Variability And Mechanisms Of The Upper Layer Salinity In Southern Subtropical Indian Ocean

Salinity,like temperature,is a basic marine element.Some studies have shown that variation of salinity in the southern subtropical Indian Ocean have certain effects on ocean currents,sea surface height,ocean dynamics,marine life and primary productivity.Therefore,by understanding the variability and mechanisms of the upper layer salinity in subtropical Indian Ocean,there are some important indications and references for understanding the hydrological cycle around Indo-Pacific warm pool,the corresponding areas of typical climate variability and biogeochemical elements.We are interested in the variation of the upper layer salinity in the south Indian Ocean,especially the southern subtropical Indian Ocean.The observational data(ARGO)and mode data(ECCOv4r3)were used to analyze the seasonal,interannual spatiotemporal variations of the upper layer salinity.The analytical methods of salinity budget were used to analysis the effects of atmospheric processes and ocean dynamic processes.Compared to other mode data,ECCO also provides the results of diffusion and mixing,which makes the salinity budget in each point to be fully closed.Therefore,it can help us quantitatively assess the impact of each process.Based on the research done,we analysis the impact of variation of the upper 300m salinity on sea level change between 2005 and 2013.The results show that the variation of the upper 300m salinity in the southeast Indian Ocean mostly explains the expansion(88.03%)caused by the salinity change of the whole water column,which has an important influence on the increase of the sea surface height Interannual variations was dominating the variation of sea surface height in southwest Indian Ocean.At the same time,it is also found that there is a significant negative correlation between the upper 300m salinity anomalies and sea surface height anomalies(R=-0.88),and the correlation gradually decreases to the west(R=-0.21).The variation of salinity in vertical direction is analyzed.It is clear that the upper 200m is the optional depth for us to analyze the salinity budget in the period of 2005-2013.According to the results we analyzed,the ITF transports fresh water from the South China Sea and the Maritime Continent region to southeast Indian Ocean,which causes the salinity decreasing by the horizontal advection.The meridional velocity anomaly is the most important factor for the fresh phenomenon while the contribution of the local E-P is mainly to enhance the upper salinity in this region.We also found that the transport of low-frequency flow in the southeast Indian Ocean is much larger than that of vortex-induced advection.The mixed layer is an important interface for material exchange between the upper ocean and the lower atmosphere,and to a certain extent can represent the variation of salinity in the upper ocean.According to the analysis of the variation of salinity in mixed layer between 1992 and 2015,the variation of mixed layer salinity in Southeast Indian Ocean is mainly dominated by interannual variation,while the mixed layer salinity in the southwest Indian Ocean has obvious seasonal variation.In the long-term average of salinity budget,there is inversely phased in space for the contributions firom the local E-P and the ocean dynamic processes.The subsurface dynamic process and horizontal advection process in ocean dynamic processes are respectively main impact factor for low latitude and southern subtropical Indian Ocean.In the seasonal scale,a certain opposite signal exists between ocean dynamic processes and atmosphere processes in the Southeast Indian Ocean,which could reduce the amplitude of the variation of mixed layer salinity.Therefore,the seasonal signal in the southeast Indian Ocean is not significant.Local E-P and ocean dynamics have the same effect in austral summer and winter,which can increase the variation amplitude of the mixed layer in the southwest Indian Ocean,and the both have a certain offsetting effect to weaken the amplitude of variation of mixed layer salinity in austral autumn and spring.Hence,the obvious seasonal cycle could be seen in the southwest Indian Ocean.In the interannual scale,the ocean dynamic process largely explains the interannual variation of mixed layer salinity in the southeast Indian Ocean(34.7%),while the variation of mixed layer salinity is mainly regulated by local E-P(32.5%).The low-frequency changes of the mixed layer in the southeast Indian Ocean are related to ENSO.During the El Nino period,the Wallker circulation weakens and results in increasing the salinity.But in La Nina period,the phenomenon is opposite.The precipitation in the atmospheric process dominates the variation of mixed layer salinity in the north of the 150° S,while the horizontal advection in the ocean process affects the variation of salinity in the south of the 15° S.