西太平洋暖池6月对流增强的成因及其预测意义

Origin of Enhanced Warm Pool Convection in June over the Western Pacific and Its Importance to Prediction

基于1979~2018年观测向外长波辐射资料和其他多种再分析资料,研究了西太平洋暖池6月对流增强的原因。合成分析结果表明,由于La Niña影响造成春季暖池海温偏高,为6月暖池对流增强提供了热力基础,而大气内部扰动特别是热带西风增强能从动力上影响到6月对流的发展。此外,6月对流增强通过局地海气相互作用持续影响到7~8月的对流变化,导致暖池对流形成显著的季节内振荡,因此6月对流增强对其后月份对流异常变化有重要的预测意义。1984年6月的个例分析表明,热带西风增强能导致暖池对流发展,二者存在明确的超前滞后关系。与此相反,虽然1989年是一个强La Niña年,春季暖池海温也偏高,但由于6月热带西风偏弱,暖池对流难以发展,夏季对流的季节内振荡也不显著。因此,6月暖池对流增强受到前期海温偏高和大气扰动的共同作用。另一方面,暖池对流偏强与偏弱并非一种完全反对称关系,暖池对流偏弱还受到其他因子的影响。

This study focuses on the enhanced warm pool convection in June over the western Pacific based on the observed outgoing longwave radiation(OLR)data and other reanalysis datasets during 1979–2018.The composite results indicate that a higher sea surface temperature(SST)in the preceding spring due to La Niña events provides a thermal condition for the enhanced warm pool convection in June.Furthermore,the warm pool convection in June is dynamically forced by atmospheric perturbation,especially strengthened tropical westerlies.The enhanced convection in June may also affect the convection anomaly from July to August through local air–sea interactions,which results in a considerable intraseasonal variation for the warm pool convection.Thus,convection in June is important for the prediction of the monthly convection anomaly in the succeeding months.A selected example in June 1984 further indicates that the development of the warm pool convection is excited by the enhanced tropical westerly.Moreover,there is a definite lead–lag relationship between the tropical westerly and the warm pool convection.In contrast,although the warm pool SST is higher in spring due to a strong La Niña in 1989,the warm pool convection in June is not developed due to a weak tropical westerly.Consequently,the intraseasonal variation during summertime was not much greater.Therefore,the combined effects of a higher SST in spring and a strong atmospheric perturbation play important roles in enhancing the warm pool convection in June.On the other hand,the enhanced convection is not antisymmetric to the suppressed convection in the warm pool because the latter is also affected by some other factors such as El Niño.