Impacts of monsoon break events in the western North Pacific on the cross-equatorial flows over the Maritime Continent

Abstract

This study examines the possible impacts of western North Pacific (WNP) monsoon break events on the cross-equatorial flows (CEFs) over the Maritime Continent using daily data from ERA5 for 1979–2020. It reveals that WNP monsoon break events can cause significant weakening of CEF in the both upper and lower troposphere, i.e., weaker southerlies in the lower level and northerlies in the upper level. These CEF anomalies are associated with a baroclinic structure of circulation anomalies over the WNP, consisting of upper-level cyclonic and lower-level anticyclonic anomalies. Furthermore, statistical analysis indicates that WNP monsoon break events correspond more closely to weakening of upper-level CEF, in comparison with lower-level CEF: 92% of WNP monsoon break events correspond to upper-level CEF weakening, while this decreases to 70% for lower-level CEF. This relatively lower proportion for lower-level CEF is largely attributable to the impact of the warm phase of ENSO.摘要本文利用ERA5逐日再分析资料, 探讨了1979–2020年间西北太平洋季风中断事件对海洋性大陆越赤道气流的影响. 合成结果表明, 西北太平洋季风中断事件会造成高, 低空越赤道气流减弱, 即高层南风异常, 低层北风异常, 与此相关的环流异常表现为西北太平洋高层气旋, 低层反气旋的斜压结构. 特别的是, 西北太平洋季风中断对高空越赤道气流的影响更为显著, 92%的季风中断事件都导致高空越赤道气流减弱, 而只有70%的事件造成低空越赤道气流减弱, 这是由于低空越赤道气流同时还受到赤道中东太平洋海温异常的调控.