Influences of Sea Surface Temperature and Land Surface Process on the Interannual Variability of Asian Summer Monsoon

Song Yang and K.-M. Lau
Laboratory for Atmospheres, NASA-Goddard Space Flight Center
Greenbelt, Maryland 20771, U.S.A.

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.

The Asian monsoon is an important climate system that affects the live of billions of people in the world. Undoubtedly, a better understanding of the mechanisms for the interannual variability of the monsoon is important for improving our skill of seasonal-to-interannual predictions for the monsoon. Using the Goddard Laboratory for Atmospheres general circulation model, we have carried out several experiments to understand the influences of sea surface temperature (SST) anomalies and land surface process on the interannual variability of Asian summer monsoon. (Land surface process is measured by the changes in snow amount and soil moisture content.) We have identified the dominant signals associated with these influences and delineated the relative contributions of these boundary forcing functions to the variability of the monsoon.

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.

Dr. Song Yang
NASA-Goddard Space Flight Center
Mail Code 913
Greenbelt, MD 20771, U.S.A.