Study Of The Variation Characteristics Of Tropical Indian Ocean Evaporation And Their Influence Mechanism On SST

Ocean evaporation is an important component of global water cycle and sea surface energy balance.Under climate warming,the tropical Indian Ocean has experienced a rapid warming.The tropical Indian Ocean sea surface temperature(SST)anomaly leads to the changes in ocean evaporation.At the same time,ocean evaporation can in turn affect the Indian Ocean SST anomaly through air-sea interaction.This study mainly focuses on the relationship between tropical Indian Ocean evaporation and SST and explores the variation characteristics of tropical Indian Ocean evaporation from different time scales,and their influence mechanism on SST.Firstly,the consistency and differences among the four datasets of ERA5,OAFlux,MERRA2 and JRA55 describing the tropical Indian Ocean evaporation variation are comprehensively evaluated,and ERA5 is found to be more representative.The long-term trends and interdecadal variations of evaporation anomaly in the tropical Indian Ocean and their main factors are further analyzed.Then,the relationship between Indian Ocean evaporation variation and SST is revealed through sea-air interaction,and the mechanisms affecting this relationship are explored.Finally,the role and contribution of southern Indian Ocean in the evolution and formation of the Indian Ocean dipole(IOD)are given from the perspective of evaporation affecting SST.The major conclusions are summarized as follows:(1)Comparison analysis of the characteristics of the tropical Indian Ocean evaporation described by the four datasetsThe annual evaporation in the tropical Indian Ocean in the four datasets of JRA55,MERRA2,ERA5 and OAFlux decreases in order,among which ERA5 has the least difference from the four datasets ensemble.The aerodynamic diagnostic equations of ocean evaporation are used to further reveal the causes of the differences among the datasets,and it is found that the evaporation anomalies in ERA5 and MERRA2 are mainly affected by the relative humidity term and wind speed term,while the evaporation anomalies in OAFlux and JRA55 are mainly affected by the stability term and wind speed term.In particular,JRA55 overestimates the average evaporation in the tropical Indian Ocean,which leads to a significantly large bias in the variability of the wind speed term.The stability terms in OAFlux and JRA55 correspond to the "Y" wind field in the southern Indian Ocean,and the relative humidity terms in ERA5 and MERRA2 correspond to the "Y" wind field in the southern Indian Ocean.The wind speed terms in the four datasets all correspond to the anticyclonic wind field in the subtropical southern Indian Ocean.The "Y" wind field in the southern Indian Ocean is the key factor affecting the differences in the tropical Indian Ocean evaporation anomaly in the four datasets,while the anticyclonic wind field in the subtropical southern Indian Ocean is the key factor that leads to the small difference in the tropical Indian Ocean evaporation anomalies.(2)Main features of evaporation variation in the tropical Indian OceanThe annual evaporation anomaly in the tropical Indian Ocean has a significant increasing trend during 1980-2018 in ERA5.The Newtonian cooling term,relative humidity term and wind speed term are the main driving factors of the long-term trend of the annual evaporation anomaly in the tropical Indian Ocean.The evaporation anomalies show a significant increasing trend in different seasons,with the largest in spring and the smallest in autumn.The increasing trend of evaporation anomaly in summer is less affected by the wind speed term.The interdecadal variations of the annual evaporation anomaly in the tropical Indian Ocean decreased gradually in the1980 s,increased from the 1990 s to the early 2000 s,and then decreased gradually after the early 2000 s,with the main factors being the wind speed term and the transfer coefficient term.Among them,the transfer coefficient term mainly contributes to the decrease in evaporation in the 1980 s and the increase in evaporation in the 1990 s,and the wind speed term mainly contributes to the increase in evaporation from the 1990 s to the early 2000 s and the decrease in evaporation after the early 2000 s.The interdecadal variation of annual evaporation anomaly over the tropical Indian Ocean is mainly related to the Pacific decadal oscillation(PDO),the Atlantic multidecadal oscillation(AMO)and the North Atlantic osciallation(NAO).The NAO affects the tropical Indian Ocean evaporation anomaly by influencing the interdecadal variation of the transfer coefficient term,the AMO mainly affects the interdecadal variation of the wind speed term.(3)Influence of ocean evaporation variation on the Indian Ocean multipole SST pattern in autumnThe relationship between autumn Indian Ocean evaporation and SST changed in the late 1990 s.During 1980-1990,there was a significant positive correlation between Indian Ocean evaporation and SST,with SST mainly affecting evaporation,and the Indian Ocean Basin Mode(IOBM)was the main driving factor.During 2005-2015,there was a significant negative correlation,with evaporation mainly affecting SST,which was mainly related to the Indian Ocean multipole SST pattern.The positive and negative phases of the Indian Ocean multipole SST pattern(p IOD-n SIOD,n IODp SIOD)are affected by different mechanisms.For p IOD-n SIOD,the anomalous cyclonic and anticyclonic circulations in the southern Indian Ocean lead to anomalous easterly winds in the tropical eastern Indian Ocean and anomalous westerly winds in the subtropical southwestern Indian Ocean,which are mainly affected by windevaporation-SST(WES)feedback.For n IOD-p SIOD,the anomalous cyclonic and anticyclonic circulations in the southern Indian Ocean lead to the low-level convergence on the tropical southeastern Indian Ocean and the low-level divergence on the subtropical southwestern Indian Ocean,and low-level convergence/divergenceevaporation-SST feedback is the main influence mechanism.During 2005-2015,the coupled mode of autumn Indian Ocean evaporation and SST mainly showed p IODn SIOD,indicating that WES feedback was the key mechanism of negative correlation between the Indian Ocean evaporation and SST in autumn.The results indicate that ENSO has little effect on the Indian Ocean multipole SST pattern.(4)Influence of ocean evaporation variation on the evolution of the Indian Ocean dipoleUsing the normalized time coefficients of the first mode of EOF of the SST anomalies in the southern Indian Ocean from January to March as the subtropical Indian Ocean dipole(SIOD)index,it is found that the positive correlation between the SIOD index and the following autumn IOD index is significantly strengthened after the early2000 s.The evolution of the IOD events after the SIOD event is different from the evolution of the typical IOD events,which shows a significant dipole SST anomaly in summer.The positive correlation between SIOD and IOD during 1986-1998 was mainly due to the interaction of the anomalous anticyclone and the equatorial dipole SST anomaly in the tropical southern Indian Ocean in summer,which was mainly related to ENSO forcing.In the strengthened positive correlation between SIOD and IOD during 2006-2018,under the influence of WES feedback,warm SST anomalies in the subtropical southwestern Indian Ocean sustained from spring to summer,and the maintenance of the warm SST anomalies can promote the development of zonal wavenumber-3 pattern(ZW3P)in the Southern Hemisphere mid-high latitudes in summer,leading to the strengthened negative correlation between SIOD and ZW3 P.They promoted the low-level convergence in the mid-latitude southern Indian Ocean in summer,and further led to anomalous subsidence over the central northern Indian Ocean through the local Hadley circulation.An anomalous anticyclonic circulation occurred in the northern Indian Ocean,and the easterly wind anomalies appeared on its southern flank.The anomalous easterly winds in the equatorial Indian Ocean can trigger positive IOD events in summer through Bjerknes feedback.ENSO also has little effect on the mechanism of the strengthened positive correlation between SIOD and IOD.