounga leonina) instrumented with Conductivity-Temperature-Depth Satellite-Relayed Data Loggers (CTD-SRDLs) during their annual post-moult (February -October) foraging trips. Southern elephant seals are excellent Southern Ocean samplers [START_REF] Roquet | A Southern Indian Ocean database of hydrographic profiles obtained with instrumented elephant seals[END_REF][START_REF] Mcmahon | Animal Borne Ocean Sensors -AniBOS -An Essential Component of the Global Ocean Observing System[END_REF], and in particular have provided almost all the available data within Antarctic coastal polynyas during winter [START_REF] Labrousse | Coastal polynyas: Winter oases for subadult southern elephant seals in East Antarctica[END_REF][START_REF] Malpress | Bio-physical characterisation of polynyas as a key foraging habitat for juvenile male southern elephant seals (Mirounga leonina) in Prydz Bay, East Antarctica[END_REF]. The profile data were retrieved from the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) database (http://www.meop.net/), which is a comprehensive quality-controlled database [START_REF] Roquet | A Southern Indian Ocean database of hydrographic profiles obtained with instrumented elephant seals[END_REF] that is an essential component of the global ocean observing system (GOOS). Additional data sources were explored, but sampling of coastal polynyas by more traditional oceanographic platforms (e.g. ships, floats and moorings) is comparatively sparse and not suitable.

For each coastal polynya, we analysed the year with the longest sampling record throughout the winter season. This means that data from different years are used in different polynyas: 2013 for MP, 2012 for BP and VBP, and 2011 for SP. The study area, as well as the location and date of the profiles collected by the tagged seals, is shown in Figure 1.

Hereafter, all the analyses and comparisons refer to the selected year for each polynya.

To estimate the contribution of brine rejection to the salinity increase in each polynya, we used a daily sea-ice production dataset. Sea-ice production data was derived from special sensor microwave/imager (SSM/I) and special sensor microwave imager/sounder (SSMI/S) data and atmospheric reanalysis from the ERA5 dataset. SIP data are seamlessly available for 28 years between 1992-2019. This dataset is similar to that of [START_REF] Tamura | Sea ice production variability in Antarctic coastal polynyas[END_REF], but uses different atmospheric forcing (ERA5 data are used in this study instead of ERAinterim).

Polynya identification

We identified polynyas using a combination of dynamic sea-ice concentration and static bathymetric data. The 12.5km resolution monthly SSM/I sea-ice concentration dataset (ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/psi-concentration/data/) was used to identify the polynya boundaries. In winter (from April to October) all data inside monthly sea-ice concentration contours of 75% were assigned to belong to the given polynya [START_REF] Massom | The distribution and formative processes of latent-heat polynyas in East Antarctica[END_REF][START_REF] Xu | The evolution of water property in the Mackenzie Bay polynya during Antarctic winter[END_REF][START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]. With this method, polynya surface area changes monthly. In summer (from November to March), when most of the region is ice-free, we assigned profiles that were located within the 75% contours of mean seaice concentration between April and October of the preceding winter. To avoid including data off the continental shelf, the contours are further constrained by the continental slope (defined by depths > 1500 m). The bathymetry for the polynya regions was obtained from RTopo-2.0.4 [START_REF] Schaffer | An update to Greenland and Antarctic ice sheet topography, cavity geometry, and global bathymetry[END_REF], (https://doi.org/ 10.1594/PANGAEA.905295).

Salt budget

The deepening of the mixed layer in coastal polynyas is driven by cooling and brine rejection during sea-ice production. As the mixed layer deepens, it entrains saltier water from below, further increasing the mixed layer salinity. In order to quantify the relative contribution of brine rejection, entrainment, and salt advection to the observed monthly salinity change (∆S Obs ) in each polynya, we computed a salinity budget for the monthlymean mixed layer and for the interior waters below. The method is illustrated in the schematic in Figure 2. Profiles within each month were used to compute the mixed layer depth using a density criterion of ∆σ=0.03 [START_REF] De Boyer Montégut | Mixed layer depth over the global ocean: An examination of profile data and a profilebased climatology[END_REF]. The mixed layer depth and salinity were then averaged in each month. Changes in salinity in each layer were calculated as the difference between consecutive monthly means.

Within the mixed layer, monthly changes in the observed salinity, ∆S Obs , can be decomposed as:

∆S Obs = ∆S Brine + ∆S Ent + ∆S Adv (1)
Where ∆S indicates the monthly salinity change centered on day 15 of each month, and the subscript indicates the contributing process: S Brine for the salt released during sea ice formation (always positive), S Ent for entrainment (always positive), and S Adv for lateral salt advection (positive or negative).

∆S Brine was obtained by first computing the salt flux from the remotely-sensed sea-ice production estimates (Sf SIP ) following [START_REF] Cavalieri | The contribution of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean[END_REF]: Where ρ i =920 kg m -3 is the sea ice density, V i is the sea-ice production over the polynya area in m 3 , S sw is the mean seawater salinity in the region, and S i is the frazil ice salinity (S i = 0.31xS sw ). We then computed the salinity increase in a given month (t) due to this Sf SIP :

Sf SIP = ρ i V i (S sw -S i ) • 10 3 (2)
∆S Brine t = Sf SIP (t→t+1) (ρ f w V (M L) t+1 ) (3)
Where ρ f w is the freshwater density (1000 kg m -3 ) and V (M L) is the monthly mean volume of the mixed layer.

The entrainment term was computed from the vertical temperature-salinity profiles.

The salt content (SC) of the volume of water entrained into the mixed layer (V Ent =

V (M L) t+1 -V (M L) t
) during mixed layer deepening between two successive months (t and t + 1), and its given salinity (S Ent ), is:

SC Ent = S Ent ρ sw V Ent (t→t+1) (4)
The change in salinity of the mixed layer due to entrainment is then:

∆S Ent = SC (M L) t + SC Ent (t→t+1) ρ f w V (M L) t+1 -S (M L) t (5)
Where ρ sw is the monthly mean seawater density directly computed from the profile and ρ f w is freshwater density. The ∆S Ent term was set to zero whenever the mixed layer depth remained unchanged or decreased in time.

Finally, the ∆S Adv was estimated as a residual, i.e., (∆S Obs -∆S Brine -∆S Ent ).

Below the mixed layer, ∆S Obs is only due to lateral salt advection. Observational errors, sparse seal sampling, and spatial variability could also contribute to the residual, but are not taken into account in our computation. Neglecting spatial variability within polynyas might be a strong assumption since, at least in the case of MP, water-mass distribution

shows important spatial variability due to localised differences in bathymetry [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]. However, for this study, most of the 2013 sampling in MP occurred over a limited area, thereby minimizing the effect of spatial variability. The importance of spatial variability is unknown for the other polynyas.

Results

The four polynyas analysed in this study show different physiographic characteristics.

All four are located next to ice shelves but the ice shelf size, location within the bay, melt rate, and the influence of meltwater outflows from the ice shelf cavities differ. For instance, MP is situated next to the Amery ice shelf, the third largest in Antarctica, while VBP is adjacent to the smallest ice shelf considered in this study. MP and VBP both receive meltwater exported from the ice shelf cavities, which has been found to affect DSW formation there [START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF][START_REF] Ribeiro | Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica[END_REF]. BP and SP are situated next to moderate-size ice shelves, the West and Shackleton ice shelves, respectively. These two ice shelves resemble ice tongues that may have multiple inflows and outflows from the ice shelf cavities, either within the polynyas or far away from them. Additionally, MP is located in western Prydz Bay, a region with a relatively wide continental shelf, while the other three polynyas are situated on narrow continental shelves and have their outer limits near the shelf break (Figure 1). Large ice shelves and wide continental shelves have been highlighted

as factors favoring DSW formation by some authors [START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF], while others have recently pointed out that the presence of wide continental shelves are less important than previously thought [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF].

The four polynyas also differ in size (Figure 3a): SP is the largest (7.5±3.6 x10 3 km 2 ), followed by VBP and BP that have similar sizes (6.2±2.7 x10 3 km 2 and 6.0±2.1 x10 3 km 2 respectively) and the smallest is MP (mean area 3.9±2.1 x10 3 km 2 ). The values provided in parentheses are the mean areas reported by [START_REF] Nihashi | Circumpolar mapping of antarctic coastal polynyas and landfast sea ice: Relationship and variability[END_REF] for the period 2003-2011. In our study, mean polynya areas between May and October, computed as the integral of the grid cells with an ice concentration below 75%, are 14.5x10 3 km 2 in SP, 12.4x10 3 km 2 in BP, 8.7x10 3 km 2 in VBP and 2.5x10 3 km 2 in MP. Before March, polynya size cannot be estimated since the sea ice coverage is insufficient to define a polynya. By April, three of the four polynyas (MP, BP, and VBP) experienced a significant size reduction, they then remained approximately constant until August before re-opening in September (with the exception of MP, whose size did not increase again until December). SP exhibited different dynamics; its significant size reduction occurred a month later (by May) and it kept reducing in area until July, when its extent was at its minimum. It re-opened again in August and remained approximately constant until October. The sea ice concentration data, together with the changes in the area of each polynya, reveal MP in 2013 to be a small but active polynya, which forms early in the year (March) and persists for longer (until November) compared to the other polynyas, with relatively higher sea ice concentration. In contrast, SP was formed later and re-opened earlier than the other polynyas.

Sea Ice production

The monthly mean of the spatially-averaged sea-ice production rate (Figure 4a) is independent of polynya size and provides a measure of the intensity of the brine rejected The sea ice concentration is the average within the polynya contour, defined by its sea ice concentration <75%, this means that this is the maximum possible value in panel b, only achieved if the sea ice concentration is 75% in every grid point within the polynya.

within each polynya. Therefore, this is the preferred parameter to assess the potential of a given polynya to form DSW, as sea-ice production is directly related to salinity increase.

The sea-ice production rate is more than twice as strong in MP than in the other polynyas between March and May, with its peak in April. From June onward sea-ice production decreases, but it remains the highest of all four polynyas until September. Over the whole winter (between March and October, inclusive), the accumulated sea-ice production in MP is more than twice that of the other three polynyas: 13.4 m in MP, 5.5 m in BP, 6.5 m in SP and 6.6 m in VBP.

Seasonal water-mass transformation

The water mass properties in each polynya between March and October (inclusive) are shown in Figure 5 where the main water masses are labeled and the time is color-coded (see Table 1 for water mass definitions). Note that while in SP and VBP there are data from the beginning of March, in MP and BP the time series begins at the middle and the end of March, respectively (see Figure 6). The saltiest DSW is observed in MP, with a maximum salinity of about 34.7 reached in mid September, the end of the sampling period here (Figures 5a and6a). DSW is also present in BP in September, with maximum salinity near 34.6 (Figures 5b and6c).

In contrast, no DSW is observed in SP and VBP over the period of observations, which extends until late August and November, respectively (Figures 5c,d and 6 e, g). In the latter polynyas, salinity of waters near the surface freezing point (also named Low Salinity Shelf Waters) reaches a maximum of ≈34.45 and ≈34.5, respectively. Besides the presence or absence of DSW, all four polynyas differ greatly in terms of water-mass distribution. mCDW is only present in MP in autumn, with a relatively cool maximum temperature of -1.5 • C. mCDW was only observed when the seal sampled the eastern side of MP (Figure 1a), so it may reflect spatial variability of the water masses within the polynya [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF] and Prydz Bay [START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF]. In BP, mCDW is practically absent and only sporadically observed in SP. In contrast, mCDW is widespread and particularly warm (maximum temperatures > 0.5 • C) in VBP, in agreement with previous observations encompassing wider or different periods [START_REF] Ribeiro | Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica[END_REF][START_REF] Kitade | Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica[END_REF]. Ice Shelf Water (ISW, Table 1) is present in MP and BP, the two DSW-forming polynyas, but is relatively salty/dense in MP (density ranging between 27.5 and 27.7 kg m -3 ) compared to the fresher/lighter ISW in BP (density between 27.4 and 27.6 kg m -3 ) (Figures 5a,b). ISW is absent in SP and a relatively warm and fresh variety is observed in VBP.

The properties of the AASW, the freshest water mass of the water column, also vary between polynyas. April AASW is warmer and saltier in MP and VBP where its minimum salinity ranges between 33.8 and 34 (Figures 5a,d and 6 a, g). In MP, warm AASW was only sampled in the eastern part of the polynya (Figure 1a), with different water-mass properties from the western part [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF] . In contrast, AASW is colder and fresher in BP The vertical distribution of the water masses with time provides additional information on how they are transformed over the winter (Figure 6 left column, a, c, e, g). In Figure 6 each water mass is represented with a different color palette. To avoid over-plotting, we have used the daily average of all data available within each polynya in their particular year. This representation highlights the role of the different water masses in setting the stratification (N 2 ) of the water column (Figure 6b, d, f, h) and thus, influencing the formation and evolution of DSW.

In MP, remnant DSW below the mixed layer (S=34.5, blue dots in Figure 6a) is observed throughout the entire sampling period except between mid April and mid May, when ISW (pink) is observed in the bottom layer instead. This ISW is as salty as the remnant DSW so it is similarly dense. Full convection is achieved and new DSW is formed in MP episodically from early June, and more consistently from July. This DSW gets progressively saltier until mid-September, the end of the sampling record (Figures 5a and6 a).

In BP only episodic remnant DSW is found in late summer, but most of the seafloor is occupied by relatively fresh (S=34.4) ISW in April and early May. Similarly fresh mCDW is observed during the same period. Full convection is achieved in late June, but new DSW is not consistently formed until August, reaching maximum salinity about 34.55 by the end of September (Figures 5b and6c). In both MP, and BP, the shallow bathymetry in comparison with the two other polynyas might have also contributed to achieve full convection earlier in the year.

In SP the water column is fully occupied by AASW, except for the bottom layer (> 500m depth) which is occupied by mCDW. Despite a relatively weak stratification compared with the other polynyas (Figure 6f), full convection is never achieved and salinity by mid-August (end of the time series) remains too low to form DSW (Figures 5c and6e). Due to data limitations, we cannot know if there was DSW formation later in the year, and the seals did not sample SP in winter in any other year.

In the relatively deep VBP embayment, the water-column structure consists of AASW overlying a thick layer of warm mCDW during the entire March to November period (with a data gap in July-August). The mixed layer deepens progressively from early April until the end of June, when it reaches more than 500 m. However, by this time, full convection

is not yet achieved, and mCDW still underlies the AASW above. By September, after the winter data gap, the mixed layer is very variable, but it does not reach the bottom of the dive profile. Despite the lack of detailed observations spanning the entire winter period, there is evidence that the mixed layer starts shallowing again by the beginning of October.

The stratification of the water column, as represented by N 2 , is a measure of the stability of the water column. Weaker stratification favors full convection and DSW formation.

Stratification is of similar magnitude and follows similar temporal variability in MP and BP (with maximum slightly stronger in BP, Figure S1), it is slightly weaker in SP, and much stronger in VBP (Figure 6 b, d, f, h). However, this must be interpreted in combination with the water-mass properties in order to gain insight into the processes governing DSW formation. The weaker stratification in SP (Figure 6f, S1) reflects the dominance of AASW within the water column, resulting in water properties that change more smoothly over depth. However, mixed layer salinity is very low and, even if the stratification is weak, the buoyancy flux is not strong enough to induce convection. In BP, SP and VBP, the maximum stratification between March and May (Figure S1) corresponds with the abrupt vertical salinity changes associated with very fresh AASW (S<33.6) occupying the mixed layer. In MP, this fresh AASW is absent, and the stratification is mainly set by the contrast between relatively salty AASW within the mixed layer, and the saltier DSW or ISW below (Figure 6).

The role of preconditioning

Low initial salinity needs stronger transformation to achieve the threshold DSW salinity of 34.5. Therefore, mixed layer salinity at the beginning of the winter is potentially an important factor influencing DSW formation. The difference between the salinity within the mixed layer and right below (another indirect measure of stratification) (Figure 7), determines the buoyancy flux needed for achieving convection. For example, at similar seaice production rate, mixing with saltier waters below as the winter draws on will increase the mixed layer salinity faster.

In MP, two features favorable to DSW formation occur by early April: (i) the mixed layer waters are already relatively salty (S≈34 compared with ≈33.7 in BP, 33.85 in VBP Despite the initial salinity differences between the mixed layer and below, BP, SP, and VBP all exhibit a rapid salinification during the month of April. In MP, the salinity increase occurs at slightly lower rate in the first half of April, and slows down from mid April. This contrasts with the sea-ice production rate observed in MP (Figure 4a) that was double that of the other polynyas. The lower rate of salinity increase in MP in April might be related to the spatial variability of the sampling, as the seal traveled from the eastern to the western side of the polynya during this month. Our previous study highlighted important differences in water-mass properties between the eastern and western areas within MP [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]. By May, the differences between polynyas has reduced. The salinity of both layers is similar in MP, BP and VBP to the beginning of July, when full convection is achieved in both MP and BP (Figure 6a-d) and the rate of salinity increase slows down in all four polynyas.

Salinity balance

To better understand the processes driving the observed monthly changes in salinity (∆S obs ), and therefore the potential for DSW formation in each polynya, we calculated three terms contributing to the salt balance: brine rejection due to sea-ice production, entrainment during mixed layer deepening, and lateral advection (as a residual), using equations 1 to 5 (see Methods).

In MP (Figure 8a), brine rejection from sea-ice production dominates the observed salinity increase, which is partially offset by advection of relatively fresh waters within the mixed layer. The inferred advection of fresh water during most of the winter might reflect meltwater inflow as reported previously [START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF], 2016), or the transport of fresh AASW into the polynya. Advection in the mixed layer appears decoupled from that in the interior (where advection of salty water occurs). This contrasts with the other three polynyas where salt advection follows a similar pattern throughout the water column.

Below the mixed layer, the strong positive salt advection suggests inflow of salty water masses such as DSW or mCDW directly into MP. Since the presence of mCDW has not been detected (Figure 6a), we hypothesize that MP might be receiving DSW in the interior layer from other upstream Prydz Bay polynyas (such as BP) [START_REF] Herraiz-Borreguero | Basal melt, seasonal water mass transformation, ocean current variability, and deep convection processes along the Amery Ice Shelf calving front, East Antarctica[END_REF][START_REF] Williams | The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay[END_REF] or possibly the neighbouring Darnley polynya [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF].

Spatial variability within MP could also explain our results since bathymetry strongly influences the water-mass distribution below the mixed layer [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]. Entrainment accounts for about a third of the ∆S obs in April, less than a fifth in May, and is reduced to zero thereafter when the mixed layer depth approaches the seafloor (Figure 8). This is a common feature across the four polynyas: entrainment makes a larger contribution to the salinity increase in the mixed layer in early winter, when the salinity contrast between the mixed layer and the interior is large. As the mixed-layer salinity increases during winter and approaches that of the interior, the salinity increase from entrainment decreases. Additionally, as the mixed layer deepens, the salt added during entrainment results in a smaller salinity increase overall.

In BP the salinity increase in the mixed layer from April to June is driven by a combination of brine rejection, salt advection and, to a lesser extent, entrainment. From June to September, brine rejection acts to increase salinity, but is almost completely offset by fresh advection between July and August. Based on the salinity profiles in Figure 6c, the inferred advection might reflect inflow of relatively fresh AASW into the mixed layer once DSW has started to form.

In SP and VBP, strong salinity increase due to inferred advection of salty waters is observed in autumn (Figure 8c,d). This might be caused by the inflow of mCDW or a salty AASW. In VBP, mCDW is widespread, whereas AASW is the dominant water mass in SP, and mCDW is rarely observed there (Figure 6e, g). Rapid mixing between AASW and mCDW could explain the rapid increase in mixed layer salinity observed in both polynyas prior to May that is not explained by brine rejection. This scenario is also consistent with the temperature increase observed in the interior AASW from April to May (Figure S2, Supplementary Information). From May onward, the salinity increase in SP and VBP is moderate (< 0.1), and mainly attributed to brine rejection and salt advection in SP, and to a combination of all terms in VBP. In the latter, the sea-ice production is still important in September, after the sampling gap, but the salinity increase due to brine rejection is largely offset by fresh advection.

The total salinity increase within the mixed layer between April and August (the common period across at least three polynyas) was 0.49 in MP, 0.50 in BP and, 0.67 in SP (Figure 8). Due to the observational data gap this cannot be assessed in the same way for VBP. The fact that the total salinity increase between April and August is the smallest in MP, which forms DSW, and largest in SP, which does not, is revealing. This suggests that a combination of factors determines whether or not DSW is formed in a given polynya, as discussed below.

for DSW formation. The high salinity in March could reflect (i) preconditioning of the interior waters by the presence or advection of DSW formed during the previous winter, (ii) sea ice formation commencing earlier than in the other polynyas, and (iii) less sea-ice melt in MP over summer. Option (i) would be consistent with the inferred advection of salty water below the mixed layer. However, this is surprising given evidence that glacial meltwater from the Amery Ice Shelf reaches MP [START_REF] Herraiz-Borreguero | Basal melt, seasonal water mass transformation, ocean current variability, and deep convection processes along the Amery Ice Shelf calving front, East Antarctica[END_REF]. One possibility is that MP does indeed receive meltwater input below the mixed layer, but that this contribution is overwhelmed by lateral advection of salty waters into the region.

The presence of salty water below the mixed layer (either remnant DSW or from lateral advection) might contribute to a saltier mixed layer through vertical entrainment. However, the salt budget suggests that the contribution of entrainment to the observed salinity increase was small in MP. Thus, even if the interior water salinity is high, it is not likely to explain the high salinity observed in Autumn in MP.

The observation that the mixed layer (i.e. AASW) salinity in MP was higher at the end of March than in the other polynyas provides support for option (ii) (sea-ice formation commencing earlier than in the other polynyas). The salinity increase in autumn in MP was also consistent with the estimated brine rejection (Figure 7). While warm surface waters in late summer can bias estimates of sea-ice production [START_REF] Tamura | Estimation of thin ice thickness and detection of fast ice from SSM/I data in the Antarctic Ocean[END_REF], the March surface temperature approaches the freezing point in all four polynyas (Figure S2), providing good confidence in the sea-ice formation estimates used here. In addition, the rapid increase in sea-ice concentration in MP from 0% in February to 40% in March (Figure 3) in MP, compared with 30% in BP, and <10% in SP and VBP for the same period, provides further evidence of early and strong sea ice formation in MP.

High mixed layer salinity in MP at the start of winter might also be related to less seaice melt over summer due to enhanced sea-ice export (option (iii)). This is consistent with the strong and persistent katabatic winds blowing sea ice offshore in this region [START_REF] Massom | The distribution and formative processes of latent-heat polynyas in East Antarctica[END_REF]. To test this idea, we computed monthly sea-ice divergence in each polynya (Figure S3). The sea-ice velocity vectors suggest that sea-ice was constantly exported from MP to the north, where there is sea-ice convergence. However, the results are inconclusive due to insufficient spatial resolution of the sea ice drift dataset to resolve polynyas.

In contrast to MP, in the other polynyas the changes in monthly-mean salinity did not track sea-ice production. The other terms in the salt balance (entrainment and lateral advection) are often as large or larger than sea ice formation (Figure 8). In BP, the salinity of the mixed layer increased more rapidly from March to May than in MP, despite weaker sea ice formation, due to salt input by entrainment and lateral advection. However, the initial salinity was much lower than in MP, therefore, the salinity increase through the winter was only sufficient to form a light variety of DSW. While BP and MP exhibited a number of similar characteristics (e.g. shallow bathymetry, early full convection, absence of MCDW, and similar stratification), the weaker sea ice formation and fresh conditions at the start of winter conspire to limit DSW formation in BP. The fresher and lighter ISW observed at BP might also increase stratification, inhibiting deep convection there. For instance, maximum stratification in BP corresponds with the period where ISW observed (Figure S1).

While sea-ice production rates in SP and VBP were similar to that in BP, DSW was not formed in SP and VBP. In SP, DSW formation was hindered by the presence of a thick layer of very fresh AASW from late March. One explanation for this particularly fresh AASW could be inflow of meltwater from either the Shackleton Ice Shelf or from the Denman glacier flowing west under the Shackleton ice shelf, but we cannot corroborate this with the data in hand. The overall winter increase in mixed layer salinity was much larger at SP than at MP or BP (the data gap means this can't be assessed for VBP), despite sea ice production lower than MP or similar to BP, largely as a result of very strong salt advection from April to May.

However, the combined salt input by brine rejection and advection was still insufficient to overcome the initial low salinity of the AASW, drive full convection, and form DSW.

VBP, like SP, showed a strong increase in mixed layer salinity at the start of the winter as a result of lateral advection (Figure 8). The mixed layer salinity at the start of the winter was not as low as at SP, but was lower than at MP. The VBP polynya therefore did not have to overcome as large a mixed layer salinity deficit as at SP. However, the presence of mCDW below the surface mixed layer produced strong stratification [START_REF] Kitade | Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica[END_REF][START_REF] Ribeiro | Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica[END_REF] that provided a barrier to full-depth convection and DSW formation.

At both SP and VBP, the absence of remnant DSW means that the waters below the mixed layer was relatively fresh and the salinity of the entire water column must be increased by a larger amount than at MP (and to a lesser extent, BP) to reach the threshold for DSW formation.

In addition to the hydrographic properties and the sea-ice production rate, physiographic features influenced the DSW formation in the four polynyas. MP is located on a wide continental shelf, while the other three polynyas are located on narrower shelves.

Narrow continental shelves allow less time for DSW retention and salinification before the dense water cascades offshore [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF]. The physiographic features of MP (shallow waters, wide continental shelf and presence of a large ice shelf) have been previously identified as favorable for DSW production [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF], although a wide shelf is not a requirement for DSW formation [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF]. The polynya size has influence on the total amount of DSW formed in a given polynya if the density threshold is reached. However, it is unrelated to the sea-ice production rate, therefore, polynya size is not a direct factor influencing on DSW formation.

Our analysis is limited to specific years, so presence or absence of DSW in our observations does not preclude DSW formation at a different time. Indeed, it has been shown that polynyas exhibit interannual variability (e.g., [START_REF] Portela | Seasonal transformation and spatial variability of water masses within MacKenzie polynya, Prydz Bay[END_REF]), especially regarding the structure of the water masses present. However, the absence of remnant DSW at SP and VBP, and the fresh waters observed at depth there, suggest that DSW did not form in the year prior to our observations in those polynyas. In any case, our focus here is on identifying the physiographic, oceanographic and cryospheric factors that determine why DSW forms in some coastal polynyas and not others. By focusing on specific polynyas in particular years, we are able to explore in detail the factors that control DSW formation. This work therefore complements recent circumpolar assessments spanning a longer time period that investigate more general features [START_REF] Amblas | Physiographic influences on dense shelf-water cascading down the Antarctic continental slope[END_REF][START_REF] Narayanan | Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water[END_REF].

We (1972). Waters of the continental margin off Adélie Coast,

Figure 1 .

 1 Figure 1. Location of vertical profile sampling (color-coded by time) within four East Antarctic coastal polynyas: a) 2013 Mackenzie (MP), b) 2012 Barrier (BP) c) 2011 Shackleton (SP) and d) 2012 Vincennes Bay (VBP).

Figure 2 .

 2 Figure 2. Schematic showing the processes considered in our salt budget computation as driving salinity change in the mixed layer and in the interior waters below. In this example, the process is schematised over three months (t1 -t3).

Figure 3 .

 3 Figure 3. a) Area and b) sea ice concentration of the four polynyas investigated in this study.

Figure 4 .

 4 Figure 4. Monthly mean of the Sea Ice Production rate in each polynya between March and October.

  and SP in April (minimum salinity of 33.4-33.5) (Figures5b, c and 6 c, e). Fresh AASW is also observed in VBP in March (S<33.6). However it rapidly becomes saltier in April (S>33.8). The fresh surface layer in autumn reflects summer ice melt, and it cools and salinifies as the winter progresses. Thus, the properties of the AASW in autumn (March-April) depend on the oceanic heat loss and brine rejection in each polynya and on the amount of melt that occurred over the previous summer. Saltier (therefore denser) AASW in autumn is indicative of an earlier or more intense sea-ice production and/or less ice melt.This results in the preconditioning of the water column that would reduce stratification and favor further convection.

Figure 5 .

 5 Figure 5. Potential temperature -salinity diagram in the four polynyas: (a) Mackenzie, (b) Barrier, (c) Shackleton, and (d) Vincennes Bay. The dashed lines are the isopycnals and the thick black contours highlight the isoneutrals of 28 and 28.27 kg m 3 that delimit the mCDW

Figure 6 .

 6 Figure 6. Time line of water column salinity (left column), highlighting the different water masses and their seasonal transformation across the four polynyas, and their stratification given by N 2 (right column). (a-b) Mackenzie, (c-d) Barrier, (e-f) Shackleton, and (g-h) Vincennes Bay. For clarity, each water mass is shown with a different color palette. Grey dots represent the transitional water masses before the minimum DSW salinity is reached at a temperature near the freezing point (sometimes defined as Low Salinity Shelf Water (LSSW)(Gordon & Tchernia, 1972)). The bathymetry is shaded in grey and the yellow dots (black in right column) represent the mixed layer depth as computed with a ∆σ criterion of 0.03 kg m -3 (de Boyer[START_REF] De Boyer Montégut | Mixed layer depth over the global ocean: An examination of profile data and a profilebased climatology[END_REF] .

Figure 7 .

 7 Figure 7. Temporal evolution of mean salinity within the mixed layer (red line), and within the 100 m below the mixed layer (blue line) or to the seafloor where shallower. (a) Mackenzie, (b) Barrier, (c) Shackleton, and (d) Vincennes Bay.

Figure 8 .

 8 Figure 8. Monthly observed salinity increase and the relative contribution of brine rejection due to sea-ice production, entrainment, and advection for (a) Mackenzie (MP), (b) Barrier (BP), (c) Shackleton (SP), and (d) Vincennes Bay (VBP). The salt budget is computed within the dynamic mixed layer (bars); below the mixed layer (dashed line) the only contribution is advection.

  show that DSW formation in Antarctic coastal polynyas is influenced by preconditioning (e.g. salinity and stratification at the start of winter), lateral advection, and physiographic factors, as well as sea ice formation rates. By demonstrating the sensitivity of DSW formation to multiple factors, our study provides novel insights into how DSW formation might respond to future changes in climate. Changes in wind strength and direction that lead to changes in polynya size and sea ice production, are an obvious potential driver of change in DSW formation. Changes in wind forcing may also drive changes in circulation that alter the salinity and stratification of the water column (i.e. preconditioning) and hence the susceptibility to full-depth convection. Increased glacial melt will likely enhance stratification and limit DSW formation, with potential feedback on glacial melt[START_REF] Silvano | Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water[END_REF]. Climate variability and change may disrupt the delicate balance of factors that control DSW formation. Previous observations and paleoceanographic evidence show that DSW formation has varied on timescales from years to millennia[START_REF] De Boer | Effect of global ocean temperature change on deep ocean ventilation[END_REF][START_REF] Smith | The presence of polynyas in the Weddell Sea during the Last Glacial Period with implications for the reconstruction of sea-ice limits and ice sheet history[END_REF]). Future changes in climate are likely to alter the factors influencing DSW formation, and therefore bottom water formation and the deep overturning circulation, with important consequences for climate, ventilation of the deep ocean, and ocean-driven melt of Antarctic ice shelves.AcronymsMP Mackenzie polynya BP Barrier polynya SP Shackleton polynya VBP Vincennes Bay polynya DSW Dense shelf Water AASW Antarctic Surface Water mCDW modified Circumpolar Deep Water ISW Ice Shelf Water AABW Antarctic Bottom Water Assessment Report of the Intergovernmental Panel on Climate ChangeScience Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergover , 2018 (August), 1-257. Gordon, A. L., & Tchernia, P. L.

Table 1 .

 1 Water-mass classification based on[START_REF] Herraiz-Borreguero | Basal melt, seasonal water mass transformation, ocean current variability, and deep convection processes along the Amery Ice Shelf calving front, East Antarctica[END_REF], 2015);Williams et al. (2016) and[START_REF] Orsi | A recount of Ross Sea waters. Deep-Sea Research Part II[END_REF].

	Water mass	Salinity	Potential temperature	Neutral density
	AASW	S < 34.4	θ > T f	γ < 28
	mCDW	-	θ > T f + 0.1	28 < γ < 28.27
	ISW	-	θ < T f -0.05	-
	DSW	S > 34.5 T

f -0.05 < θ < T f + 0.1 γ > 28.27 * T f : surface freezing point.

Acknowledgments

The seal CTD-SRDL tags and deployment were funded and supported through a collaboration between the French Polar Institute (program 109: PI. H. Weimerskirch and 1201: PI. C. Gilbert and C. Guinet), the SNO-MEMO and CNES-TOSCA and the Integrated Marine Observing System. Australia's Integrated Marine Observing System (IMOS) is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). It is operated by a consortium of institutions as an unincorporated joint venture, with the University of Tasmania as Lead Agent. www.imos.org.au. The Australian Research Council provided financial support through Discovery Project DP180101667, under which E.P. is supported. S.B is supported under ARC DECRA grant DE180100828. The marine mammal data were collected and made freely available by the International MEOP Consortium and the national programs that contribute to it. (http://www.meop.net).

Here we investigated several factors influencing DSW formation in four East Antarctic polynyas with different characteristics. We analysed the sea-ice production rate, water-mass distribution and seasonal transformation, and mixed layer depth. Using a salt budget, we estimated the relative contribution of brine rejection, entrainment, and salt advection to the monthly salinity change in and below the mixed layer in each polynya. The particularly high sea-ice production rate in MP clearly favoured DSW formation there. The accumulated sea-ice production over the whole winter was twice as large in MP as in the other three polynyas (Figure 4). The difference was particularly large between March and May, when sea-ice production was strongest in MP. However, we found that DSW formation was not linked to sea-ice production alone. While sea-ice production remained high throughout the winter in MP, the increase in the mixed-layer salinity between April and August was the lowest of all polynyas (Figure 8a). The mixed layer salt budget suggests that the strong sea-ice production was partially offset by fresh advection during this period.

The fresh advection likely reflects glacial meltwater entering MP from the cavity beneath the Amery Ice Shelf, which undergoes a net basal melt rate of 57.4±25.3 Gt yr -1 (1±0.4 m yr -1 ) (Herraiz- Borreguero et al., 2016). Another possibility is that MP received inflow of fresh AASW from neighboring areas with more sea-ice melt. However, this seems less likely given that the direction of sea ice drift was offshore (Figure S3).

Formation of DSW in MP was favored by the relatively high salinity observed throughout the water column from the beginning of the sampling record in March. The high salinity at the end of autumn means that less salt needs to be added to reach the salinity threshold