Wang, W.-C., H.-H. Hsu, W.-S. Kau, X.-Z. Liang, L. Ho, C.-T. Chen, A. N. Samel, C.-H. Tsou, P.-H. Lin, and K.-C. Ko, 1998: GCM simulations of the east Asia climate. Proceedings of the Third East Asia-West Pacific Meteorology and Climate Conference, C.P. Chang (ed.), World Scientific Publication Corp., 562 pp.

General circulation models (GCMs) have been used extensively to study global climate change and its regional distribution due to increasing atmospheric greenhouse gas concentrations and sulfate aerosols associated with human activities. One important issue related to the continued development and application of GCMs for climate change studies is the ability for GCMs to simulate observed regional climate characteristics. An understanding of the causes for models’ inadequacy in simulating these characteristics will lead to their further improvement.

East Asia is located in the southeastern part of the Eurasian continent. It is bordered in the east by the Pacific Ocean, and in the southwest by the Tibetan Plateau which penetrates into the middle troposphere. These unique geographic features produce distinct climate characteristics over East Asia. Observational analyses indicate that this region has the strongest heat sources/sinks in the Northern Hemisphere, and that the Tibetan Plateau creates vigorous dynamic and thermal forcing. Both factors induce substantial atmospheric responses that result in well-defined, seasonally varying general circulation features. In winter, the primary features are the lower-tropospheric continental high, the upper-tropospheric East Asia westerly jet, the East Asia trough and the stratospheric Aleutian high. In summer, the dominant features include the lower-tropospheric continental heat low and the upper tropospheric/lower stratospheric Tibetan high.

These stationary circulation systems produce several unique regional climate characteristics over East Asia, including (1) the mid-tropospheric Zonal Index Cycle (ZIC) which describes the vacillation between "high (zonal) index" and "low (meridional) index" circulation patterns; (2) Seasonal Abrupt Changes (SACs) in June and October caused by non-linear atmospheric responses to large-scale external forcing, such as seasonal variations in solar insolation, and the dynamic and thermal effects of the Tibetan Plateau; and (3) the East Asia Monsoon (EAM) characterized by heavy precipitation in summer and cold surges during winter. Note that ZIC, SAC and EAM are not independent features. For instance, the onset of the summer and winter EAM is closely associated with SAC. In addition, rainfall identified with the prevailing zonal circulation patterns is caused by processes that differ substantially from those related to extratropical cyclones and tropical deep convection. The summer EAM rainband is typically a quasi-stationary large-scale feature that produces continuous precipitation. Embedded within the rainband are mesoscale vortices, which produce intense local rainfall. It is well known that these EAC characteristics experience substantial intraseasonal and interannual variability.

To study the capability of GCMs to simulate the climate over East Asia, an international research program entitled, "General Circulation Model Simulations of the East Asia Climate (EAC)" was initiated in the Fall of 1994 within the Atmospheric Model Intercomparison Project (AMIP). The program’s objectives are to assess the ability of current GCMs to simulate the EAC and to study the mechanisms and geophysical factors that cause the intraseasonal to interannual variability. A GCM-to-observation comparison was conducted to examine the climatological mean and variability of individual climate parameters (e.g., sea level pressure, surface temperature, wind, precipitation, outgoing longwave radiation, 500 mb height, snow/ice cover) and their mutual associations. These findings will subsequently be used to design sensitivity experiments to identify the mechanisms important to the EAC. Here, we report some results focusing on the seasonal variation of precipitation.