Simulations of the Indo-Pacific Warm Pool by IPCC Models

The Indo-Pacific warm pool is the region where tropical deep convection is concentrated. The latent heat release from deep convection powers the Walker and Hadley circulation in the atmosphere which in turn drive the currents in the upper ocean. The Hadley circulation and its counterpart in the ocean extends the influence of the warm-pool to the extratropical region. Understanding what determines the SST of the tropical Pacific warm pool is a central issue in the study of global climate dynamics. Evaluation of the simulations of the Indo-Pacific warm pool by IPCC models, however, reveals systematic biases. The size of the warm pool in the models is too small; the mean warm pool SST and the maximum SST over tropical Indo-Pacific (60°E-250°E, 30°S-30°N) are too high. The trend in the warm pool size is smaller than in the observation, so is the mean warm pool SST, whether measured over the last century, the last 50 years, or the last 30 years. The trend in the tropical Indo-Pacific maximum SST in the models, on the other hand, tends to be higher than in the observation. Rainfall, latent heat flux (evaporation), shortwave radiation, and net heat flux over warm pool are analyzed to understand the cause of these biases. It is found that all the models overestimate the rainfall. Most of these models also overestimate the latent heat flux. Most of the models, however, are found to have a negative trend in rainfall, in contrast with a positive trend in observation over the last 21 years of the 20th century during which the rainfall data are available. The latent heat flux has a positive trend in both models and observation, but the trend in the models is much weaker than the trend in observation over the same period. The net heat flux has a negative trend in both models and observation from 1984 to 1999 during which the observation data is available. However, the negative trend in the models in much larger than in observation.