On mechanisms that drive Walker type circulations within the context of a general circulation model

The so-called east-west cells of the global tropics observed by Madden and Julian were documented within the context of the Community Climate Model. There was a substantial agreement between observations and the simulated east-west cells. A comparison of a no-mountain experiment with a control run revealed the contribution of the orography to the location and strength of the upward branch of the Walker cell. It was found that the mountains indirectly enhanced the upper level divergence and the uplifting over the Maritime Continent through the position of the Asian Jet.The model sensitivity to SST was studied by analyzing a run with prescribed SST anomalies which are representative of a mature phase of El Nino episode. The analysis included another run with prescribed warm SST anomalies over the eastern Pacific. Both simulations were successful in reproducing anomalous Walker cells over the region of warm SST and convective heating. The importance of global moisture and local evaporation was evaluated through a set of experiments in which the evaporation was suppressed either globally or over regions of maximum uplifting over Indonesia. For the case of the evaporation globally suppressed the resulting circulation consisted of a giant sea-breeze with ascending and descending branches over the hot land and cooler ocean respectively. For the case of evaporation gradually reduced over Indonesia, the model responded with a substantial decrease in the uplifting and rainfall there, suggesting that local evaporation was the primary mechanism for initiating precipitation. Correlations among the moisture budget fields reaffirmed the importance of local evaporation in producing precipitation in the ascending branch of the Walker cell