Link between the Land-Sea Thermal Contrast and the Asian Summer Monsoon

The land-sea thermal contrast is the foundation of the occurrences of the Asian summer monsoon (ASM). Using multiobservational datasets and model simulations covering the period 1960-2018, this study investigates the relationship that links the thermal contrast between the Asian landmass and the Indian Ocean (TCAI) to the ASM and evaluates the effects of the various driving factors that influence the TCAI in the climate models. The tropospheric TCAI is highly consistent with the ASM circulation, with a stronger ASM circulation and larger tropospheric TCAI before the 1970s and after the late 1990s than those seen from the 1970s to the late 1990s. The tropospheric TCAI is also closely related to rainfall over the Indian landmass and the south-flood-north-drought pattern over eastern China. Thus, the tropospheric TCAI reliably reflects the state of the ASM system and could be used as a new ASM index that directly describes the land-sea thermal contrast. The variation of the surface TCAI is highly consistent with that of the tropospheric TCAI. The model simulation suggests that the main drivers of the surface TCAI are anthropogenic forcing and the Atlantic multidecadal oscillation (AMO). Anthropogenic forcing dominates SAT variations over the Indian Ocean, whereas the AMO plays a major role in SAT variations over the Asian landmass, which introduces a large uncertainty into the projections of the ASM.