An overview of Antarctic polynyas: sea ice production, forcing mechanisms, temporal variability and water mass formation

Polynyas are irregular open water bodies within the sea ice cover in polar regions under freezing weather conditions.In this study,we reviewed the progress of research work on dynamical forcing,sea ice production(SIP),and water mass formation for both coastal polynyas and open-ocean polynyas in the Southern Ocean,as well as the variability and controlling mechanisms of polynya processes on different time scales.Polynyas play an irreplaceable role in the regulation of global ocean circulation and biological processes in regional ocean ecosystems.The coastal polynyas(latent heat polynyas)are mainly located in the Weddell Sea,the Ross Sea and on the west side of protruding topographic features in East Antarctica.During the formation of coastal polynyas,which are mainly forced by offshore winds or ocean currents,brine rejection triggered by high SIP results in the formation of high salinity shelf water,which is the predecessor of the Antarctic bottom water-the lower limb of the global thermohaline circulation.The open-ocean polynyas(sensible heat polynyas)are mainly found in the Indian sector of the Southern Ocean,which are formed by ocean convection processes generated by topography and negative wind stress curl.The convection processes bring nutrients into the upper ocean,which supports biological production and makes the polynya regions an important sink for atmospheric carbon dioxide.The limitations and challenges in polynya research are also discussed.

Formation and Transportation of High-Salinity Water Produced in Polynyas South of the St.Lawrence Island

The authors studied variations of temperature and salinity in seawater under sea ice using hydrologic data collected from polynyas south of the St.Lawrence Island during March of 2008 and 2009.The results indicate that the high-salinity water found during the cruises of 2008 and 2009 was due to the formation of polynyas.The salinity observed in 2008 was higher than that in 2009 as a result of higher salt production in 2008.The spatial distributions of high-salinity cores differed between the two cruises.In March 2008, a southeastward flow was formed under the persistent northerly wind in the observation region, which transported the high-salinity water produced by the polynyas to the southeast.The similar flow, however, did not exist in March 2009 because the northerly wind over the study area was interrupted by a southerly wind.Accordingly, the polynyas and the high-salinity water pro-duced by them existed for a short time.As a result, the high-salinity water in 2009 did not spread very far, and stayed within the polynyas.In addition, during the 2009 cruise, two stages of observations in the polynyas showed the core of high-salinity water was shifted to the southwest of the St.Lawrence Island.This result suggested that a southwestward flow might have existed in the area at the onset of the northerly wind, which was consistent with the alongshore and/or offshore flows caused by the northerly wind.

Distribution of modified Circumpolar Deep Water and its threat in Vincennes Bay,East Antarctica

The Antarctic Bottom Water formation site Vincennes Bay,East Antarctica is experiencing a substantial intrusion of modified Circumpolar Deep Water(mCDW),which may inhibit the formation of Dense Shelf Water(DSW)and drive basal melting of the ice shelves.Based on hydrographic data obtained from March to November in 2012,we evaluated the spatial spread of mCDW over the continental shelf region of Vincennes Bay and the associated temporal evolution of water properties,as well as the sea ice formation effect on water column in the coastal polynya.Results show that two branches of mCDW occupied the deep layers of the continental shelf,distinguished by the potential density(smaller than 27.8 kg/m^(3) or not)when potential temperatureθ=0.5°C in theθ-salinity space.The warmer and less dense branch observed on the east plateau,accessed the eastern ice shelves in the coastal polynya to drive basal melting of ice shelves.In contrast,the other colder and denser branch in the mid-depression reached the western Underwood Ice Shelf.DSW formation was detectable in the coastal polynya during September-November,proving the occurrence of deep convection.Surface heat loss and brine rejection during the intensive sea ice formation contributed to the destratification of the water column in the coastal polynya.It was estimated that at least 1.11±0.79 TW heat carried by mCDW into the inner part of the polynya.

The connection of phytoplankton biomass in the Marguerite Bay polynya of the western Antarctic Peninsula to the Southern Annular Mode

Antarctic coastal polynyas are biological hotspots in the Southern Ocean that support the abundance of hightrophic-level predators and are important for carbon cycling in the high-latitude oceans.In this study,we examined the interannual variation of summertime phytoplankton biomass in the Marguerite Bay polynya(MBP)in the western Antarctic Peninsula area,and linked such variability to the Southern Annular Mode(SAM)that dominated the southern hemisphere extratropical climate variability.Combining satellite data,atmosphere reanalysis products and numerical simulations,we found that the interannual variation of summer chlorophyll-a(Chl-a)concentration in the MBP is significantly and negatively correlated with the spring SAM index,and weakly correlated with the summer SAM index.The negative relation between summer Chl-a and spring SAM is due to weaker spring vertical mixing under a more positive SAM condition,which would inhibit the supply of iron from deep layers into the surface euphotic layer.The negative relation between spring mixing and spring SAM results from greater precipitation rate over the MBP region in positive SAM phase,which leads to lower salinity in the ocean surface layer.The coupled physical-biological mechanisms between SAM and phytoplankton biomass revealed in this study is important for us to predict the future variations of phytoplankton biomasses in Antarctic polynyas under climate change.

Characterization of the unprecedented polynya events north of Greenland in 2017/2018 using remote sensing and reanalysis data

Based on an ice concentration threshold of 90%,it has been identified that two polynya events occurred in the region north of Greenland during the 2017/2018 ice season.The winter event lasted from February 20 to March 3,2018 and the summer event persisted from August 2 to September 5,2018.The minimum ice concentration derived from Advanced Microwave Scanning Radiometer 2(AMSR2)observations was 72%and 65%during the winter and summer events,respectively.The occurrence of both events can be related to strengthened southerly winds associated with an increased east-west zonal surface level air pressure gradient across the north Greenland due to perturbation of mid-troposphere polar vortex.The relatively warm air temperature during the 2017/2018 freezing season in comparison with previous years,together with the occurrence of the winter polynya,formed favourable pre-conditions for ice field fracturing in summer,which promoted the formation of the summer polynya.Diminished southerly winds and increased cover of new ice over the open water were the dominant factors for the disappearance of the winter polynya,whereas increased ice inflow from the north was the primary factor behind the closure of the summer polynya.Sentinel-1 Synthetic Aperture Radar(SAR)images were found better suited than AMSR2 observations for quantification of a new ice product during the polynya event because the SAR images have high potential for mapping of different sea ice regimes with finely spatial resolution.The unprecedented polynya events north of Greenland in 2017/2018 are important from the perspective of Arctic sea ice loss because they occurred in a region that could potentially be the last“Arctic sea ice refuge”in future summers.

Specific Relationship between the Surface Air Temperature and the Area of the Terra Nova Bay Polynya,Antarctica

Antarctic polynyas play an important role in regional atmosphere?ice?ocean interactions and are considered to help generate the global deep ocean conveyer belt.Polynyas therefore have a potential impact on the Earth’s climate in terms of the production of sea ice and high-salinity shelf water.In this study,we investigated the relationship between the area of the Terra Nova Bay polynya and the air temperature as well as the eastward and northward wind based on the ERA5 and ERAInterim reanalysis datasets and observations from automatic weather stations during the polar night.We examined the correlation between each factor and the polynya area under different temperature conditions.Previous studies have focused more on the effect of winds on the polynya,but the relationship between air temperature and the polynya area has not been fully investigated.Our study shows,eliminating the influence of winds,lower air temperature has a stronger positive correlation with the polynya area.The results show that the relationship between the polynya area and air temperature is more likely to be interactively influenced.As temperature drops,the relationship of the polynya area with air temperature becomes closer with increasing correlation coefficients.In the low temperature conditions,the correlation coefficients of the polynya area with air temperature are above 0.5,larger than that with the wind speed.

Sources of particulate organic matter in the Chukchi and Siberian shelves: clues from carbon and nitrogen isotopes

The stable isotopic composition(δ13C andδ15N)and carbon/nitrogen ratio(C/N)of particulate organic matter(POM)in the Chukchi and East Siberian shelves from July to September,2016 were measured to evaluate the spatial variability and origin of POM.Theδ13CPOC values were in the range of−29.5‰to−17.5‰with an average of−25.9‰±2.0‰,and theδ15NPN values ranged from 3.9‰to 13.1‰with an average of 8.0‰±1.6‰.The C/N ratios in the East Siberian shelf were generally higher than those in the Chukchi shelf,while theδ13C andδ15N values were just the opposite.Abnormally low C/N ratios(<4),lowδ13CPOC(almost−28‰)and highδ15NPN(>10‰)values were observed in the Wrangel Island polynya,which was attributed to the early bloom of small phytoplankton.The contributions of terrestrial POM,bloom-produced POM and non-bloom marine POM were estimated using a three end-member mixing model.The spatial distribution of terrestrial POM showed a high fraction in the East Siberian shelf and decreased eastward,indicating the influence of Russian rivers.The distribution of non-bloom marine POM showed a high fraction in the Chukchi shelf with the highest fraction occurring in the Bering Strait and decreased westward,suggesting the stimulation of biological production by the Pacific inflow in the Chukchi shelf.The fractions of bloom-produced POM were highest in the winter polynya and gradually decreased toward the periphery.A negative relationship between the bloom-produced POM and the sea ice meltwater inventory was observed,indicating that the net sea ice loss promotes early bloom in the polynya.Given the high fraction of bloom-produced POM,the early bloom of phytoplankton in the polynyas may play an important role on marine production and POM export in the Arctic shelves.

Variability of size-fractionated phytoplankton standing stock in the Amundsen Sea during summer

The size-fractionated composition of phytoplankton greatly influences the transfer efficiency of biomass in pelagic food chains and the biological carbon flux from surface waters to the deep sea.To better understand phytoplankton abundance and composition in polynya,ice zone,and open ocean regions of the Amundsen Sea Sector of the Southern Ocean(110°W-150°W),its size-fractionated distribution and vertical structure are reported for January to February 2020.Vertical integrated(0-200 m)chlorophyll(Chl)a concentrations within Amundsen polynya regions are significantly higher than those within ice zone(t test,p<0.01)and open ocean(t test,p<0.01)regions,averaging 372.3±189.0,146.2±152.1,and 49.0±20.8 mg·m^(−2),respectively.High Chl is associated with shallow mixed-layer depths and near-shelf regions,especially at the southern ends of 112°W and 145°W.Netplankton(>20μm)contribute 60%of the total Chl in Amundsen polynya and sea ice areas,and form subsurface chlorophyll maxima(SCM)above the pycnocline in the upper water column,probably because of diatom blooms.Net-,nano-,and picoplankton comprise 39%,32%,and 29%of total Chl in open ocean stations,respectively.The open-ocean SCM migrates deeper and is below the pycnocline.The Amundsen Sea SCM is moderately,positively correlated with the euphotic zone depth and moderately,negatively correlated with column-integrated net-and nanoplankton Chl.

Thinner Sea Ice Contribution to the Remarkable Polynya Formation North of Greenland in August 2018

In August 2018,a remarkable polynya was observed off the north coast of Greenland,a perennial ice zone where thick sea ice cover persists.In order to investigate the formation process of this polynya,satellite observations,a coupled iceocean model,ocean profiling data,and atmosphere reanalysis data were applied.We found that the thinnest sea ice cover in August since 1978(mean value of 1.1 m,compared to the average value of 2.8 m during 1978-2017) and the modest southerly wind caused by a positive North Atlantic Oscillation(mean value of 0.82,compared to the climatological value of-0.02) were responsible for the formation and maintenance of this polynya.The opening mechanism of this polynya differs from the one formed in February 2018 in the same area caused by persistent anomalously high wind.Sea ice drift patterns have become more responsive to the atmospheric forcing due to thinning of sea ice cover in this region.

Horizontal distribution of tintinnids(Ciliophora)in surface waters of the Ross Sea and polynya in the Amundsen Sea(Antarctica)during summer 2019/2020

Information on tintinnid horizontal distribution in the Antarctic Continental Zone is scarce.During the summer of 2019/2020,tintinnid diversity and horizontal distribution in surface waters were investigated in the Ross Sea and Amundsen Sea polynya.Eight tintinnid species were found and the dominant species showed obvious horizontal distribution characteristics.In the Ross Sea,three tintinnid community groups were identified.Cymatocylis cristallina and Laackmanniella prolongata(group I)were dominant species and were mainly distributed in stations closer to the coast than were species in the other two groups.Codonellopsis gaussi(group II)and Cy.convallaria(group III)were mainly distributed in nearshore and offshore stations,respectively.In the Amundsen Sea polynya,the dominant species Cy.cristallina,L.prolongata and Salpingella faurei(group I)were mainly distributed in stations closer to the coast than were species in the other two groups.Cy.convallaria(group III)was mainly distributed in offshore stations.The distribution area where C.gaussi and C.cristallina were found in high abundance and abundance proportion of loricae with protoplasts was divided by the approximate boundary of the Antarctic Slope Front Current and Coastal Current in the Ross Sea.The highest abundance proportion in the Ross Sea was the 32-36μm lorica oral diameter(LOD)size class(75.7%),and the 36-40μm LOD size class(56.0%)was found in the Amundsen Sea polynya.Temperature-salinity-plankton diagrams of the two seas revealed that temperature may be the main reason for species distribution.Our results contribute to a better understanding of horizontal distribution of the microbial food web,and serve as a baseline for future studies of pelagic community change in the Antarctic Continental Zone.

Characteristics of particle fluxes in the Prydz Bay polynya, Eastern Antarctica

The settling of particulate carbon in seawater is a key component of the ocean carbon cycle. We deployed a set of sediment trap in the polynya of Prydz Bay from December 2010 to December 2011 to investigate the seasonal variations in particle fluxes. There was a clear seasonal variation in the particle fluxes, with maximum and minimum fluxes recorded during the summer and winter, respectively. The average total flux over the sampling period was 193.58 mg m^(-2)d^(-1), and the average fluxes of organic carbon(C_(org)), inorganic carbon(C_(inorg)), and biogenic silica(Si_(bio)) were 721.78, 28.67, and 2382.80 μmol m^(-2) d^(-1), respectively. Si_(bio)was the main contributor to the total mass flux, and strongly correlated with C_(org). The high Si_(bio)/C_(org)molar ratios(>1) suggest that C_(org)was transported to deep sea in association with Si_(bio). By comparing remote sensing data of sea ice and chlorophyll in the upper water column, we found that the dynamics of carbon fluxes were closely related to changes in sea ice. Algae in sea ice may have a key role in biological pump processes in early summer. Apart from the ice algae bloom period, variations in carbon fluxes generally corresponded with phytoplankton blooms in the upper water. The ballast effect controlled the particle settling velocity and the efficiency of the biological pump. Sea ice rafts initiated the first particle export event and enhanced the particle settling efficiency during melting period. As diatoms might become less dominant in the ice-free area, sea ice loss may cause the efficiency of the biological pump efficiency to decrease over the long term.

Impacts of atmospheric and oceanic factors on monthly and interannual variations of polynya in the East Siberian Sea and Chukchi Sea

As a key region of Northeast Passage,the polynya along the Siberian coast in the East Siberian and Chukchi Seas is important to local dynamic and thermodynamic processes,sea ice production and marine ecosystem.The detailed variations of polynya and the contributions of atmospheric and oceanic factors to the polynya have not been explored quantitatively.AMSR-E satellite data from January to April during the period 2003-2011 were used to study the impacts of wind stress and ocean heat transport on variations of polynya in the East Siberian Sea and Chukchi Sea.The study region was divided into six domains.Four sets of AMSR-E data with resolutions of 6.25 km and 12.5 km were compared based on two algorithms of sea ice concentration(referred to as 6.25 km-IC and 12.5 km-IC)and sea ice thickness(referred to as 6.25 km-h and 12.5 km-h).The monthly and yearly polynya areas in the four cases and six domains had remarkable differences.The two cases of 6.25 km-h and 12.5 km-h had larger areas of polynya than the other two cases of 6.25 km-IC and 12.5 km-IC.The difference in polynya area between the 6.25 km-h and 12.5 km-h cases was much smaller than the difference between the 6.25 km-IC and 12.5 km-IC cases.The study of atmospheric and oceanic mechanisms on polynya is influenced significantly by the sensitivity of polynya areas.In general,the impact of wind stress and ocean heat transport on the polynyas had noticeable monthly and interannual variations and was dependent on the locations of the polynyas.The alongshore and offshore wind had stronger correlations with the polynya area than ocean heat transport.Although the higher resolution(6.25 km)AMSR-E data are best for the study of atmospheric and oceanic impacts on polynya area,the coarse resolution(12.5 km)AMSR-E data based on sea ice thickness can also be used.Wind direction dominated the polynya area in the East Siberian Sea and wind speed dominated the polynya area in the Chukchi Sea.The variation in ocean heat transport was influenced mainly by variation in volume transport rather than variation in water temperature.