Decadal Variation Of Oceanatmosphere Feedback In The Tropical Indian Ocean And Its Influencing Factors

The ocean-atmosphere feedback in the tropical Indian Ocean maintains the climate of the Indian Ocean and plays an important role in the formation and development of the local ocean-atmosphere coupling mode.Thermocline-sea surface temperature(SST)feedback,SST-zonal wind feedback and zonal wind-thermocline feedback are the main processes of the ocean-atmosphere feedback,collectively referred to as Bjerknes positive feedback.During global warming,the elements associated with these oceanatmosphere feedbacks have undergone significant decadal changes.As a result,the intensity of ocean-atmosphere feedback has also changed,which has affected the frequency and predictability of ocean-atmosphere coupling modes.At present,many researches on tropical ocean-atmosphere feedbacks have been done.The most existing researches focused on the seasonal variability and interannual characteristics of the ocean-atmosphere feedbacks in Pacific and Atlantic,while few studies concentrated on the decadal variation and the basic seasonal cycle of oceanatmosphere feedbacks in the tropical Indian Ocean.In this paper,the seasonal differences and decadal variation of the ocean-atmosphere feedback in the tropical Indian Ocean will be discussed by using observational data and reanalysis data.Then,the main influencing factors affecting the decadal variation of the local air-sea feedback will be clarified.The specific conclusions are as follows:(1)The thermocline-SST feedback in the tropical Indian Ocean occurs significantly both in the Southeast and the Southwest Indian Ocean.The feedback process in the southeastern Indian Ocean mainly occurs in summer and autumn,while the feedback process in the southwestern Indian Ocean mainly occurs in winter and spring.The thermocline-SST feedback in the Indian Ocean has experienced dramatic decadal variations over the last 40 years.The feedback intensified in the late twentieth century and then weakened during the hiatus in global warming at the early twenty-first century.Thermocline depth and upwelling are the main factors affecting the decadal variation of the thermocline-SST feedback in the tropical Indian Ocean.But the relative importance of different factors is different in the tropical southeastern and southwestern Indian Oceans.When the thermocline is uplifted in the tropical southeastern Indian Ocean,the strength of the thermocline-SST feedback will be enhanced.In the opposite,the coupling strength will be weakened.In the tropical southwest Indian Ocean,the strength of the upwelling mainly affects the feedback strength with a delay from February to March,whereas the depth of the thermocline mainly affects the feedback strength from April to June on a decadal timescale.(2)The SST-zonal wind feedback in the tropical Indian Ocean mainly occurs from July to November.During global warming,the SST-zonal wind feedback in the tropical Indian Ocean showed a decadal increasing trend.During the global warming hiatus,it showed a decadal decreasing trend.This feedback is significantly asymmetry on the decadal time scale.Its negative process occurs more than the positive process,but the feedback intensity of its negative process is weaker than the positive process.The decadal variation of the SST-zonal wind feedback is mainly influenced by the SST and the static stability of the atmospheric troposphere in the tropical southeastern Indian Ocean.Before 1995,SST and the atmospheric tropical stability in the tropical southeastern Indian Ocean were important factors for the decadal enhancement of the SST-zonal wind feedback.However,after 2000,the atmospheric tropical stability was the main factor.(3)The zonal wind-thermocline feedback preogress in the tropical Indian Ocean is strongest in the tropical eastern and western Indian Ocean.From 1980 to 2019,the feedback existed in summer,autumn and winter,also showed decadal variation trend.The intensity of the negative process of the zonal wind-thermocline feedback in the tropical Indian Ocean is stronger than that of the positive process.The feedback is stronger when the equatorial easterly wind anomoly is strengthened with the shoaling thermocline.The decadal variation of this feedback is mainly affected by the zonal wind speed,thermocline depth and upwelling intensity.The relative importance of each influencing factor varies in different regions.In the tropical eastern Indian Ocean,the intensity of the easterly wind anomoly,the depth of local thermocline and the speed of upwelling are main factors of the decadal variation of zonal wind-thermocline feedback.In contrast,the tropical western Indian Ocean local feedback more responsed to the thermocline,less responsed to zonal wind anomalies or upwelling.