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  • View in gallery
    Fig. 1.

    (a) Average Antarctic SIC in September for the period 2002–19. (b) SIC on 27 Sep 2018. (c) SIC on 4 Nov 2018. Black box indicates the CS sector.

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    Fig. 2.

    The numbers of polynya days counted for the period 2002–19. (a) Total polynya days for the SIC threshold value of 0.7. (b) Total polynya days for the SIC threshold value of 0.15. (c)–(f) Total polynya days counted for each month from June to November for the SIC threshold value of 0.7. Note the different scales of the color bars. Isobaths of 3500 m are shown as gray lines in each panel.

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    Fig. 3.

    (a) Time series of the open-ocean polynya areas in the CS (60°–90°E) for the period 2002–19. Black (red) lines correspond to the SIC threshold value of 0.7 (0.15). (b) Time series of the open-ocean polynya areas in the CS (60°–90°E) for 2018 and around the Maud Rise (13°W–18°E) for 2017. Light gray lines (light red lines) are the time series of the polynya areas in the CS for the other years identified by the SIC threshold value of 0.7 (0.15). Green (magenta) lines correspond to the 2017 Maud Rise polynya (2018 CS polynya). Solid (dotted) lines correspond to the SIC threshold value of 0.7 (0.15). Note that the ticks on the horizontal axis mark the first date of each month.

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    Fig. 4.

    (a)–(l) The depth of the −0.5°C isotherm derived from EN4 data from April to March averaged over 2002–19. Yellow light lines in (c)–(g): 40-day contour lines of the total polynya days in June–October between 2002 and 2019. Red solid lines: SB from Orsi et al. (1995); red dotted line: SB derived from EN4 data; black dotted lines: sACCf from Orsi et al. (1995); green lines: sea ice edge; white lines: monthly mean AD over 2002–19; see text for the definitions of SB, sACCf, and AD.

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    Fig. 5.

    Monthly mean dynamic ocean topography for the period 2011–18 in (a) July, (b) August, and (c) September. Isobaths of 3500 m are shown as the black lines. Red solid lines: SB from Orsi et al. (1995); red dotted lines: SB derived from EN4 data; white lines: AD.

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    Fig. 6.

    Sea ice thickness (color shading) in (a) May and (b) June 2012 [red solid lines: SB from Orsi et al. (1995); red dashed lines: SB derived from EN4 data; white lines: AD; cyan dotted lines: 40-day contour lines of the total polynya days in July between 2002 and 2019; black line: isobath of 3500 m]. (c)–(e) Ice velocity vector (black arrows) and 10-m wind vector (gray arrows) overlaid on sea ice speed on 10–12 Jul 2012. Color shading: sea ice speed (cm s−1). Red stars: location of low pressure centers; yellow lines: SIC contour lines of 0.15; black dots: polynyas identified using the SIC threshold of 0.7; white line: monthly mean AD in July over 2002–19. (f),(g) Locations of seal CTD profiles on 10, 12, and 21 Jul. The color shading is SIC on 12 Jul 2012. (h)–(j) Vertical structures of potential density, salinity, and potential temperature.

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    Fig. 7.

    The seal observed potential temperature profiles on (a) 13 Jul, (b) 24 Aug, and (c) 12 Sep 2012. Red, green, and blue lines with different symbols represent the profiles at different locations shown in (d) 13 Jul, (e) 24 Aug, and (f) 12 Sep 2012. Black lines with different symbols: monthly potential temperature profiles from EN4. The locations of the black symbols in (d)–(f) are consistent with that shown in (a)–(c). Cyan lines in (d)–(f): SIC contour lines of 0.7.

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    Fig. 8.

    (a) The ice-free polynyas from July to October in 2012 at intervals of six days. Colored lines: SIC contour lines of 0.15 excluding those that connected to the coast and the open ocean; black box: canyon and drift region. The numbers on the left of color bar indicate the number of days from 3 Jul, and the dates are shown on the right of the color bar. (b) The daily changes in the total area of the polynyas (blue) and average wind stress curl (red) for July–October 2012 over the black box shown in Fig. 8a. (c)–(f) Wind stress curl (color shading; N m−3) overlaid on SIC on 8–11 Oct 2012. Green lines: SIC contour lines of 0.15; black dots: polynyas correspond to the SIC threshold value of 0.7; the gray lines from south to north are the 2800-, 3000-, and 3500-m isobaths, to depict the submarine topography.

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    Fig. 9.

    Monthly mean currents (black arrows) at a depth of 3325 m over 2005–10 from SOSE in (a) July and (b) September. Cyan lines: 40-day contour lines of the total polynya days in (a) July and (b) September between 2002 and 2019; green line: the ice-free (SIC < 0.15) polynya occurred on 10 Oct 2012 in Fig. 8e. The black box is the same as in Fig. 8a.

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    Fig. 10.

    A schematic diagram for the mechanisms of the formation of the open-ocean polynyas in the Cooperation Sea. Red solid lines: SB from Orsi et al. (1995); red dotted line: SB derived from EN4 data; shading color: mean state of 200-m potential temperature in September along with the cross section of 75°E (contours) down to 300-m depth; black line with arrows: sACCf from Orsi et al. (1995); green line: sea ice edge in September over 2002–19; white line: AD in September over 2002–19; shaded area: total polynya days of more than 40 days in September between 2002 and 2019 identified by the SIC threshold value of 0.7; blue arrows: Antarctic Slope Current; purple arrows: an overflow cascades through the Wild Canyon from the upstream Burton basin. Clockwise circular arrows indicate the present of cyclonic eddies.

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    Fig. A1.

    Antarctic SIC on 1 Dec 2017 from (a) AMSR2 and (b) NSIDC, on 24 Nov 2017 from (c) AMSR2 and (d) NSIDC, and on 25 Aug 2010 from (e) AMSR-E and (f) NSIDC.

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Open-Ocean Polynyas in the Cooperation Sea, Antarctica

Qing QinaKey Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources, Hohai University, Nanjing, China
bCollege of Oceanography, Hohai University, Nanjing, China

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Zhaomin WangaKey Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources, Hohai University, Nanjing, China
bCollege of Oceanography, Hohai University, Nanjing, China
cSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

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Chengyan LiucSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

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Chen ChengcSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

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Abstract

Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. Here, we show that more persistent open-ocean polynyas occur in the Cooperation Sea (CS) (60°–90°E), a sector of the Southern Ocean off the Prydz Bay continental shelf, between 2002 and 2019. Polynyas are formed annually mainly within the 62°–65°S band, as identified by sea ice concentrations less than 0.7. The polynyas usually began to emerge in April and expanded to large sizes during July–October, with sizes often larger than those of the Maud Rise polynya in 2017. The annual maximum size of polynyas ranged from 115.3 × 103 km2 in 2013 to 312.4 × 103 km2 in 2010, with an average value of 188.9 × 103 km2. The Antarctic Circumpolar Current (ACC) travels closer to the continental shelf and brings the upper circumpolar deep water to much higher latitudes in the CS than in most other sectors; cyclonic ocean circulations often develop between the ACC and the Antarctic Slope Current, with many of them being associated with local topographic features and dense water cascading. These oceanic preconditions, along with cyclonic wind forcing in the Antarctic Divergence zone, generated polynyas in the CS. These findings offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean. Future efforts should be particularly devoted to more extensively observing the ocean circulation to understand the variability of open-ocean polynyas in the CS.

Significance Statement

An open-ocean polynya is an offshore area of open water or low sea ice cover surrounded by pack ice. Open-ocean polynyas are important for driving the physical, biogeochemical, and biological processes in the Southern Ocean. Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. The purpose of this study is to document the existence of more persistent open-ocean polynyas in the Cooperation Sea (60°–90°E) and explore the atmospheric and oceanic forcing mechanisms responsible for the formation of the open-ocean polynyas. Our results would offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Zhaomin Wang, zhaomin.wang@hhu.edu.cn; Chengyan Liu, liuchengyan@sml-zhuhai.cn

Abstract

Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. Here, we show that more persistent open-ocean polynyas occur in the Cooperation Sea (CS) (60°–90°E), a sector of the Southern Ocean off the Prydz Bay continental shelf, between 2002 and 2019. Polynyas are formed annually mainly within the 62°–65°S band, as identified by sea ice concentrations less than 0.7. The polynyas usually began to emerge in April and expanded to large sizes during July–October, with sizes often larger than those of the Maud Rise polynya in 2017. The annual maximum size of polynyas ranged from 115.3 × 103 km2 in 2013 to 312.4 × 103 km2 in 2010, with an average value of 188.9 × 103 km2. The Antarctic Circumpolar Current (ACC) travels closer to the continental shelf and brings the upper circumpolar deep water to much higher latitudes in the CS than in most other sectors; cyclonic ocean circulations often develop between the ACC and the Antarctic Slope Current, with many of them being associated with local topographic features and dense water cascading. These oceanic preconditions, along with cyclonic wind forcing in the Antarctic Divergence zone, generated polynyas in the CS. These findings offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean. Future efforts should be particularly devoted to more extensively observing the ocean circulation to understand the variability of open-ocean polynyas in the CS.

Significance Statement

An open-ocean polynya is an offshore area of open water or low sea ice cover surrounded by pack ice. Open-ocean polynyas are important for driving the physical, biogeochemical, and biological processes in the Southern Ocean. Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. The purpose of this study is to document the existence of more persistent open-ocean polynyas in the Cooperation Sea (60°–90°E) and explore the atmospheric and oceanic forcing mechanisms responsible for the formation of the open-ocean polynyas. Our results would offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Zhaomin Wang, zhaomin.wang@hhu.edu.cn; Chengyan Liu, liuchengyan@sml-zhuhai.cn

1. Introduction

The variability of sea ice cover plays an important role in polar and global climate changes by modulating the fluxes of momentum, heat, freshwater, and carbon between the atmosphere and the ocean (Kottmeier et al. 1997; Thomas and Dieckmann 2008; Fransson et al. 2011). Polynyas are particularly important in driving the interactions between the atmosphere and the ocean. A polynya is an area where the sea ice concentration (SIC) is significantly lower than that of its surrounding area (Smith et al. 1990). An open-ocean polynya is a polynya that occurs offshore, which is why it is also often referred to as an offshore polynya, in contrast to a coastal polynya whose boundaries are partly confined to the coast, landfast ice, and ice shelf fronts. Open-ocean polynyas, also known as sensible heat polynyas, usually form in regions of warmer water upwelling from the deeper ocean (Comiso and Gordon 1996). The occurrence of open-ocean polynyas has aroused interests in physical, biogeochemical, and biological communities. Previous studies have found significant impacts of open-ocean polynyas on ocean circulation (e.g., Martin et al. 2013) and coastal and bottom water properties (Wang et al. 2017), atmospheric circulation (Weijer et al. 2017), primary production (Delmont et al. 2014), and Antarctic marine ecosystem (Yager et al. 2016).

Antarctic sea ice variability is strongly driven by atmospheric circulation (Enomoto and Ohmura 1990; Raphael 2007; Gordon et al. 2007; Holland and Kwok 2012; Wang et al. 2014; Turner et al. 2017; Wang et al. 2019). At high southern latitudes, there exists a circumpolar low pressure zonal belt, namely, the atmospheric circumpolar trough, consisting of the collective tracks of transient cyclones. Due to the net effect of many individual cyclones in the atmosphere (Eayrs et al. 2019) and the divergence of induced Ekman transport at the surface, this zonal belt is also known as the “Atmospheric Convergence Line” (e.g., Enomoto and Ohmura 1990) and the “Antarctic Divergence” (AD) (e.g., Sverdrup et al. 1942; Wakatsuchi et al. 1994). Owing to the cyclonic wind forcing in the AD, the divergent surface Ekman transport leads to an upwelling of warm deep water (Kottmeier et al. 1997; Heil and Allison 1999; Wakatsuchi et al. 1994). The effect of this wind-driven upwelling has been suggested to accelerate the melting in the marginal ice zone in the austral springtime, significantly mediating the seasonal cycle across the sea ice extent (Enomoto and Ohmura 1990; Eayrs et al. 2019). Wakatsuchi et al. (1994) found that in the eastern Indian Ocean and western Pacific Ocean sector (100°–130°E), where abundant cyclonic eddies exist and the Antarctic Circumpolar Current (ACC) is closer to the continent owing to bathymetric steering, the wind-driven upward heat flux greatly restrains the northward advance of the sea ice cover, resulting in much less severe sea ice expansion than in other sectors.

When the sea ice edge is able to cross the AD during its northward advance phase, sea ice divergence can potentially lead to the formation of polynyas under certain conditions. The divergent sea ice motion within the AD tends to generate sea ice conditions with low concentration and thin thickness. This process can enhance the heat loss to the cold atmosphere. Subsequent freezing leads to brine rejection and thus greater surface water density, which may trigger convection to some extent. If deep convection can be triggered, warm and saline deep water can then be brought to the surface. The surface heat loss to the atmosphere continues to result in a buoyancy loss and thus maintains convection. When insufficient deep ocean heat is supplied to the surface, sea ice formation resumes and convection ceases, leading to the disappearance of open-ocean polynyas (Gordon 1991; Martin et al. 2013; Kurtakoti et al. 2018). However, in contrast to the existing circumpolar features of cyclonic wind forcing, open-ocean polynyas occur only in certain specific locations.

Open-ocean polynyas have long been observed around the Maud Rise (a seamount situated at 65°S, 3°E) in the Weddell Sea. Since the first use of passive microwave satellites for retrieving sea ice extent in 1972, satellite observations have been used to examine Antarctic sea ice cover variability (Zwally and Gloersen 1977; Carsey 1980). Throughout the winters of 1974–76, a large open-ocean polynya termed the Weddell Sea polynya occurred near the Maud Rise (Gordon and Comiso 1988). The open water area within the polynya varied between 2 and 3 × 105 km2 (Carsey 1980). However, since then, no polynyas of similar size and duration have been observed in the Weddell Sea. Much smaller polynyas (∼104 km2) often occur near the Maud Rise (Comiso and Gordon 1987; Gordon and Comiso 1988; Comiso and Gordon 1996; Martinson 1991; Drinkwater 1996). Such smaller polynyas are referred to as the Maud Rise polynyas as opposed to the Weddell Sea polynya of the mid-1970s. More recently, in 2016 and 2017, the largest Maud Rise polynyas (maximum open water area of ∼298.1 × 103 km2, with SIC < 0.15 occurring on 1 December 2017) (Jena et al. 2019) have been formed near the Maud Rise seamount since 1976.

Cyclonic wind forcing has been identified as a key driver in the formation of open-ocean polynyas in the Weddell Sea. Warm Circumpolar Deep Water (CDW) is a necessary precondition for the occurrence and maintenance of open-ocean polynyas near the Maud Rise in the Weddell Sea (Dufour et al. 2017; Kurtakoti et al. 2018; Lindsay et al. 2008; Martin et al. 2013). Warm deep water is maintained in the subsurface through salinity-controlled stratification. The topographic effect of the Maud Rise, including the generation of Taylor columns and free and trapped waves induced by tidal forcing (Beckmann et al. 2001), causes a preconditioning of the ocean structure (Ou 1991; Alverson and Owens 1996; Holland 2001). It has been suggested that the negative phase of the southern annular mode associated with its multidecadal variability weakens the upper-ocean stratification by making the atmosphere colder and drier at high southern latitudes (Gordon et al. 2007). These offered a favorable condition for enhanced cyclonic wind forcing to trigger the formation of the Weddell Sea polynya (Enomoto and Ohmura 1990; Alverson and Owens 1996; Comiso and Gordon 1996; Cheon et al. 2015). Despite the southern annular mode being in a more positive phase compared to the mid-1970s, intense cyclonic wind forcing was found to have initiated the Maud Rise polynyas in 2016 and 2017, along with concurrent upper-ocean preconditioning (Campbell et al. 2019).

Since 1973, recurrent open-ocean polynyas have also been formed in the Cosmonaut Sea, offshore of Enderby Land (Comiso and Gordon 1987, 1996; Gordon and Comiso 1988; Arbetter et al. 2004). The open water area within the polynya varied between 1.1 and 5.2 × 104 km2, and the polynyas lasted from a few days to several weeks, with considerable interannual variability (for a more detailed review, see Morales Maqueda et al. 2004). Arbetter et al. (2004) and Bailey et al. (2004) suggested that cyclones forced the formation of polynyas by causing sea ice divergence. Stress shear at the ocean surface can lead to upwelling, which would maintain polynyas. However, Comiso and Gordon (1996) proposed that the interaction between the eastward ACC and the westward Antarctic Slope Current (ASC) resulted in upwelling, which provided an oceanic origin for the polynya formation.

In summary, the important role of cyclonic wind forcing in the AD has been confirmed in the formation of open-ocean polynyas. Preconditioning of the oceanic structure has also been found to be a necessity. These atmospheric and oceanic conditions determine the locations of open-ocean polynyas. Previous studies to date have only addressed open-ocean polynyas at two locations, namely, near the Maud Rise in the Weddell Sea and in the Cosmonaut Sea. In this study, we show that in the Cooperation Sea (CS) (60°–90°E, marked by the black box in Fig. 1), a sector of the Southern Ocean off the Prydz Bay continental shelf, there exists a more persistent feature of open-ocean polynyas generated by appropriate atmospheric and oceanic forcing conditions. The data and methods are described in section 2. Section 3 presents the spatial and temporal features of the polynyas. The large-scale and synoptic forcing conditions are analyzed in sections 4 and 5, respectively. We conclude this study in section 6.

2. Data and methods

Table 1 summarizes the datasets used in this study. The AMSR-E (Advanced Microwave Scanning Radiometer for EOS) and AMSR2 (Advanced Microwave Scanning Radiometer 2) were obtained from the Institute of Environmental Physics, University of Bremen. These SIC data were retrieved using the Arctic Radiation and Turbulence Interaction Study (ARTIST) sea ice algorithm (Spreen et al. 2008) and applied to microwave radiometer data from the sensors AMSR-E on the NASA satellite Aqua and AMSR2 on the JAXA satellite GCOM-W1. Due to their high resolution, we used these two datasets to detect the open-ocean polynyas in the CS.

Table 1

Datasets used in this study.

Table 1

SIC data (Sea Ice Index, version 3) from the National Snow and Ice Data Center (NSIDC) were also used to compare results derived from AMSR-E and AMSR2 (see appendix). These SIC data were retrieved from the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave Imager (SSMI), and the Special Sensor Microwave Imager/Sounder (SSMIS) (Fetterer et al. 2017). The sea ice index is based on the NASA Team algorithm (Cavalieri et al. 1997).

The sea ice motion data are from the reconstructed daily 25-km EASE-Grid Sea Ice Motion dataset (Tschudi et al. 2019).

Sea ice thickness data from SMOS (Soil Moisture and Ocean Salinity) (Huntemann et al. 2014) were used to examine the thickness distribution during the early stages of the freezing season. The ice thickness is inferred from the difference between the intensities observed at vertical and horizontal polarization (Huntemann et al. 2014). As a result, these thickness data are more accurate when the sea ice is thin, with 0–20 cm being quite accurate and stable (Huntemann et al. 2014).

The daily 10-m winds and wind stress were derived from ERA5, which was generated using 4D-Var data assimilation in CY41R2 ECMWF’s Integrated Forecast System (Hersbach et al. 2018). A 10-m wind was used to illustrate the effect of storms on the formation of the open-ocean polynyas in the CS, and wind stress was used to calculate wind stress curl to quantify the intensity of storms and their dynamic effects on sea ice divergence and Ekman pumping.

The daily mean surface net longwave radiation flux, mean surface net shortwave radiation flux, mean surface sensible heat flux, and mean surface latent heat flux were derived from ERA5 (Hersbach et al. 2018). Dynamic ocean topography (DOT) data, acquired by the Synthetic Aperture Radar Interferometric Radar Altimeter instrument on CryoSat‐2, were used to describe the surface circulation features in the CS. For ice-covered areas, the dataset was processed according to the method of Peacock and Laxon (2004). Seasonal differences caused by different retrackers between the open ocean and the lead were first determined and then added to the lead data to rectify the bias (Armitage et al. 2018). The data were corrected for local deviations. The uncertainty of the retrieved DOT was 1.5 cm (Dotto et al. 2018; Naveira Garabato et al. 2019).

Monthly EN4 data (Good et al. 2013) from 83°S to 90°N were used to examine warm deep water patterns. The latest version of this dataset (EN.4.2.1) was used. The EN4 dataset was generated using in situ observations, including Argo (Argo 2000), ASBO (Arctic Synoptic Basin-wide Oceanography), GTSPP (Global Temperature and Salinity Profile Program), and WOD13 (World Ocean Database 2013) (Good et al. 2013).

Ocean current data from the Southern Ocean State Estimate (SOSE) (Mazloff et al. 2010) were analyzed to understand the link between polynyas and local circulation. The SOSE model is an evolved version of the Massachusetts Institute of Technology GCM (MITgcm) (Mazloff et al. 2010).

Seal-CTD data were collected by the Integrated Marine Observing System (IMOS). The MEOP (Marine Mammals Exploring the Oceans Pole to Pole) consortium brings together several national programs to produce a comprehensive quality-controlled database of oceanographic data obtained from instrumented marine mammals.

Open-ocean polynyas were identified as pixels with a SIC less than a threshold and distinguished from coastal polynyas by ensuring that these pixels were not connected to the coast. We also ensured that these pixels were not connected to the open ocean, thereby excluding regions of sea ice marginal zones and low SIC embayments. Thus, the identified open-ocean polynyas are located in the middle of the ice pack. The area of the polynya was computed by summing up the area of these pixels. A SIC threshold of 0.7 was used to detect the presence of polynyas and to calculate the extent of polynyas, but a much smaller value of 0.15 was also used to identify ice-free polynyas. We note that various thresholds have been used to identify open-ocean polynyas and calculate their extent, with values ranging from 0.15 (e.g., Jena et al. 2019) to 0.8 (e.g., Arbetter et al. 2004; Geddes and Moore 2007). In the following, if not specified, “polynya” refers to the open-ocean polynya identified using a threshold of 0.7.

The zero line of monthly eastward wind stress defines the core location of the AD in this study, as described by Yamazaki et al. (2020). The southern boundary of the ACC (SB) is defined as the southern limit of the Upper Circumpolar Deep Water (UCDW), which is marked by the oxygen minimum layer (Orsi et al. 1995). The UCDW usually rises to near 200 m with temperatures above 1.5°C (Orsi et al. 1995) and is primarily found from the eastern South Pacific Ocean to western Indian Ocean (Callahan 1972; Warren 1981). Based on the above reference, we computed the SB in the region from 30°E to 180° using the EN4 monthly ocean temperatures. In the CS, there are differences between the SB computed from the EN4 data and that derived from Orsi et al. (1995), where similar differences were also found by Yamazaki et al. (2020) and Bestley et al. (2020). The ACC consists of multiple fronts associated with jets, which are separated by relatively quiescent zones. The southernmost front is termed the southern ACC front (sACCf). We utilized the position of the sACCf from Orsi et al. (1995) to illustrate the poleward migration of the ACC and to link its location to the formation of polynyas.

3. Spatial and temporal characteristics of polynyas in the Cooperation Sea

a. An illustration of polynyas in the Cooperation Sea

Antarctic sea ice often experiences a slow-growing season from March to early September and a rapid melting season from November to early February (Gordon 1981). According to Parkinson (2014), the average Antarctic sea ice extent reaches its annual maximum of 18.5 × 106 km2 in September. Figure 1a shows the mean SIC for September 2002–19 from AMSR-E and AMSR2 satellite data. Apparently, in some sectors (e.g., the western Weddell Sea and the Ross Sea), the sea ice edge advanced farther north than in other sectors (e.g., the Bellingshausen Sea, the Amundsen Sea, the Indian Ocean, and the western Pacific sector). Moreover, certain regions showed lower SIC than their surrounding areas, such as those near the Maud Rise, near Cape Ann in the Cosmonaut Sea, and in the CS (Fig. 1a), implying the presence of open-ocean polynyas in these regions.

While the occurrence of recurrent open-ocean polynyas near the Maud Rise in the Weddell Sea and near Cape Ann in the Cosmonaut Sea has been well addressed, the presence of open-ocean polynyas in other regions has not been well documented and explored. We further show the SIC pattern for 27 September 2018 in Fig. 1b to illustrate the polynyas in the CS, as shown by the relatively low SIC in the middle of pack ice. The area of the polynya is estimated to be 69.5 × 103 km2 by summing the areas of those pixels with SICs less than 0.7 on 27 September 2018 (see section 2). The extent of the polynya then expanded considerably during the melting season, followed by the formation of an ice-edge embayment on 4 November 2018 (Fig. 1c). Based on data from 39 drifting buoys deployed from 1985 to 1996 in the East Antarctic sea ice zone, two areas with high sea ice drift speeds were found near (62.5°S, 77°E) and (62.5°S, 87°E), while ice drift speeds were slower and SIC was higher along the northern edge (Heil and Allison 1999). A secondary peak of geochemical fluxes in midwinter and early spring also existed following the primary peak during the austral summer, as indicated by data from sediment traps deployed within the Southern Indian Ocean seasonal ice zone between 1998 and 2001 (Pilskaln et al. 2004). The similar results were also found by Rigual-Hernández et al. (2019) that two unexpected peaks of particle export increase during the austral winter and spring by analyzing the annual variability of the total diatom, biogenic silica, and particulate organic carbon fluxes at offshore Prydz Bay, within the polynyas we studied. Our results can thus provide a reasonable explanation for this unexplained biochemical phenomenon. Therefore, the revealed polynya activities in the Cooperation Sea are also important for biogeochemical and biological processes in this region.

b. Variability of open-ocean polynyas in the Cooperation Sea

Figure 2 shows the total polynya days at each pixel (defined by the total number of days for which the pixel was within a polynya) for the period 2002–19. During 2002–19, polynyas occurred more frequently in the CS, the Cosmonaut Sea, and the Weddell Sea than in other sectors, with the CS being the sector characterized by the most persistent polynyas (Fig. 2a). The maximum number of polynya days reached 435 days in the CS, compared to 378 days in the Cosmonaut Sea and 369 days in the Weddell Sea. However, when using an SIC threshold of 0.15, the maximum number of polynya days reduces to 75 days in the CS, 96 days in the Cosmonaut Sea, and 190 days in the Weddell Sea (Fig. 2b). For the number of polynya days counted for each month (Fig. 2c), polynyas existed from July to November in the CS and the Cosmonaut Sea, while they were much less visible in July and August in the Weddell Sea.

Fig. 1.
Fig. 1.

(a) Average Antarctic SIC in September for the period 2002–19. (b) SIC on 27 Sep 2018. (c) SIC on 4 Nov 2018. Black box indicates the CS sector.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

Fig. 2.
Fig. 2.

The numbers of polynya days counted for the period 2002–19. (a) Total polynya days for the SIC threshold value of 0.7. (b) Total polynya days for the SIC threshold value of 0.15. (c)–(f) Total polynya days counted for each month from June to November for the SIC threshold value of 0.7. Note the different scales of the color bars. Isobaths of 3500 m are shown as gray lines in each panel.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

Figure 3 shows the time series of the total polynya area in the CS between 2002 and 2019. The total area reached its largest value of 312.4 × 103 km2 on 25 August 2010, which is close to the maximum size of the polynya that reappeared around the Maud Rise on 24 November 2017 (328.0 × 103 km2 for SIC < 0.7). However, we note that for a threshold of 0.15, the total polynya area reached its largest value of 170.5 × 103 km2 in the CS on 3 November 2018, while the maximum size of the Maud Rise polynya reached 388.7 × 103 km2 on 2 December 2017. These results reflect that the SIC in the Maud Rise polynya was much smaller in 2017 than that in the CS during the melting phase.

Fig. 3.
Fig. 3.

(a) Time series of the open-ocean polynya areas in the CS (60°–90°E) for the period 2002–19. Black (red) lines correspond to the SIC threshold value of 0.7 (0.15). (b) Time series of the open-ocean polynya areas in the CS (60°–90°E) for 2018 and around the Maud Rise (13°W–18°E) for 2017. Light gray lines (light red lines) are the time series of the polynya areas in the CS for the other years identified by the SIC threshold value of 0.7 (0.15). Green (magenta) lines correspond to the 2017 Maud Rise polynya (2018 CS polynya). Solid (dotted) lines correspond to the SIC threshold value of 0.7 (0.15). Note that the ticks on the horizontal axis mark the first date of each month.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

The annual maximum polynya areas were all above 100 × 103 km2 and even greater than 200 × 103 km2 in 2002, 2004, 2010, 2012, 2014, 2018, and 2019 (Fig. 3a). Even for the threshold value of 0.15, the maximum polynya area reached 170.5 × 103 km2 on 3 November 2018 during the melting phase (Fig. 3a). Large polynyas often reached their maximum total area in October (2002, 2004, 2007, 2008, 2009, 2014, 2016, 2017, 2018, 2019), and even earlier in August in some years (2003, 2005, 2010, 2012).

To clearly see the seasonal variability, we show the daily polynya areas from 1 January to 31 December for each year, with 2018 being highlighted, as shown in Fig. 3b. In the CS, the polynyas began to appear in April and expanded to relatively large sizes between July and October. For example, on 23 May 2018, polynyas were scattered between 65° and 80°E (not shown). These polynyas lasted only a few days and had a fairly small total area (30.1 × 103 km2 on 28 May 2018). Between 12 and 27 June 2018, sizable polynyas were formed, with a maximum total area exceeding 85.3 × 103 km2 on 18 June 2018 (Fig. 3b). Since mid-July 2018, the frequency of large polynya events had increased, particularly during July–August 2018. Subsequently, during September–October 2018, the polynyas expanded to even large sizes. As a typical feature of this sequence, the area of polynyas around the Kerguelen Plateau increased, with the total area reaching its peak at 156.1 × 103 km2 on 21 September 2018. The last sharp increase in the size of the polynyas began on 12 October 2018, with the total area reaching an annual maximum of 281.7 × 103 km2 on 23 October 2018 (Fig. 3b). Then, the polynya rapidly decreased in size due to further melting and consequent connection to the open ocean. When using the threshold value of 0.15, the development of polynyas more or less followed a similar pattern throughout the year, but with a much smaller total area, with the largest polynya size occurring on 7 November 2018 (total area of 170.5 × 103 km2).

It is worth noting that during the most wintertime of 2002–19, the polynya areas in the CS were considerably larger than that of the Maud Rise polynya in 2017. When the threshold of 15% is applied, the areas of the Cooperation Sea polynyas (see the light red line in Fig. 3b) during July–October were generally close to or even larger than the area of the Maud Rise polynya in 2017 (see the green dotted line in Fig. 3b) in some years (2005, 2010, 2012, 2013, 2014, 2016, 2017, 2018, 2019). Figure 3a shows that the areas of the detected Cooperation Sea polynyas using the threshold of 0.15 are often larger than 0.5 × 105 km2 (which is the size of the Maud Rise polynya when it expanded to a large size in September and October; see the green dotted line in Fig. 3b) during the period of 2002–19, suggesting a persistent feature of open-ocean polynyas in the CS.

4. Large-scale atmospheric and oceanic forcing

The contribution of subsurface heat reservoirs to the formation of open-ocean polynyas has been well recognized in previous studies. The occurrence and persistence of open-ocean polynyas relies on the salt input from warm deep water to the mixed layer surpassing the freshwater released by melting sea ice, which is thought to be the cause of the large Weddell Sea polynya in the 1970s (Gordon 1982). In 2016 and 2017, the eddy-induced doming of isotherms over the Maud Rise seamount was found to be the primary cause of the Maud Rise polynya (Jena et al. 2019). Processes involving wind-driven upwelling and mixed layer deepening have been suggested to be responsible for the significant increase in heat and salt exchanges, which contribute to limiting winter sea ice thickness (Gordon and Huber 1990; Comiso and Gordon 1996). In early winter, the shallower subsurface heat reservoir provides a precondition for the formation of open-ocean polynyas (Cheon et al. 2015; Kurtakoti et al. 2018). As the upper-ocean water column becomes unstable, relatively warm and saline UCDW rises to the surface to melt sea ice, resulting in areas of thin sea ice and low SIC (Martinson et al. 1981; Gordon and Comiso 1988; Goosse and Zunz 2014).

a. Atmospheric forcing

Between the cores of the circumpolar westerlies and the polar easterlies, there exists a divergence zone owing to the divergent Ekman drift, which is often referred to as the AD (Sverdrup et al. 1942; Wakatsuchi et al. 1994).

Figure 4 indicates that the average location of the AD from 2002 to 2019 well coincides with the polynya sites in the CS (Figs. 4c–g). As the sea ice edge (Fig. 4) advanced northward across the AD, wind shear drove substantial sea ice divergence in the CS beginning in June, and it was during this month that relatively large polynyas began to form (see Fig. 3b). Cyclonic wind forcing can also promote upwelling of warm deep water to limit surface freezing. The divergence of sea ice, along with a possible reduction in new ice formation, may have contributed considerably to the formation of polynyas in the CS.

Fig. 4.
Fig. 4.

(a)–(l) The depth of the −0.5°C isotherm derived from EN4 data from April to March averaged over 2002–19. Yellow light lines in (c)–(g): 40-day contour lines of the total polynya days in June–October between 2002 and 2019. Red solid lines: SB from Orsi et al. (1995); red dotted line: SB derived from EN4 data; black dotted lines: sACCf from Orsi et al. (1995); green lines: sea ice edge; white lines: monthly mean AD over 2002–19; see text for the definitions of SB, sACCf, and AD.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

b. Oceanic circulation features

The SB is defined as the poleward limit of the UCDW (at a temperature of 1.5°C near 200 m depth) (Orsi et al. 1995) (see section 2), which well describes the poleward extent of the UCDW. The location of the SB is primarily determined by the structure of the ACC and the subpolar gyres, which are dynamically constrained by the topographic features of the Southern Ocean (SO).

The ACC in the CS is mainly zonal upstream of the largest topographic feature in the SO, the Kerguelen Plateau, after being considerably deflected toward the Antarctic continent in the eastern branch of the Weddell Gyre around 30°–40°E (Bestley et al. 2020; Yamazaki et al. 2020) (see also sACCf in Fig. 4, black dotted line). The sACCf bifurcates due to the constriction of the Kerguelen Plateau (Bestley et al. 2020), resulting in the southern branch of the sACCf passing through the northern Princess Elizabeth Trough. The southern branch of the sACCf then turns sharply northward to join the western boundary current at the eastern edge of the plateau (Bestley et al. 2020; Yamazaki et al. 2020; Aoki et al. 2010).

The shape of the subpolar gyres largely modulates the location of the sACCf. The ACC travels closer to the continental shelf in the CS than in most other sectors around the Antarctic (Fig. 4). In sharp contrast, the locations of the sACCf in the Weddell and Ross Seas are quite far north, and are pushed by the western branches of the two largest cyclonic subpolar gyres. Thus, the SB is located at much lower latitudes in the Weddell and Ross Seas than in the other sectors, resulting in their locations being far away from those of the AD located at higher latitudes (Fig. 4).

The shape of the subpolar gyres also largely controls the sea ice extent around the Antarctic (see Fig. 1). The northward advance of the sea ice edge in the Weddell and Ross Seas reaches the farthest offshore around the Antarctic. In contrast, in the Cosmonaut Sea, the Indian Ocean and the western Pacific Ocean sector, the Amundsen Sea and the Bellingshausen Sea, the northward advance of sea ice is somewhat limited. Moreover, the opposite flow direction from the ASC to the ACC offers a favorable condition for creating a cyclonic circulation in the CS (Heywood et al. 1999). Figure 5 shows the mean DOT in the Prydz Bay–CS region for July, August, and September 2011–18. The relatively low DOT extended from the interior of the bay to around 63°S in July and 64°S in August and September in the CS, confirming the existence of a cyclonic gyre. The large-scale cyclonic circulation in this basin was also reproduced by a high-resolution ocean general circulation model (Aoki et al. 2010).

Fig. 5.
Fig. 5.

Monthly mean dynamic ocean topography for the period 2011–18 in (a) July, (b) August, and (c) September. Isobaths of 3500 m are shown as the black lines. Red solid lines: SB from Orsi et al. (1995); red dotted lines: SB derived from EN4 data; white lines: AD.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

c. Distribution of warm deep water

Within the AD zone, the Ekman pumping effect tends to cause upwelling of warm deep water. Therefore, the relative location of the AD to the SB is critical for the heat and salt exchanges between the UCDW and its surface waters (Wakatsuchi et al. 1994). In other words, if the UCDW reaches the AD region owing to the southward migration of the ACC, it is expected that warmer and saltier waters in the UCDW are more likely to be uplifted and may even be exchanged with colder and fresher surface water (Bindoff et al. 2000).

The average depth of the −0.5°C isotherm for the period 2002–19 derived from EN4 from April to March confirms the shallowing of the warm subsurface water in the CS (Fig. 4). Figure 4 further demonstrates the persistent shallowing of the warm subsurface water in the CS from April to September. At 75°E, the shallowest depth of the −0.5°C isotherm was 62.6 m, occurring at 61°S in July; for August and September, the depths were 58.0 and 76.6 m, respectively, both occurring at 62°S.

Between 60° and 90°E, the shallowest −0.5°C isotherm was around 70°E from April to June, shifted to around 75°E from July to September, and then moved to around 79°E in October and November. This warm water depth, which coincides with the AD position, is also an indicator of the more probable location where polynyas occur.

Figure 4 also shows the distribution of shallow warm water from 100° to 150°E, which coincides with the location of the AD. Strong Ekman pumping, as well as abundant eddies in this region, brought warm water to the surface to restrain the northward ice advance (Wakatsuchi et al. 1994), resulting in the much less profound sea ice advance than in the other sectors (see also Fig. 1a). A cyclonic eddy train was also observed from 100° and 130°E, using the multiple satellite radar altimeters of CryoSat‐2, Jason‐2, Jason‐3, and Sentinel‐3A (Mizobata et al. 2020), which provided an evidence of cyclonic eddy activity in the region.

Another offshore area with shallow warm water lies between 30° and 40°E in the Cosmonaut Sea, which is particularly evident during July–September. However, a notable difference from the situation in the CS was that the warm water between 30° and 40°E was deeper in May and June than in the CS. This means that thicker sea ice is likely to be formed during these two months. This precondition of sea ice thickness may limit the development of subsequent polynyas. This is probably the reason why polynyas in the Cosmonaut Sea are less frequent and persistent.

Thus, these atmospheric and oceanic circulation features offer unique conditions for the formation of the persistent polynyas in the CS. These conditions create the necessary preconditions for synoptic atmospheric and oceanic processes to generate relatively persistent polynya events in the CS.

It can also be easily seen that the warm water around the Maud Rise and in the central region of the Weddell Gyre is also shallow. The dynamic topographic effect of the Maud Rise and the strong Weddell Gyre created such a condition, which is widely recognized as a precondition for the formation of the Weddell Sea polynya and the Maud Rise polynya.

5. Synoptic atmospheric and oceanic forcing

a. Role of storms

Intense storms can lead to strong sea ice divergence. The effect of storms on the formation of polynyas is expected to be more visible in regions with thinner sea ice. To illustrate this effect, we first examined the sea ice thickness distribution in May and June 2012 (Figs. 6a,b). In 2012, the depth of the −0.5°C isotherm in the CS was close to the average value for the period 2002–19 (the absolute value of the difference in the central region of the CS was generally close to 10 m; not shown). During the early stages of the sea ice formation season, relatively shallow warm water restricted sea ice growth and then often maintained relatively low sea ice thickness and concentration in the region of upward Ekman pumping, i.e., the AD zone. Indeed, in May and June, sea ice was relatively thin along 64°S and surrounded by thicker sea ice to the south and north (Figs. 6a,b). Relatively large polynyas were formed in July 2012 (Figs. 6c–e) due to the persistent features of the shallower warm deep water (Figs. 4b–d and 7) and the storm-induced upward Ekman pumping. The seal-collected CTD profiles show the vertical structure of the potential temperature in the polynyas on 13 July, 24 August, and 12 September 2012 (Fig. 7). During these days, the thermocline in the polynyas was shallow, and is generally consistent with the corresponding monthly profiles from EN4. Note that the water temperature below the thermocline was above 1°C and even close to 2°C for some profiles around 200 m in the Cooperation Sea (Figs. 7a–c), while the water around 200 m was cooler than 1°C in the Maud Rise polynya (Jena et al. 2019, their Fig. 1c). Though the convective depth was found to be around 100 m in our study associated with the Cooperation Sea polynyas, the shallower and warmer deep water than in the Maud Rise polynya and the cyclonic wind forcing in the Cooperation Sea favored the delivery of warm water to the mixed layer. So, the heat delivery from the deeper ocean to the mixed layer was also involved in the formation of the polynyas in the Cooperation Sea, which was similar to (but weaker than) the heat delivery by deep convection. However, the mixed layers were much deeper than those from the EN4 data. Some other notable differences also exist in the EN4 profiles, particularly those for12 September 2012.

Fig. 6.
Fig. 6.

Sea ice thickness (color shading) in (a) May and (b) June 2012 [red solid lines: SB from Orsi et al. (1995); red dashed lines: SB derived from EN4 data; white lines: AD; cyan dotted lines: 40-day contour lines of the total polynya days in July between 2002 and 2019; black line: isobath of 3500 m]. (c)–(e) Ice velocity vector (black arrows) and 10-m wind vector (gray arrows) overlaid on sea ice speed on 10–12 Jul 2012. Color shading: sea ice speed (cm s−1). Red stars: location of low pressure centers; yellow lines: SIC contour lines of 0.15; black dots: polynyas identified using the SIC threshold of 0.7; white line: monthly mean AD in July over 2002–19. (f),(g) Locations of seal CTD profiles on 10, 12, and 21 Jul. The color shading is SIC on 12 Jul 2012. (h)–(j) Vertical structures of potential density, salinity, and potential temperature.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

Fig. 7.
Fig. 7.

The seal observed potential temperature profiles on (a) 13 Jul, (b) 24 Aug, and (c) 12 Sep 2012. Red, green, and blue lines with different symbols represent the profiles at different locations shown in (d) 13 Jul, (e) 24 Aug, and (f) 12 Sep 2012. Black lines with different symbols: monthly potential temperature profiles from EN4. The locations of the black symbols in (d)–(f) are consistent with that shown in (a)–(c). Cyan lines in (d)–(f): SIC contour lines of 0.7.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

Figures 6c–e illustrate the effects of storms on the polynya formation during 10–12 July 2012. Apparently, there was a passing storm in the Cooperation Sea on 11 July 2012 (Fig. 6d), which is the successive of the eastward moving storm on 10 July 2012 (Fig. 6c). In the western portion of the storm (around 70°E), the winds are generally offshore, resulting in a zone of sea ice divergence. In contrast, in the eastern portion of the storm, the winds are generally onshore, resulting in sea ice convergence. Therefore, the polynyas developed under the western portion of the storm. The further eastward movement of the storm and the resulted sea ice divergence in the western portion of the storm led to the eastward expansion of the polynya in the Cooperation Sea on 12 July 2012 (Fig. 6e).

Before the formation of the polynyas in the Cooperation Sea, the erosion of the thermocline was weak on 10 July 2012 (Fig. 6j). Since the heat entrainment rate is small during the initial phase of the polynyas, it is also reasonable to infer that it is mainly the wind-induced sea ice divergence that initiated the polynyas.

Once the polynyas occurred, the ocean surface stress could increase substantially, owing to the removal of the sea ice insulation effect, which could enhance the upwelling across the mixed layer base. Despite the enormous heat loss to the atmosphere, the enhanced heat entrainment could maintain the surface water to be above the freezing point (>−1.8°C, see Fig. 6j). The seal CTD data observed the entire duration of the polynyas from 10 to 21 July 2012 (the locations of these seal CTD profiles on 10, 12, and 21 July are shown in Figs. 6f and 6g and the colors indicate the date of the observations). From 10 to 21 July, with the deepening of the mixed layer (from 66 to 100 m), the average heat entrainment rate is estimated as 262.3 W m−2. The average sea–air net heat flux from ERA5 over 10 to 21 July 2012 is 78.1 W m−2, so the temperature in the mixed layer could stay above freezing point and hence the polynyas could sustain for a certain time. The heat entrainment rate is calculated by ρCpΔTwe, where ΔT is the temperature jump across the mixed layer base, we is the average entrainment rate given by he/t (if the vertical advection is ignored), he is the mixed layer depth change rate due to the deepening over the period t (t = 10–21 July), ρ is the density of the seawater (1027.35 kg m−3), and Cp is the heat capacity of the seawater (3890 J °C−1 kg−1).

Comparing the potential density and salinity profiles on 12 July 2012 (Figs. 6h,i, green lines) with 21 July 2012 (Figs. 6h,i, blue lines), we can see the deepening of the mixed layer and the associated entrainment of warm and salty deeper water. By entraining salty water into the mixed layer, the mixed layer could become further dense and hence be further deepened, forming a positive feedback associated with the convection. After 21 July, presumably due to sea ice transport rather than local sea ice formation (as the temperature in the mixed layer is above freezing point; see Fig. 6j), the polynyas closed; consequently, the mixed layer returned to the shallow case at some locations, by the weakened wind stress effect and the sea ice melting. Therefore, the polynyas were engendered by both the dynamic effect, i.e., the wind-induced sea ice divergence, particularly during the initial phase, and the thermodynamic effect, i.e., the entrainment of heat from deeper ocean.

Associated with the CS polynyas, the convective depth was found to be around 100 m in our study, while the deep-reaching, but short lived, convective event occurred in the Maud Rise polynya in 2017 (see Fig. 4d in Campbell et al. 2019). This suggests that the CS polynya has a limited effect on the exchanges between the deep ocean and the atmosphere, compared to the Maud Rise polynya and the Weddell Polynya.

b. The role of eddies

Some polynyas formed in the region featured by canyons and drifts. These polynyas usually occurred near Wild Canyon (∼67.5°E) or Wilkins Canyon (∼68.5°E) in July and sometimes extended eastward to the Prydz Channel Fan (64°S, 73°E) in September and October (Fig. 8a). Figure 8a shows the distribution of canyons at ∼64°S, rising from the abyssal plain at ∼4000 m to the shallowest depth of ∼2500 m. The head of Wilkins Canyon (Vanney and Johnson 1985) is situated just west of Prydz Channel Fan and north of Fram Bank. Wilkins Canyon runs north from the shelf edge before turning northeast at ∼65.8°S (Shipboard Scientific Party 2001). Wild Canyon, which lies further west of the Prydz Channel Fan, is a steep canyon with its head on the continental slope and extends northwest to the open ocean.

Fig. 8.
Fig. 8.

(a) The ice-free polynyas from July to October in 2012 at intervals of six days. Colored lines: SIC contour lines of 0.15 excluding those that connected to the coast and the open ocean; black box: canyon and drift region. The numbers on the left of color bar indicate the number of days from 3 Jul, and the dates are shown on the right of the color bar. (b) The daily changes in the total area of the polynyas (blue) and average wind stress curl (red) for July–October 2012 over the black box shown in Fig. 8a. (c)–(f) Wind stress curl (color shading; N m−3) overlaid on SIC on 8–11 Oct 2012. Green lines: SIC contour lines of 0.15; black dots: polynyas correspond to the SIC threshold value of 0.7; the gray lines from south to north are the 2800-, 3000-, and 3500-m isobaths, to depict the submarine topography.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

Associated with the rugged terrain, some polynyas were often observed along the 3500 m isobath in the region marked by the box in Fig. 8a. In 2012, in this region, long-lasting polynyas existed for more than 120 days since 3 July 2012, despite their highly variable sizes. For the sake of clarity, the positions and their relative sizes were illustrated by the SIC contour lines of 0.15 for every 6 days from 3 July to 31 October 2012 (Fig. 8a). Due to missing data between 4 October 2011 and 3 July 2012, the first polynya in this region could only be detected around (64.5°S, 69°E) near the mouth of Wilkins Canyon on 3 July 2012. The total area of the polynyas inside the box on this particular day was 18.3 × 103 km2, with the ice-free area (SIC < 0.15) being 1.6 × 103 km2. At first, during the first half of July, ice-free polynyas occurred mainly on the eastern side of Wilkins Canyon. Subsequently, ice-free polynyas moved westward and occurred on the eastern side of Wild Canyon. During the second half of August, they reached their northmost position and then moved eastward until the first half of October. At this time, they settled near the west of the Prydz Channel Fan around (64°S, 73°E) and then moved slightly westward until the end of October.

Figure 8b shows the time series of the total polynya area and the area-averaged wind stress curl over the box in Fig. 8a for the period from 3 July 2012 to 31 October 2012. Several relatively large polynya events occurred during this period, that is, around 10 July, 16 August, 29 September, and 24 October 2012. On 10 July 2012, the total area of polynyas in this region reached its maximum of 43.9 × 103 km2, with an ice-free area of 3.8 × 103 km2. Apparently, these large polynya events immediately followed the strong cyclonic wind forcing, particularly for the polynya events around 16 August and 29 September 2012. Although these results confirm the important role of wind forcing in the formation of the polynyas, the formation or expansion of some polynyas might not be necessarily associated with cyclonic wind forcing; for example, the polynya event around 10–14 October 2012 was not storm related.

The wind stress curl appeared to be positive around the polynya between 8 and 11 October 2012 (Figs. 8c–f), and the regional mean wind stress curl was close to zero on these days (Fig. 8b). On 8 October 2012, low SIC was maintained west of the Prydz Channel Fan (Fig. 8c). One day later, the SIC decreased and the ice-free area appeared (Fig. 8d). Subsequently, even with a positive wind stress curl, the area of the ice-free polynya expanded slightly on 10–11 October 2012 (Figs. 8e,f). This result suggests that in this region, the polynya could expand solely under the influence of oceanic processes.

To identify the oceanic process potentially responsible for the polynyas in this region, we examined the simulated ocean circulation by SOSE. The monthly mean climatological horizontal current velocity (Figs. 9a,b, black arrows) at a depth of 3325 m during 2005–10 indicates the possible presence of the cyclonic circulation, for example, around (63.2°S, 69°E), (63.2°S, 71°E) and (64.7°S, 73.5°E). The 40-day contour lines of the total polynya days in July (Fig. 9a, cyan lines) and September (Fig. 9b, cyan lines) between 2002 and 2019 are consistent with the locations of the cyclonic circulation. Figures 9a and 9b show that the cyclonic circulation was present at these locations for most of the period 2005–10.

Fig. 9.
Fig. 9.

Monthly mean currents (black arrows) at a depth of 3325 m over 2005–10 from SOSE in (a) July and (b) September. Cyan lines: 40-day contour lines of the total polynya days in (a) July and (b) September between 2002 and 2019; green line: the ice-free (SIC < 0.15) polynya occurred on 10 Oct 2012 in Fig. 8e. The black box is the same as in Fig. 8a.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

The dynamics of upwelling over canyons and drifts north of the shelf break have been examined in previous studies through observations, numerical models, and laboratory models (Hickey 1997; Klinck 1996; Allen et al. 2003). According to Allen et al. (2001), near-surface isopycnals may be elevated when flow passes over a canyon. As the flow crosses the canyons, the water column first stretches and generates cyclonic vorticity. The flow then becomes compressed as it flows over the other side of the canyon, thereby generating anticyclonic vorticity. In addition, as dense shelf water cascades through Wild Canyon from the upstream Burton basin, the fluid column stretches and creates cyclonic eddies (Nakayama et al. 2014). Around (65.5°S, 68°E), near Wild Canyon, cyclonic eddies were observed by Japanese and Australian cruises (Wakatsuchi et al. 1994). Around (65°S, 75°E), near the Prydz Bay channel fan, cyclonic eddies were observed by an XCTD (Williams et al. 2010). Another cyclonic eddy centered at around (65°S, 80°E) was observed by Smith et al. (1984). A cyclonic eddy is often formed or strengthened during the stretching of the water column, and the northward movement of these eddies from the slope region to the deep ocean favors its development.

At a further offshore location, near the locations of the sACCf, the SOSE monthly mean climatological horizontal current velocity (Figs. 9a,b, black arrows) at a depth of 3325 m from 2005 to 2010 indicates the existence of cyclonic circulation around (63.2°S, 66.7°E) and (63°S, 73°E).

The location of the SB (Fig. 4) suggests that warm deep water could reach latitudes where the oceanic cyclonic circulation also existed (Fig. 9). The cyclonic circulation could help set the favorable oceanic condition for moving warm and saline water to the mixed layer, which contributed to making low SIC and thin sea ice.

6. Conclusions

Using high-resolution sea ice concentration data from AMSR-E and AMSR2 (see Table 1 and appendix) for the period 2002–19, we documented the spatial and temporal patterns of the open-ocean polynyas in the Cooperation Sea (CS). Most of the polynyas were formed in a zonally elongated band (62°–65°S, 68°–80°E) north of the 3500-m isobaths, with an average central position around (64°S, 75°E). Some polynyas occurred in the region closer to the continental slope characterized by canyons and drifts. The polynyas in the CS were poorly connected and had relatively small ice-free areas compared to those in other regions, such as the Weddell Sea polynya and the Maud Rise polynya. The polynyas generally began to emerge in April, expanded to relatively large sizes during July–October, and reached their maximum sizes in August and October. The sizes of polynyas in the CS were often larger than those of the Maud Rise polynya in 2017 during the most winter months of 2002–19. The annual maximum total area of the polynyas ranged from 115.3 × 103 km2 in 2013 to 312.4 × 103 km2 in 2010, with an average value of 188.9 × 103 km2.

We use Fig. 10 to illustrate the atmospheric and oceanic forcing mechanisms responsible for the formation of the open-ocean polynyas in the CS. Since the ACC travels closer to the continental shelf in the CS than in many other sectors around the Antarctic, the UCDW migrates poleward and closer to the continental slope, as indicated by the locations of the sACCf and the SB. In this way, the AD is situated over the UCDW. The cyclonic wind forcing associated with the AD causes sea ice divergence and promotes the upwelling of warm subsurface water, which is usually shallower in the CS than in other sectors. Such oceanic and atmospheric conditions favor the formation of the open-ocean polynyas in the CS.

Fig. 10.
Fig. 10.

A schematic diagram for the mechanisms of the formation of the open-ocean polynyas in the Cooperation Sea. Red solid lines: SB from Orsi et al. (1995); red dotted line: SB derived from EN4 data; shading color: mean state of 200-m potential temperature in September along with the cross section of 75°E (contours) down to 300-m depth; black line with arrows: sACCf from Orsi et al. (1995); green line: sea ice edge in September over 2002–19; white line: AD in September over 2002–19; shaded area: total polynya days of more than 40 days in September between 2002 and 2019 identified by the SIC threshold value of 0.7; blue arrows: Antarctic Slope Current; purple arrows: an overflow cascades through the Wild Canyon from the upstream Burton basin. Clockwise circular arrows indicate the present of cyclonic eddies.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

It is worth emphasizing the important role of the other two oceanic features in the formation of the open-ocean polynyas in the CS. First, the sACCf is close to the ASC in the CS, which creates a gyre circulation (Heywood et al. 1999). Second, at smaller scales, the CS is also characterized by eddy and cyclonic circulations. The eddy or cyclonic circulation usually forms directly north of the canyons and drifts in the slope region. As the ASC moves through the canyon, the stretching of the water column leads to the formation of eddies; moreover, periodic eddies can be formed when dense shelf water cascades through canyons (Nakayama et al. 2014). Observational and numerical results also indicate the presence of further offshore eddies in the CS.

This study offers a more complete circumpolar view of open-ocean polynyas in the SO. During the 2002–19 period, the open-ocean polynyas in the CS were much more persistent than the Maud Rise polynya and the polynya in the Cosmonaut Sea, primarily due to the unique and more favorable atmospheric and oceanic conditions in the CS. In the Weddell and Ross Seas, the SB lies far north of the AD, in contrast to the situation in the CS. This is probably the reason for very few, if any, polynya events in these two regions, despite the local topographic effects of the Maud Rise and the variability in stratification modulated by large-scale atmospheric and oceanic circulations. In the other regions, such as the Bellingshausen–Amundsen Seas and the sector between 90° and 150°E, the SB lies close to the AD. For the Bellingshausen–Amundsen Seas, where no open-ocean polynyas occurred, the warm deep water is much less shallow, presumably due to the absence of ASC and hence weaker eddy activity than in the CS; in contrast, in the sector between 90° and 150°E, eddy activity is stronger, which restrains the sea ice edge close to the AD and thus eliminates the possible occurrence of open-ocean polynyas.

This study further highlights the important role of oceanic conditions in the formation of open-ocean polynyas. It is, however, a formidable challenge to observe changes in ocean circulation and quantify their relative contribution to driving the changes in the open-ocean polynyas to the changes in atmospheric circulation in the CS, including changes in ACC, ASC, bottom water formation, and associated cyclonic circulation. Understanding the key drivers of changes in polynyas is essential to faithfully model and predict the physical, biogeochemical, and biological processes in the Southern Ocean.

Acknowledgments.

This work was supported by the National Natural Science Foundation of China (NSFC) Projects (41941007; 41876220) and the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (311021008). We thank Dr Fabien Roquet and the anonymous reviewer for their comments that helped improve the original manuscript.

Data availability statement.

AMSRE and AMSR-2 SIC data were obtained through the Institute of Environmental Physics, University of Bremen (https://seaice.uni-bremen.de/data/amsr2/asi_daygrid_swath/n6250/). SMOS thin sea ice thickness data were obtained through the Institute of Environmental Physics, University of Bremen (https://seaice.uni-bremen.de/data/smos/). NSIDC SIC data were obtained from NSIDC (https://nsidc.org/data/seaice_index/archives). The sea ice motion data are from the reconstructed sea ice velocity (daily 25 km EASE-Grid Sea Ice Motion data set; https://nsidc.org/data/nsidc-0116). ERA5 data were obtained from the European Centre for Medium-Range Weather Forecasts (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels). CryoSat-2 SSH data were obtained from the European Space Agency (https://earth.esa.int/web/guest/data‐access/) and processed at CPOM (UCL). EN4 data were obtained from the Met Office Hadley Centre for Climate Change (https://www.metoffice.gov.uk/hadobs/en4/download-en4-2-1.html). SOSE data were obtained from the Scripps Institution of Oceanography (http://sose.ucsd.edu/sose_stateestimation_data_05to10.html). MEOP Seal-CTD data were collected by the Integrated Marine Observing System (IMOS) (https://www.meop.net/database/meop-databases/density-of-data.html).

APPENDIX

Comparisons between AMSR2 and NSIDC Sea Ice Concentration Datasets

AMSR-E and AMSR2 SIC data have a higher resolution (∼3.125 km × 3.125 km) than NSIDC SIC data (∼25 km × 25 km), which is dependent on the channel frequency and instrument series. The AMSR-E and AMSR2 SIC data were retrieved using the ARTIST Sea Ice (ASI) algorithm (Spreen et al. 2008). The SIC data (Sea Ice Index, version 3) from the National Snow and Ice Data Center (NSIDC) emerged based on the NASA Team algorithm (Cavalieri et al. 1997). Different algorithms can also explain the large differences between the two datasets, especially for marginal ice zones (Alekseeva et al. 2019).

As a matter of fact, there are remarkable differences between these two datasets, particularly near the sea ice edges, as illustrated by the SIC on 1 December 2017 (Figs. A1a,b), 24 November 2017 (Figs. A1c,d), and 25 August 2010 (Figs. A1e,f). Using SIC data from NSIDC, the Maud Rise open-ocean polynya was found to have expanded to its maximum extent on 1 December 2017 (Fig. A1b), with an estimated polynya area (SIC < 0.15) of ∼298.1 × 103 km2 (Jena et al. 2019). In contrast, the estimated polynya area was ∼337.2 × 103 km2 using SIC data from AMSR2 (Fig. A1a). Using the SIC threshold of 0.7 and the AMSR2 dataset, the Maud Rise polynya expanded to its maximum extent on 24 November 2017 (Fig. A1c) with an estimated polynya area of ∼328.0 × 103 km2; however, on 24 November 2017, the estimated polynya area was ∼246.7 × 103 km2 using NSIDC data.

Fig. A1.
Fig. A1.

Antarctic SIC on 1 Dec 2017 from (a) AMSR2 and (b) NSIDC, on 24 Nov 2017 from (c) AMSR2 and (d) NSIDC, and on 25 Aug 2010 from (e) AMSR-E and (f) NSIDC.

Citation: Journal of Physical Oceanography 52, 7; 10.1175/JPO-D-21-0197.1

On 25 August 2010, the SIC patterns in the CS were very different between these two datasets (Figs. A1e,f). Near the sea ice edge, the NSIDC SIC was below 0.6; in contrast, the AMSR-2 SIC was greater than 0.7. Therefore, using the threshold of 0.7, the identified polynyas using the AMSR-2 dataset cannot be detected by using the NSIDC dataset on 25 August 2010.

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