Interannual Variability Of Water Characteristics From Surface To Deep In The Indian Ocean Sector Of The Southern Ocean And Its Causes

The meridional overturning circulation of the Southern Ocean has an important role in global thermohaline circulation,and changes in the thermohaline properties of water masses within the Indian Ocean sector have important implications for global climate change.The Southern Ocean has been changing rapidly in recent decades,with a wide range of impacts on global climate,and a large number of studies have focused on the interdecadal desalination and warming of the Antarctic Bottom Water（AABW）,while changes in the properties of surface to upper water masses are still lacking.Several contemporary studies have confirmed the existence of thermohaline variability in the Southern Ocean water column from the surface to the bottom,and have suggested a correlation with polar glacial melting,changes in atmospheric forcing,and changes in the structure of shelf waters.However,little is known about the characteristics and causes of higher frequency interannual variability of water masses,including winter water（WW）,which is most vulnerable to external forcing near the surface,upper circumpolar deep water（UCDW）,which is characterised by extreme values of potential temperature below the Antarctic surface water,and Antarctic intermediate water（AAIW）,which is characterised by very small values of salinity below the westerly zone.This paper investigates the interannual variability and response to external large-scale forcing of WW,UCDW and AAIW water masses along the 110°E section in a north-south region,bounded by 60°S,by combining repeated observations and ERA5 reanalysis data collected along the 110°E section in January each year from 2011 to 2020 by a Japanese Antarctic research team.The results show that the overall trends in WW,UCDW and AAIW are relatively weak compared to the significant interannual variability.10 years north of 60°S the low-salt core temperature shows a south-to-north increasing trend,with a decrease north of45°S,while the salinity shows a south-to-north increasing trend over the observed decade.10 years south of 60°S the cold core temperature shows a south-to-north increasing trend,accompanied by a corresponding salinity increase.The temperature of the cold core south of 60°S tends to increase from south to north over the 10-year period,accompanied by a decrease in salinity（except for the 60°S site）.The WW characteristics in the southern part of the ocean are characterised by significant temporal and spatial variability,and are dominated by interannual variability.The WW cold core temperature increased at a rate of 0.098°C·decade^-1,corresponding salinity increased at a rate of 0.017 decade^-1,neutral density increased at a rate of 0.012 kg m^-3·decade^-1,and dissolved oxygen increased at a rate of 4.8 The WW core temperature trend increased from south to north over the decade,accompanied by a decrease in corresponding salinity（except at 60°S）.The AAO index has a significant positive correlation（R=0.69）and the AAO index has a negative correlation（R=-0.61）with the latitude of the local latitudinal wind shift,indicating that a larger（smaller）AAO index corresponds to a southward（northward）shift of the dispersion zone and an increase（decrease）in WW core temperature in the seasonal ice zone.The change in local net precipitation distance level is opposite to the change in WW core salinity distance level,with a negative net precipitation distance level（enhanced freshwater transport to the atmosphere）contributing to an increase in WW core salinity distance level after 2016.On the other hand,the local eddy kinetic energy distance level is negatively correlated with the WW thickness distance level（R=-0.70）,and it is assumed that changes in the intensity of the cyclonic eddies that persist in this area of the sea trigger changes in upward pumping,which in turn affects changes in WW layer thickness.The properties of the UCDW in the southern study area are dominated by long-term trends,with the thermocline potential temperature of the UCDW warming at a rate of0.135°C·decade^-1,corresponding to a decrease in salinity at a rate of 0.109 decade^-1,a decrease in neutral density at a rate of 0.093 kg·m^-3·decade^-1,and a lower rate of dissolved oxygen reduction at 0.562μmol·kg^-1-The annual mean AAO distance level had a positive correlation with the UCDW core temperature distance level（R=0.41）and a negative correlation with the UCDW core salinity distance level（R=-0.54）.the EKE distance level had the highest correlation with the UCDW L1.8°C distance level had the highest correlation（R=0.49）.The positive（negative）phase AAO drives the southernmost end of the westerly zone south（north）,with a corresponding south（north）shift of the southern ACC front,corresponding to an increase（decrease）in the UCDW core temperature.The effect of the sea surface buoyancy forcing element on the variability of the UCDW is small and negligible.The observed region north of 60°S is dominated by the westerly wind band.The westerly belts are not regionally consistent,and they are concentrated in the Indian Ocean seas where the maximum winds are obtained at 50oS,near the latitudinal position that happens to be the southernmost where the AAIW is detected.The monthly mean AAO index for this region has a significant positive correlation with the monthly mean wind stress（R=0.54,p<0.01）,wind stress cyclicity（R=0.40,p<0.01）and a significant negative correlation with the EPV（R=-0.43,p<0.01）.This suggests that positive（negative）phase AAO corresponds to an increase（decrease）in local wind stress and wind stress spin and a decrease（increase）in Ekman pumping velocity.The properties of the AAIW in the northern study area are dominated by interannual variability,with the AAIW hypersaline core potential temperature warming at a rate of0.397°C·decade^-1,hypersaline core salinity increasing at a rate of 0.027 decade^-1,neutral density decreasing at a rate of 0.027 kg m^-3·decade^-1,and dissolved oxygen decreasing at a rate of 7.759μmol·kg^-1·decade^-1,and the detected 27.1 kg·m^-3 isotope was located at a latitudinal southward shift rate of 0.062°decade^-1.The annual mean AAO distance level had a negative correlation with the L_27.1kg·m-3 of AAIW（R=-0.49）,and the annual mean wind stress distance level had a negative correlation with the L_{27.1 kg·m}-3 distance level（R=-0.41）and a positive correlation with the AAIW core salinity distance level（R=0.50）,it is hypothesized that the driven increase in wind stress at the sea surface leads to a certain degree of southward shift in AAIW production sites and an increase in AAIW salinity.The present work will help to extend the study of the interannual variability in the nature of water masses and the water mass water exchange characteristics in the Indian Ocean sector of the Southern Ocean,and thus provide insight into the specific response of the Southern Ocean marine water column to high-frequency changes in external forcing.