Lau, W. K.-M. and S. Yang, 1996a: Seasonal variation, abrupt
transition, and intraseasonal variability associated with the Asian summer
monsoon in the GLA GCM. Journal of Climate, 9, 965-985.
In this paper, a description of the global and regional features of
the Asian summer monsoon in the Goddard Laboratory for 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 underdeveloped 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.