Relationship between the Tibetan Plateau-tropical Indian Ocean thermal contrast and the South Asian summer monsoon

The impact of land-sea thermal contrast on the South Asian summer monsoon (SASM) was investigated by calculating the atmospheric heat sources (AHS) and baroclinic component with ERA5 data for the period 1979-2019. Using diagnostic and statistical methods, it was found that the thermal contrast between the Tibetan Plateau (TP) and the tropical Indian Ocean (TIO) affects the South Asian monsoon circulation through the meridional temperature gradient in the upper troposphere. The seasonal changes of the AHS of the TP and TIO are reversed. In summer, the TP is the strongest at the same latitude whereas the TIO is the weakest, and the thermal contrast is the most obvious. The heat sources of the TP and TIO are located on the north and south side of the strong baroclinic area of the SASM region, respectively, and both of which are dominated by deep convective heating in the upper troposphere. The TP-TIO regional meridional thermal contrast index (QI) based on the AHS, and the SASM index (MI) based on baroclinicity were found to be strongly positively correlated. In years of abnormally high QI, the thermal contrast between the TP and TIO is strong in summer, which warms the upper troposphere over Eurasia and cools it over the TIO. The stronger temperature gradient enhances the baroclinicity in the troposphere, which results in a strengthening of the low-level westerly airflow and the upper-level easterly airflow. The anomalous winds strengthen the South Asian high (SAH), with the warmer center in the upper troposphere, and the enhanced Walker circulation over the equatorial Indian Ocean. Finally, the anomalous circulation leads to much more precipitation over the SASM region. The influence of abnormally low QI is almost the opposite.