Background
The general circulation model (GCM) has 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 of the GCMs to simulate the presently observed regional climate characteristics. An understanding of the causes of the models' inadequacy in simulating these characteristics will lead to improvement of these models.
East Asia is located in the southeast 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 forcings; both factors induce substantial atmospheric responses that result in a well-defined, seasonally varying general circulation features. In winter, the primary features are the lower-tropospheric continental high, the upper-tropospheric east Asian westerly jet, the east Asian 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) the Seasonal Abrupt Changes (SACs) in June and October caused by non-linear atmospheric responses to large-scale external forcings, such as seasonal variations in solar insolation, and the dynamic and thermal effects of the Tibetan Plateau; and (3) the East Asian Monsoon (EAM) characterized by heavy precipitation in summer and cold surges in winter. Note that ZIC, SAC and EAM are not independent features. For instance, the onset of 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. Note that additional details and relevant references were included in the AMIP I proposal.
To study the capability of GCMs in simulating the climate over this region, an international research project entitled, "General Circulation Model Simulations of the East Asian Climate (EAC)" was initiated in the Fall of 1994 (Subproject No. 25) within Phase-I of the Atmospheric Model Intercomparison Project (AMIP). Since its inception, three workshops have been organized: October 18-20, 1994 at SUNY-Albany, New York; November 16-18, 1995 at the National Central University, Chung-li, Taiwan; and April 28-30, 1997 at the Nanjing Institute of Meteorology, Nanjing, China.
Significant progress have been made, in particular the summer monsoon, including the atmospheric circulation characteristics during onset, the role of middle latitude jet streams in affecting rainfall and its movement, the energy and moisture components in the Bay of Bengal, South China Sea and the Tibetan-Plateau. The active participating groups in EAC include:
- Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing
National Climate Center, China Meteorological Administration, Beijing
Department of Atmospheric Sciences, National Taiwan University, Taipei
Department of Earth Sciences, National Taiwan Normal University, Taipei
Atmospheric Sciences Research Center, State University of New York, Albany
Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore
National Laboratory, Livermore
The project has two main objectives: To assess the ability of current GCMs to simulate the EAC, including vacillations between high and low zonal index circulation, abrupt seasonal change in the atmospheric circulation and the east Asian monsoon.
To study the mechanisms and factors that cause the intraseasonal to interannual variability in the EAC, which include the dynamic and thermal effects of the Tibetan Plateau, anomalous surface boundary conditions (e.g., temperature, snow cover, soil moisture) over the Eurasian continent.
General Consideration
The study will be conducted by examining: (1) Systematic biases of the climatological mean and interannual variability of individual climate parameters, which include sea level pressure, surface temperature, wind, precipitation, outgoing longwave radiation, 500 mb height, snow/ice cover; (2) Consistency of the biases between the individual parameters; and (3) the geophysical factors and mechanisms which have dominate influences over EAC. Statistical analyses of these parameters will be performed to examine their variations and internal consistency while GCM sensitivity experiments will be conducted to identify the causes for the model biases for their further improvements.
In addition to the comparisons discussed above, it will be useful to compare moist potential vorticity (MPV), a variable which can characterize the evolution of lower tropospheric motions. MPV consists of a dynamic component (the specific absolute vorticity) and a thermodynamic component (the 3-D gradient of equivalent potential temperature). It has been demonstrated that its isobaric presentation is an excellent indicator of summer monsoon frontal rainfall.
Specific Considerations
The ZIC and SAC are large-scale phenomena that can be resolved by the resolution of the participating AMIP GCMs. It is straightforward to define the ZIC and to identify the temporal and spatial variations of SAC. Statistical techniques (such as spectrum analysis, EOF analysis, correlation analysis and EP flux) will be used to investigate ZIC and SAC variability.
It is difficult, however, to define the EAM in a GCM. We plan to study first the East Asian Summer Monsoon (EASM) with focus on the onset. For the period of mid-April through mid-June (pre-onset, onset and post-onset), Hovmoller diagrams of climatological five-days running means of a variety of parameters (temperature, precipitation, cloud, moisture and wind) for several critical regions will be compared. Note that proper spatial scales need to be considered in averaging the GCM outputs over the specified latitudinal or longitudinal zones. We plan also to study, in addition to the climatological means, the characteristics of individual years.
- The available AMIP II data are listed in the Appendix A of AMIP Newsletter
No. 8 (August 1996). The spatial domain covers (60E-180E; 20S-70N).
Our needs for the monthly mean values are included in Tables 1 and 2 and
will not repeated here. To study some of the key characteristics
such as the monsoon onset, we request the 6-hourly data listed in Table
3 (all) and Table 6 (partial).
- Liang, X.-Z., W.-C. Wang and M.
P. Dudek, 1995: Interannual variability of regional climate and its
change due to the greenhouse effect. Global and Planetary Change,
10, 217-238.
Liang, X.-Z., A. N. Samel, and W.-C. Wang, 1995: Observed and GCM simulated decadal variability of monsoon rainfall in east China. Climate Dynamics, 11, 103-114.
Samel, A. N., S.-W. Wang, and W.-C. Wang, 1995: A comparison between observed and GCM simulated summer monsoon characteristics over China. J. Climate, 8, 1690-1696.
Liang, X.-Z., W.-C. Wang, and M. P. Dudek, 1996: Northern hemispheric interannual teleconnection patterns and their changes due to the greenhouse effect. J. Climate, 9, 465-479.
Wang, W.-C., H.-H. Hsu, W.-S. Kau,
X.-Z. Liang, Lin Ho, C.-T. Chen, A. N. Samel, C.-H. Tsou, P.-H. Lin, and
K.-C. Ko, 1997: GCM simulations of the
east Asia climate. Proceedings
of the Third East Asia-West Pacific
Meteorology and Climate Conference,
Chung-li, Taiwan, May 16-18, 1996.
For further information, contact Wei-Chyung Wang or the AMIP Project Office (amip@pcmdi.llnl.gov).
Last update: 25 February 1998. This page is maintained by mccravy@pcmdi.llnl.gov
UCRL-MI-127350