Local and non-local atmospheric effects of abnormal soil moisture over Indochina during May and June

Abstract

Indochina is one of the key areas affecting the East Asian summer monsoon and its thermal condition can modulate the prevailing wind and water vapour transport, especially in the early stages of establishment of the summer monsoon. Previous studies emphasized the climatic effect of the seasonal-averaged soil moisture (SM), but the subseasonal effects of land surface processes over Indochina are not clear yet. This study investigated the subseasonal effect of SM in Indochina on precipitation in southern China (SC) during May and June together with its relevant mechanism based on both statistical analysis and numerical experiments. Results show that increased SM in Indochina in May is closely associated with increased local (Indochina) precipitation and suppressed non-local (SC) precipitation in June. The positive SM anomaly over Indochina can persist from May to June, cooling the atmosphere and increasing the specific humidity in the lower troposphere. Meanwhile, the potentially unstable lower troposphere and the ascending motion over Indochina result in strengthened upward moisture advection and condensational heating, which are conducive to forming an abnormal cyclonic circulation and increased precipitation over Indochina. To the northeast, an easterly wind anomaly appears in SC, leading to weakened moisture transport and decreased precipitation. When the SM decreases over Indochina, the opposite results are obtained. Moreover, through increasing and decreasing SM in Indochina in May, numerical experiments showed similar results to those of reanalysis data and further indicated that the moist and thermal processes of the SM anomaly over Indochina play distinct roles in affecting the atmosphere and regional precipitation. The moisture process mainly influences local precipitation, whereas the thermal process can change the precipitation non-locally. This study highlights the significant local and non-local impacts of SM in Indochina and could contribute to understanding the physical mechanisms of SM effects.