Evolution Of Water Structure And Characteristics Of The Coastal Polynya During The Freezing Season On The Continental Shelf Of Vincennes Bay,Antarctica

Vincennes Bay is an important source region of Antarctic Bottom Water（AABW）due to its active coastal polynya and high sea ice production.Relatively warm and saline modified Circumpolar Deep Water（mCDW）can cross the continental slope to reach the ice shelves front of Vincennes Bay,which results in basal melt of the ice shelves and influences the evolution of water structure in the continental shelf,especially the formation process of Dense Shelf Water（DSW）,which is the predecessor of AABW.In addition,the heat carried by mCDW can offset the heat loss of the coastal polynya,prevent the vertical mixing of water column,and inhibit the deep convection during winter time,but low salt DSW can still be produced.Although current research has some understanding of the evolution of water structure in the Vincennes Bay shelf,the spatial distribution of mCDW and the processes of sea ice formation and atmospheric forcing in the coastal polynya are still unclear.Using the profile data obtained by elephant seal CTD from March to November 2012,we comprehensively analyzed the evolution process of water structure under the influence of mCDW intrusion during the freezing season.Then we analyzed the spatial distribution of mCDW in different neutral density layers,quantitatively evaluated the heat content and salt content carried by mCDW,and proposed the possible pathway of mCDW from the mouth of Vincennes Bay to the innermost parts of the bay.Finally,we discussed the effects of sea ice formation and atmospheric forcing on water structure in coastal polynya by using sea ice concentration,sea ice thickness data and ERA5 reanalysis data set.Results showed mCDW intrusion was significant in the deep water column from the mouth to the innermost parts of the bay,and affected other water masses in the shelf.Due to the cooling and freshening effect of shelf water,the core temperature and salinity of mCDW are higher temperature and salinity in zones A（the bay mouth area with a bottom depth less than 1000 m）and B（the middle region of this bay）,while lower in zones C（the front of the western Ice Shelf）and D（the region in the coastal polynya）.The intrusion degree of the continental shelf was obvious in autumn（March-May）,but weak in winter and spring（June-November）.Heated by warm mCDW below,Antarctic Surface Water（AASW）in the upper layer of the water column had a higher temperature in the initial freezing season.With the deepening of the freezing season,the cooling and brine rejection from the sea surface extended to the interior of the seawater from the upper layer,leading to the gradual weakening of the stratification.However,the stratification remained unbroken until early June at 400 dbar in the innermost parts of the bay,with almost no change in heat content.Zones C and D had different water structure characteristics,and zone D had stronger mCDW intrusion than zone C.From April to late May,low salt Ice Shelf Water（ISW）(34.5-34.6 g kg^-1)persisted only in the depth range of 300-600 dbar in zone C,and a high temperature coreΘ>-1.7℃appeared both above and below the ISW.The temperature of water column in zone D is more than 0.2℃higher than that in zone C.From March to early April,higher salt ISW(34.55-34.65 g kg^-1)existed at 400 dbar.Therefore,the basal melt of the ice shelves in zone D may occur in a deeper range,while the basal melt of the ice shelves in region C may be shallower.According to the spatial distribution of mCDW in three neutral density layers of28.0-28.1 kg m^-3,28.1-28.2 kg m^-3 and 28.2-28.27 kg m^-3,the salinity and temperature of mCDW decrease from northeast to southwest.The depth increases gradually from the mouth and east of the bay（400-500 dbar）to the mid-depression and innermost（500-800dbar）.Combined with theΘ-S_A characteristics in zone B and the frontal transects of the east and west ice shelves,there were two mCDW branches in the middle of the bay:the east branch was located on the eastern plateau,which was relatively warmer and lower dense;the west branch was located in the mid-depression,which was relatively cold and higher dense.The east branch reached the coastal polynya,pushing the basal melt of the eastern three ice shelves.The west branch reached the front of the western Underwood Ice Shelf.In early April,mCDW transported at least 1.74×10¹⁹ J heat content and1.62×10¹⁴ kg salt content into the shelf.During the late freezing season（September to November）,the water characteristics of coastal polynya（zone D）tended to be uniform vertically,and DSW occupied the bottom of the water column,which directly reflected the existence of deep convection in winter.At the same time,the high heat loss(3.92×10¹⁹ J)and salinity input（3.05×10⁹ kg）in the coastal polynya resulted in the loss of buoyancy of water column,which contributed significantly to the instability of water column stratification.By evaluating the effect of Ekman pumping on the upper ocean,it can be seen that the negative wind stress curl(-2.6×10^-6--1.5×10^-6N m^-2)lead to the divergent motion of sea ice and promoted the formation of sea ice;positive Ekman vertical velocity caused upwelling in the upper ocean with an order of magnitude of O(10^-5)m s^-1,which accelerated the vertical mixing of seawater and was conducive to the production of DSW.