Study On Recurrence Of Wintertime SST Anomalies And Mode Water In The Southern Indian Ocean

Based on the GECCO2 reanalysis data from 1958 to 2016,this paper studies the spatiotemporal characteristics of the Southern Indian Ocean sea surface temperature anomalies（SSTA）winter-to-winter recurrence through lag correlation analysis,and investigate the roles of oceanic reemergence mechanism and atmospheric forcing.Since the process of winter-to-winter recurrence of SSTA is similar to the process of subduction and outcrop of mode water,this paper mainly uses the SODA3.3.1 data from 1958 to 2016 to systematically analyzes the spatial distribution and seasonal,interannual and interdecadal variation characteristics of temperature,salinity,density and volume of the Subantarctic Mode Water（SAMW）in the South Indian Ocean,explores the physical mechanism of interannual variation of the SAMW.The results show that the winter-to-winter recurrence of SSTA occurs south of 15°S in the Southern Indian Ocean,especially between Madagascar and southwest Australia（70-100°E,15-45°S）.The recurrence mainly occurs in the winter of the following year,but there are also some areas where the recurrence occurs in the following fall and continue to the subsequent winter.Further analysis shows that the difference between the deep winter mixed layer and shallow summer mixed layer,known as the oceanic reemergence mechanism,is the main cause of the recurrence of SSTA in the Southern Indian Ocean.In addition,the net heat flux also directly contributes to the recurrence of SSTA in southern Madagascar and southwestern Australia.The South Indian Ocean（SIO）SAMW is divided into three major density classes corresponding to different regions of formation:the light SAMW,26.6<σ≤26.7;the medium SAMW,26.7<σ≤26.8;and the dense SAMW,26.8<σ≤26.9.The results of SODA data show that from 2005 to 2015,the annual average volume of the SAMW decreased,among which the dense SAMW decreased by 0.187×10¹⁵m³,the light SAMW increased by 0.116×10¹⁵m³,while the medium SAMW did not change significantly.The volume change of the dense SAMW is opposite to that of the light SAMW,and the reduced volume is more,so the total volume of the SAMW decreases.The variation trend of SAMW volume in the SIO is opposite to that of the overall volume of subarctic mode water,which may be due to different study areas rather than different definition methods.Changes in the SAMW volume are closely related to the MLD in austral winter,since it takes～3 years for the SAMW to reach 30°S after its subduction,a 3-year time lag is considered to evaluate the impact of changes in MLD on subarctic mode water.The results show that the MLD of the SAMW formation region became shallower,which is consistent with the decrease of the volume of the SAMW（3 years later than the change in MLD）.East of 80°E,the weakening of the westerly winds in the SIO reduces the evaporation-precipitation,surface heat flux,and Ekman pumping and shoals the mixed layer southwest of Australia,which leads to a volume decrease at 26.8-26.9 kg m^-3in approximately 3years.West of 80°E,the parameters exhibit the opposite change,leading to a volume increase at 26.6-26.8 kg m^-3.In general,the dipole patterns of the surface buoyancy flux and Ekman pumping changes are both induced by the westerly winds associated with the subtropical high,which dominate the dipole patterns of the MLD and SAMW volume changes in the SIO.The analysis of SODA data from 1980 to 2015 shows that,before 1999,the interannual variability of the SAMW in the southern Indian Ocean was more significant,and the amplitude of interannual variability decreased significantly after2005,while the volume of the SAMW increased significantly from 1999 to 2005.There are two main reasons for the significant increase of the SAMW.First,although the spatial distribution range of the dense SAMW do not increase significantly,the spatial distribution range of the light SAMW the medium SAMW increase significantly,resulting in the increase of total volume.The second is the increase of the SAMW thickness.The spatial distribution range and thickness increase significantly from the first time period to the second time period,resulting in the increase of the total volume of the SAMW.