Results from a multi-decadal simulation with an improved version of the Center for Ocean-Land-Atmosphere Studies (COLA) coupled atmosphere-ocean general circulation model (GCM) are presented and analyzed. Previous versions of the COLA coupled atmosphere-ocean GCM had serious deficiencies in the simulation of the annual mean, annual cycle, and interannual variability of sea surface temperature (SST) in the tropics. In order to improve these aspects of the climate simulation, the atmospheric component of the coupled model has been improved by: (1) using higher horizontal resolution, (2) replacing the moisture convergence based convection scheme with a stability based scheme, and (3) improving the empirical cloud-radiative interactions. The simulation of the climatological mean and annual cucle are reported, and a detailed analysis is made of the near equatorial heat budgets for the annual cycle of SST in the Atlantic, Pacific, and Indian Oceans. The initial spinup is also examined, as understanding the adjustments during this phase could aid in further improving the physical parameterizations.
This model is the first coupled GCM to our knowledge to simulate a realistic equatorial annual cycle of SST in all three oceans, Pacific, Atlantic, and Indian, without flux correction. However, climate drift is significant in some areas. Climate drift is dramatically different in the Atlantic and Pacific SST. This difference is related to the behavior of the low clouds in the beginning of integration. Warm errors in the high southern latitudes are also partially due to the interaction between the low clouds and the SST. The upper ocean thermal structure is found to have some large biases which are due to errors in simulated wind stress field.
An analysis of the heat budget of the upper ocean is performed in order to understand the mechanisms which determine the annual cycle in the model. In the eastern parts of the Atlantic and the Pacific, the annual cycle results primarily from the interaction between the upwelling and the surface heat flux. The seasonally varying upwelling in the Atlantic is dominated by variations in the surface heat flux, while, in the Pacific, seasonal variations in the stratification are also important. The model simulates the annual cycle of SST in the Pacific in spite of deficiencies in the simulation of the wind stress and the heat flux. The annual cycle of SST in the Indian Ocean is, to first order, controlled by the heat flux with modifications due to temperature advection in all three directions.
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 23 July 1997
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