The Tibetan Plateau-tropical Indian Ocean Thermal Contrast And Its Relationship With The South Asian Summer Monsoon

This paper used multi-source data and multiple methods to calculate the atmospheric heat sources of Tibetan Plateau(TP),and analyzed the characteristics and differences of the TP-Tropical Indian Ocean(TIO)atmospheric heat sources(Q1)and water vapor sinks(Q2)using ERA5.It defined land-sea thermal contrast indicators(QI,TIup,and QIup)to investigate their relationships with the South Asian summer monsoon,and also explore the relationship between land-sea thermal contrast and the onset of the Asian summer monsoon in 2018.The main conclusions are as follows.(1)The TP and the TIO Q1 are both dominated by the precipitation latent heat(Q2),and the seasonal variability in spatial distribution are also significant.The TP Q1 has an obvious feature of cold sources in winter and heat sources in summer,while the TIO is a heat sources all the year around and Q1 in winter is stronger than that in summer.For summer,on the west side near the ground of the TP is the heat source,which is mainly compensated by cold advection and subsidence warming.The upper troposphere turns into a cold source and is mainly compensated by subsidence warming.On the east side of the TP is a strong heat sources and strong water vapor sinks.Meanwhile,the heat sources on the west side of the TIO is relatively weak,and the entire troposphere on the east side is a strong heat source,with the largest value in the middle and upper layers.For winter,the surface near the west side of the TP is a water vapor sinks,and it is mainly compensated by the positive water vapor advection transport,and the strong advection and vertical transport effects below 300 h Pa results in a small Q1 in east side of the TP.For the TIO,it's a strong heat sources and water vapor sinks,and reaches the strongest at the upper-middle level in winter.(2)The uncertainties to estimate the thermal contrast between the TP and TIO are derived from indicators and data.The seasonal transition of TIup lags behind the QI and QIup by one month.The summer QIup and TIup has a decadal shift around2000(weak to strong shift),while QI shows a "V" shaped change(the land-sea thermal contrast between the early 1990 s and the early 2000 s is weak).Different data indicate that the interannual variability of TIup in summer varies greatly.Reanalysis data has the highest correlation with Radiosonde data,and ERA5 and JRA-55 have the highest correlation,but Radiosonde data(except IUK)and reanalysis data(except NCEP/DOE)shows that the warming trend of the TP is stronger than that of the TIO,leading to an increase land-sea thermal contrast in upper troposhere,while the results of the CMIP6 show consistent warming.(3)Establish a positive feedback mechanism of "meridial thermal contrastmonsoon circulation-monsoon precipitation" to explain the influence of the TP-TIO meridional thermal contrast on the South Asian summer monsoon.When the QI is abnormal positive,the upper troposphere meridional temperature gradient increased to strengthen the baroclinicity and monsoon circulation,resulting in abnormal rising and sinking flow on the southern side of the TP and TIO respectively,and finally produce abnormal precipitation in Bay of Bengal,Indian Peninsula and Indochina Peninsula.The situation is basically opposite when the QI is negative abnormal.For the abnormal positive QIup,the temperature in the upper troposphere has a "tripole" distribution(warm anomaly on the west side of the plateau—cold anomaly on the tropical Indian Ocean—cold anomaly on the east side of the plateau),and the monsoon circulation and precipitation anomalies are quite different from QI.(4)The land-sea thermal contrast is closely related to the onset of the Asian summer monsoon in 2018.From 5th pented in May to 1st pented in June,correspond to the onset of the Indian summer monsoon,the Bay of Bengal summer monsoon,and the South China Sea summer monsoon,respectively.The latter two correspond to the maximum meridional and zonal temperature gradients in the upper troposphere.From the 4th to the 6th pentad of May,the thermal contrast between the TP and TIO is mainly dominated by the precipitation latent heat in the TIO.Deep convective activity in the Bay of Bengal erupts on the 6th pentad of May,and the meridional latent heat difference reaches its maximum,accompanied by the upper tropospheric meridional temperature gradient reached the maximum,which led to the onset of the Bay of Bengal summer monsoon.On 1st pented of June,the convective activity and sea temperature in the Bay of Bengal decreased,resulting in a significant decrease in the latent heat and sensible heat of precipitation condensation,and the zonal latent heat difference in the TIO and the South China Sea was reduced to a minimum,so that the zonal temperature gradient reaches maximum,leading to the onset of the South China Sea summer monsoon.