Dynamic Analysis Of Salinity Balances And Freshwater Transports In The Tropical Eastern Indian Ocean

Based on observational and model data,this paper analyzes the seasonal and interannual variations of salinity in the Eastern tropical Indian Ocean,studies the cross-equatorial pathways,seasonal variation,and vertical structure of freshwater in the eastern mouth of the Bay of Bengal,and estimates the meridional heat and salt transport of the cross-equatorial currents.In the Eastern tropical Indian Ocean,if only evaporation(E)and precipitation(P)are considered,the spatial distribution of salinity cannot be fully explained.This is because ocean currents also play an important role in salinity variation.Through a budget analysis of salinity in the mixed layer,it can be found that the contribution caused by the annual freshwater flux is balanced by horizontal advection.The seasonal variation of freshwater flux is consistent with the pattern of E-P,while the seasonal variation of horizontal advection is mainly affected by seasonal ocean currents and zonal and meridional salinity gradients,with significant seasonal variations.The results of the regional salinity analysis indicate that salinity in the equatorial Indian Ocean exhibits a semi-annual cycle,primarily due to the influence of zonal advection associated with Wyrtki Jets,with maximum values(0.3psu/month)occurring in spring(April)and autumn(November).Off the Java coast,salinity shows an annual cycle of change.Salinity is affected by both horizontal advection and freshwater flux,and the meridional component dominates the seasonal variation of horizontal advection.Horizontal advection introduces low salinity,minimizing the salinity tendency(-0.1psu/month)in spring(April to May),while freshwater flux causes excessive evaporation in summer(August),resulting in the salinity tendency reaching its maximum(0.1psu/month).The analysis of the interannual variation of salinity indicates that significant positive salinity anomalies were found in the equatorial eastern Indian Ocean during the negative Indian Ocean Dipole(IOD)event.The abnormal eastward currents transport high salinity water from the Western Indian Ocean to the east,resulting in an abnormal increase in salinity,reaching its maximum value in July and August,approximately 0.2 psu/month.The contribution of freshwater flux to salinity change is relatively weak compared to horizontal advection,mainly showing a negative value,introducing additional freshwater to offset the tendency of salinity increase.During positive IOD events,the changes in salinity anomalies are,on the contrary,mainly affected by anomalies in westward currents,resulting in a decrease in salinity,with the largest contribution of about-0.3 psu/month in September-October.The freshwater flux causes excessive evaporation,leading to an increase in salinity in the region.The Lagrange particle tracking results show that there are three freshwater pathways at the eastern mouth of the Bay of Bengal,with significant seasonal variations.During the northeast monsoon,two freshwater pathways can be observed.Part of the freshwater is brought back to the Bay of Bengal by the northward Ekman currents(Path Ⅰ),while the other part is transported by the Northeast Monsoon Current to the Western Indian Ocean.During the southwest monsoon period of the following year,it crosses the equator to reach the Southern Hemisphere(Path Ⅱ).The cross-equatorial transport of freshwater occurs mainly during the southwest monsoon,carried by the southward Ekman currents.It crosses the equator along the upper layer(<50m)of the eastern boundary of the Indian Ocean into the Southern Hemisphere,and eventually joins the South Equatorial Current(Path Ⅲ).The annual average meridional salt and heat transport estimated from model(observational)data for the cross-equatorial currents are-0.06×10~9(-0.11×10~9)kg/s and-0.20(-0.38)PW,respectively.The seasonal variation of meridional transport between the equator and the Bay of Bengal is significantly correlated with Ekman transport.