|Editor's Note: This is a short research note, and the full article is being prepared by the same authors for the next issue of AMON.|
Compared to land surface process, SST variability causes a more significant change in the Asian summer monsoon. For example, the reduction in May-September monsoon rainfall averaged within 10N-25N, 50E-120E is about 1 mm/day due to the warming in the equatorial eastern Pacific and 0.2 mm/day due to increased snow and soil moisture in the Asian continent. The response of monsoon circulation appears asymmetric with respect to warm and cold SST anomalies. The monsoon becomes significantly weaker during the warm events and changes little during the cold events. Associated with the warm SST anomalies, both the Walker circulation and local Hadley circulation become significantly weaker. Consistently, the amount of atmospheric water vapor transported into tropical Asia reduces remarkably during the warm SST events. We refer to this impact as SST's direct impact on the monsoon.
The Asian summer monsoon becomes moderately weaker following a wet (enhanced snow and soil moisture) Asian continent but changes insignificantly following the seasons of reduced snow and soil moisture. Land surface process mainly influences the intensity of monsoon during late spring and early summer. Results also indicate clearly that warm SST anomalies lead to wetter conditions in the Asian continent which in turn affects the monsoon. We refer to this as SST's indirect impact on the Asian summer monsoon. Therefore, the influences of SST and land surface process on the Asian summer monsoon clearly involve mutually interactive processes. Land surface forcing reinforces the anomalies of Asian monsoon produced by warm SST forcing.