Seasonal Variation, Abrupt Transition, and Intraseasonal Variability Associated with the Asian Summer Monsoon in the GLA GCM

Abstract

In this paper, a description of the global and regional features of the Asian summer monsoon in the Goddard Laboratory Atmospheres (GLA) general circulation model (GCM) is presented based on a 10-yr (1979?1988) integration of the model under the Atmospheric Model Intercomparison Project. It is found that the earliest signal of onset of the Asian monsoon is represented by the emergence of convection over the Indo-China ?land bridge? in early May, followed by the sudden jump of the equatorial ITCZ (intertropical convergence zone) to 10°N over the South China Sea and Indian Ocean in mid-May. This sudden jump is linked to an abrupt northward shift of the ascending branch of the local Hadley circulation, possibly stemming from symmetric instability of the basic flow in May. In agreement with observations, the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic-scale activities identified as easterly waves in the tropical and subtropical western Pacific. These intraseasonal and synoptic activities strongly regulate rainfall variability over East Asia, especially during the premonsoon period. While the model simulates a similar range of variability in the broad-scale structure and evolution of the Asian monsoon as observed, there is considerable scope for improvement in the model's ability to simulate regional features of the monsoon. Compared to observed climatology, the model produces deficient rainfall over northern India but excessive rainfall over the South China Sea and the western Pacific region due to an overactive ITCZ at 10°N. The intraseasonal transition between the equatorial ITCZ and the monsoon (land) convection to the north is weaker than in nature. Moreover, the East Asian monsoon trough and Mei-yu rainband are under-developed in the model. These discrepancies are related to the limited northward extension of the model monsoon circulation and the confinement of the upper-level easterly flow near the equator. Overall, the GLA GCM provides a reasonably realistic description of the seasonal and subseasonal climatology of the Asian monsoon and yields important information that sheds new light on the dynamical underpinnings of the multiscale variabilities associated with the Asian summer monsoon.