Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Abstract

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Niño-Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900–2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Niño (La Niña) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO-BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Niño (La Niña) event exhibiting significant positive (negative) SSTAs over the tropical central-eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Niño-related (La Niña-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.

摘 要

本文基于 NCEP-1、 JRA-55 和 ERA-20C 大气再分析资料以及 HadISST 海温再分析资料, 研究了 1900–2017 年间孟加拉湾夏季风 (BOBSM) 爆发年际异常与厄尔尼诺-南方涛动 (ENSO)的关系. 统计分析表明, BOBSM 爆发早晚与 ENSO 事件的依赖关系存在年代际差异, 而且这种年代际变化特征显著地受太平洋年代际振荡 (PDO) 所调控. 在 PDO 正相位期间, El Niño (La Niña) 事件很大可能导致 BOBSM 爆发异常偏晚(早);相反, 在 PDO 负相位期间, BOBSM 爆发早晚与 ENSO 事件并无显著相关关系. 进一步研究表明, PDO 调制 BOBSM 爆发年际异常与 ENSO 依赖关系的物理机制是通过 PDO 有关的海温异常改变 ENSO 有关的海温异常空间结构, 激发不同类型的大气环流异常而影响 BOBSM 爆发时间. 在 PDO 正相位期间, PDO 有关的海温异常与 ENSO 有关的海温异常相叠加使得 El Niño (La Niña) 事件在热带中东太平洋和热带印度洋海温均呈现显著的正(负)异常, 而在热带西太平洋海温则表现为显著的负(正)异常, 从而在热带西太平洋与热带印度洋之间形成了很强的纬向海温异常梯度. 这种纬向海温异常梯度势必在对流层低层激发显著的东风(西风)异常, 同时在对流层高层伴随西风(东风)异常, 进而导致BOBSM爆发异常偏晚(早). 然而, 在 PDO 负相位期间, 无论是 El Niño 年还是 La Niña 年, 热带西太平洋的海温异常均不显著, 因而热带大洋间的纬向海温异常梯度较弱, 无法在孟加拉湾地区激发出有利于 BOBSM 爆发偏早或偏晚的显著大气环流异常.