Influence of the Eastern Pacific and Central Pacific Types of ENSO on the South Asian Summer Monsoon

Abstract

Based on observational and reanalysis data, the relationships between the eastern Pacific (EP) and central Pacific (CP) types of El Niño-Southern Oscillation (ENSO) during the developing summer and the South Asian summer monsoon (SASM) are examined. The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s. While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically, the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s. The drought-producing El Niño warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line. Accordingly, compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent, leading to suppressed convection and decreased precipitation over these regions. Moreover, anomalously less moisture flux into South Asia associated with developing EP El Niño and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Niño, may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods, respectively. El Niño events with the same "flavor" may not necessarily produce consistent Indian monsoon rainfall anomalies, while similar Indian monsoon droughts may be induced by different types of El Niño, implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.

摘要

基于观测和再分析资料,本文研究了东太平洋型和中太平洋型厄尔尼诺–南方涛动(ENSO)在发展年夏季与南亚夏季风的关系。这两种不同形态的ENSO对南亚夏季风的影响在二十世纪70年代末呈现出明显的多年代际调整。二十世纪70年代末以后,相较于东太平洋型ENSO与南亚夏季风之间的反向关系的显著减弱,中太平洋型ENSO在导致印度季风降水异常上发挥着更为重要的作用。能够引起南亚夏季干旱的中、东太平洋型厄尔尼诺暖异常通常会在赤道中太平洋160°W至日界线附近激发出异常上升气流;补偿性的异常下沉气流相应地出现在海洋大陆至印度次大陆,从而抑制了这些地区的对流和降水。此外,东太平洋型ENSO发展过程中南亚地区水汽输送的明显减少以及中太平洋型ENSO发展过程中控制印度南部的显著异常西北气流也可以进一步解释发生在1979年前、后两个时期的印度季风干旱。总而言之,同一类型的厄尔尼诺未必会引起一致的印度季风降水异常,而不同类型的厄尔尼诺也有可能导致类似的印度夏季严重干旱,这充分表明了南亚季风降水的响应对于热带太平洋海温异常强迫的具体分布形态非常敏感。