Mechanism Of The Sustained Warming In The Southwest Indian Ocean In Recent Decades

Anomalous changes in the sea surface temperature?SST?of the Indian Ocean?IO?have a significant impact on the climate in many areas,including China.In recent decades the IO has shown a strongest warming trend.In particular,this study found that the SST in the southwest Indian Ocean?SWIO?has the unique characteristics of sustained warming in recent decades,which will bring more uncertainty to climate predictions.This paper studies the spatial distribution of SWIO warming and its mechanism,and provide reference for climate prediction.The main conclusions reached are as follows:First,the SWIO's SST has experienced sustained warming from 1996 to 2016,and there is no significant seasonal difference during this period.The study found that SWIO has been warming for more than half a century,but it can be divided into two periods from 1962 to 1982and 1996 to 2016 according to the warming characteristics.The spatial distribution reveals that SWIO SSTs warmed steadily during 1996-2016,causing a warming spot of 0.4 K decade^-1 near the east of Madagascar,and the pattern of this phenomenon is not the same as the four sea temperature modes.Comparing the SST in each season?Southern Hemisphere?,we found that SWIO has a sustained warming trend in all seasons,and the significant warming areas are the same.There is no obvious seasonal difference in the phenomenon of the warming.Secondly,using the mixed layer heat budget analysis to explore the mechanism,it is found that the zonal and meridional current anomalies are the main factors.Specifically,the depth of warming is mainly concentrated in the upper 100 m.The values of the heat flux show that there is a strong anomaly of eastward currents at the western boundary,and the anomaly of the positive heat flux increased at the southern boundary.The effects of atmospheric circulation on ocean current transport indicated that the SSH has a positive trend in the west.Affected by the regression distribution of wind stress at the southern boundary,the trend of the Ekman current shows a southward transport,which reduces the climatic cold currents and warms the SWIO.In addition,some studies have shown that the ITF,sea surface heat flux,and ocean Rossby wave have no direct impact on the warming.The results of the tracer method show that the number of particles arriving at SWIO is less than 10%in most years,and the largest year is only23.93%.Therefore,ITF has no direct impact on the warming.The trend of net sea surface heat flux is negative,indicating that this factor causes the ocean to lose heat.Similarly,it is found that the Rossby waves mainly propagate heat westward at a depth of 100-150 m,but the propagation process is mainly at 10°S-5°S,and the heat process is not continuous.To sum up,SWIO has experienced sustained warming for more than a decade since 1996.The main reason for the warming is that local wind stress changes have driven the Ekman current at the southern boundary to the south,which weakened the transport of the cold ocean advection from the Southern Ocean,causing SWIO to warm.At the same time,the SSH in the central SWIO also gradually rises,driving an anticyclone sea surface geostrophic current,causing an abnormal increase in the geostrophic current to the east,which continues to increase the sea temperature in SWIO.A process of self-sustainment and self-reinforcement has led to a sustained warming in SWIO for decades.