Modeling the influence of spring Hadley circulation on the summer tropical cyclone frequency in the western North Pacific
春季Hadley环流异常对夏季西北太平洋热带气旋频数影响的数值模拟试验

The spring Hadley circulation is observed to be negatively correlated to the summer tropical cyclone frequency in the western North Pacific. This relationship is linked by the summer atmospheric circulation anomalies resulting from the anomalous spring Hadley circulation. To examine the authenticity of such a relationship indicated by the observation, a simulation on the strong spring Hadley circulation scenario by IAP9L-AGCM is performed, and the response of the environmental conditions affecting the tropical cyclone genesis is analyzed in the present study. Results show that in relation to the strong spring Hadley circulation, larger magnitude of the vertical zonal wind shear and the anomalous low-level divergence and upper-level convergence are predominated in the western North Pacific in summer. The East Asian summer monsoon is also weakened. Such an atmospheric circulation background is unfavorable for the tropical cyclone genesis, and thus the tropical cyclone frequency in the western North Pacific is decreased. The numerical simulation supports the observation, demonstrating the existence of the out-of-phase relationship between the spring Hadley circulation and the summer tropical cyclone frequency in the western North Pacific. So, the variation of the spring Hadley circulation can be a potential indicator in predicting summer tropical cyclone activities in the western North Pacific.