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

Modeling the influence of spring Hadley circulation on the summer tropical cyclone frequency in the western North Pacific

观测事实揭示出春季Hadley环流与夏季西北太平洋热带气旋频数之间存在显著的负相关关系.由春季Hadley环流异常引起的西北太平洋地区夏季纬向风垂直切变、大气辐合辐散等的异常变化是这一关系存在的内在原因.本文通过数值试验对这一关系的真实性进行了验证,即利用中国科学院大气物理研究所发展的9层大气环流模式(IAP9L-AGCM)模拟了春季Hadley环流异常偏强情景,并分析了该情景下影响西北太平洋热带气旋生成的环境场的响应.结果表明,在春季Hadley环流偏强情景下,夏季西北太平洋地区纬向风垂直切变幅度加大,低空大气异常辐散,高空大气异常辐合,东亚夏季风减弱,这种环流背景不利于热带气旋生成和发展,因此,西北太平洋热带气旋频数异常偏少.数值模拟结果与已有的诊断结果相吻合,进而证实了春季Hadley环流与夏季西北太平洋热带气旋频数负相关关系的存在.因此,春季Hadley环流信号可以用于西北太平洋热带气旋活动的气候预测。

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.