Simulation of the 1998 East Asian Summer Monsoon by the CCSR/NIES AGCM

The East Asian countries experienced an extremely wet summer in 1998. More than 150% of normal rainfall has been observed over a large portion of East Asia extending from southern China, Korean peninsula to Japan. A record-breaking flood occurred over the Changjiang River basin of China, and lasted for almost three months. Heavy rainfalls hit northern and eastern Japan, and the Korean peninsula in July and August. Observational studies indicate that the 1998 East Asian summer monsoon was characterized by suppressed convection and persistent low-level anticyclonic circulation anomalies over the subtropical western Pacific. It is confirmed by a moisture budget analysis that the seasonal mean, rather than transient, component of the moisture transport anomaly contributed mainly to the wet summer in East Asia. An atmospheric general circulation model (AGCM) was integrated with observed sea surface temperature anomalies to study the 1998 East Asian summer monsoon. The large-scale features over the East Asian monsoon region were well reproduced by the model. Experiments indicate that SST anomalies over the two key regions, the southeastern Indian Ocean and the equatorial eastern Pacific, were most influential in forming the subsidence anomaly over the subtropical western Pacific and associated low-level anticyclonic anomaly. It is indicated that high SSTs over the southeastern Indian Ocean enhanced local convection and weakened the local Hadley circulation, and that associated subsidence contributed to strengthen the low-level anticyclonic anomaly over the subtropical western Pacific. On the other hand, SSTAs over the equatorial eastern Pacific helped enhance the local convective activity and weaken the Walker circulation. The AGCM experiments indicated that the dual effect made the persistent and strong low-level anticyclonic anomaly in the subtropical western Pacific, and thereby was responsible for the anomalous 1998 East Asian summer monsoon.