sSimulation of the 1998 East Asian Summer Monsoon by the CCSR/NIES AGCM
Masahide Kimoto
Center for Climate System Research, University of Tokyo
During 1998 summer, a record-breaking flood occurred over the Chang-Jiang River basin in China. Numerous heavy rainfall events hit Korea and Japan, too. Inquiries into large-scale anomalies reveal that southwesterly monsoon flow was exceptionally strong throughout the boreal summer months (June-August). A low-level anticyclonic anomaly in the subtropical western Pacific, similar to that observed during the mature-decay phase of El Nino events, was responsible for the enhancement of the monsoonal flow. An ensemble hindcast experiment was carried out using the Center for Climate System Research/National Institute for Environmental Studies atmospheric general circulation model (CCSR/NIES AGCM) giving observed distributions of global sea surface temperature (SST) as a lower boundary condition. A 10-member ensemble consisted of atmospheric initial conditions observed between 21st and 30th of April. The hindcast successfully simulated the circulation anomalies in the tropics and subtropics in the Indian-East Asian sector. In particular, a greatly suppressed precipitation to the east of Philippines and enhanced convection to the southeast of Sumatra aloft a patch of high SST anomalies (SSTAs) were conspicuous and consistent with observations. Additional seven sets of ensemble hindcasts retaining local SSTAs in various regions revealed that two local SSTAs were crucial to support the anomalous subsidence off Philippines, and therefore were responsible for the enhancement of the East Asian monsoon: positive SSTAs over the far eastern equatorial Pacific, an aftermath of 1997/98 El Nino, and the positive SSTAs off Sumatra, a part of the Indian Dipole Mode. The strong subsidence to the east of Philippines was remotely forced by convective anomalies over these two regions and associated low-level anticyclonic anomaly contributed to the enhancement of the monsoonal flow and precipitation over an east-west belt extending from southern China to Japan.
