Thermal and Mechanical Effects of the Southeast Asian Low-Latitude Highlands on the East Asian Summer Monsoon

Abstract

Abstract The thermal and mechanical effects of the regional orography have long been recognized as the two most important factors driving the East Asian summer monsoon (EASM). The Southeast Asian low-latitude highlands (SEALLH) are a warmer and wetter highland region adjacent to the southeastern margin of the Tibetan Plateau. However, the importance of the individual contributions of the thermal and mechanical effects of the SEALLH to the EASM is still unclear. Results of numerical experiments show that the thermal effect of the SEALLH contribute to the precipitation and upper-tropospheric circulation of the EASM by roughly the same magnitude as the mechanical effect of the SEALLH, when its original height is reduced by 50%. The thermal effect of the SEALLH influences the EASM by exciting an East Asia–Pacific-like teleconnection, whereas the mechanical effect of the SEALLH impacts the EASM by exciting an equivalent barotropic Bay of Bengal–East Asia–Pacific-like teleconnection. This study could provide a new perspective for a better understanding of the EASM. Significance Statement Recent studies have shown that the mountains adjacent to the Tibetan Plateau have significant effects on the Asian summer monsoon, although these mountains are much lower in elevation and smaller in extent than the Tibetan Plateau. The Southeast Asian low-latitude highlands (SEALLH), located on the southeastern margin of the Tibetan Plateau, influence the East Asian summer monsoon (EASM) via both thermal and mechanical effects. However, the individual thermal and mechanical contributions to the EASM have not yet been clarified. Numerical experiments designed specifically for the SEALLH, which is warmer and wetter than the Tibetan Plateau, show that the thermal effect of the SEALLH on precipitation and the upper-tropospheric circulation over the EASM region is roughly equivalent to the mechanical effect of the SEALLH when its original height was reduced by 50%, but via different physical processes. The thermal effect of the SEALLH induces southerly wind anomalies between the SEALLH and the western North Pacific, influencing the EASM by exciting an East Asia–Pacific-like wave train. The mechanical effect of the SEALLH influences the EASM by exciting an equivalent barotropic Bay of Bengal–East Asia–Pacific-like wave train.