Evidence of Specific MJO Phase Occurrence with Summertime California Central Valley Extreme Hot Weather

Abstract

This study examines associations between California Central Valley (CCV) heat waves and the Madden Julian Oscillation (MJO). These heat waves have major economic impact. Our prior work showed that CCV heat waves are frequently preceded by convection over the tropical Indian and eastern Pacific oceans, in patterns identifiable with MJO phases. The main analysis method is lagged composites (formed after each MJO phase pair) of CCV synoptic station temperature, outgoing longwave radiation (OLR), and velocity potential (VP). Over the CCV, positive temperature anomalies occur only after the Indian Ocean (phases 2–3) or eastern Pacific Ocean (phases 8–1) convection (implied by OLR and VP fields). The largest fractions of CCV hot days occur in the two weeks after onset of those two phase pairs. OLR and VP composites have significant subsidence and convergence above divergence over the CCV during heat waves, and these structures are each part of larger patterns having significant areas over the Indian and Pacific Oceans. Prior studies showed that CCV heat waves can be roughly grouped into two clusters: Cluster 2 is preceded by a heat wave over northwestern North America, while Cluster 1 is not. OLR and VP composite analyses are applied separately to these two clusters. However, for Cluster 2, the subsidence and VP over the CCV are not significant, and the large-scale VP pattern has low correlation with the MJO lagged composite field. Therefore, the association between the MJO convection and subsequent CCV heat wave is more evident in Cluster 1 than Cluster 2.

摘 要

本研究探讨了加利福尼亚州中央山谷(CCV)热浪和MJO之间的关系. 这些热浪事件对经济有着重要的影响. 我们之前的研究表明在发生CCV热浪之前, 在热带印度洋和东太平洋上空通常会有对流活动, 表现为MJO位相. 本文所用的主要分析方法为滞后合成分析方法, 即研究每个MJO位相对发生后CCV台站温度, 向外长波辐射(OLR)和速度势(VP)的演变过程. 从OLR和VP的分析结果可以看出, CCV的温度正异常仅仅出现在印度洋(位相2-3)和东太平洋(位相8-1)对流发生之后. CCV极端高温日数也出现在上述两个位相对发生后的两周内. 在热浪事件出现期间, CCV上空呈现低层辐散高层辐合并且伴有显著的下沉运动, 这种结构也是位于印太海洋上空大尺度环流型的一部分. 以往的研究表明CCV热浪可粗略的分为两类, 第二类发生在北美西北部热浪之后, 然而第一类却不是. 我们用OLR和VP的合成分析方法分别分析这两种CCV热浪类型. 对于第二类热浪, 在CCV的下沉运动并不显著, 并且大尺度VP型与MJO滞后合成场之间的相关系数很小. 因此, 第一类CCV热浪呈现出比第二类更强的与前期MJO对流之间的联系.