Distinct Evolution of the SST Anomalies in the Far Eastern Pacific between the 1997/98 and 2015/16 Extreme El Niños

Abstract

The 2015/16 El Niño displayed a distinct feature in the SST anomalies over the far eastern Pacific (FEP) compared to the 1997/98 extreme case. In contrast to the strong warm SST anomalies in the FEP in the 1997/98 event, the FEP warm SST anomalies in the 2015/16 El Niño were modest and accompanied by strong southeasterly wind anomalies in the southeastern Pacific. Exploring possible underlying causes of this distinct difference in the FEP may improve understanding of the diversity of extreme El Niños. Here, we employ observational analyses and numerical model experiments to tackle this issue. Mixed-layer heat budget analysis suggests that compared to the 1997/98 event, the modest FEP SST warming in the 2015/16 event was closely related to strong vertical upwelling, strong westward current, and enhanced surface evaporation, which were caused by the strong southeasterly wind anomalies in the southeastern Pacific. The strong southeasterly wind anomalies were initially triggered by the combined effects of warm SST anomalies in the equatorial central and eastern Pacific (CEP) and cold SST anomalies in the southeastern subtropical Pacific in the antecedent winter, and then sustained by the warm SST anomalies over the northeastern subtropical Pacific and CEP. In contrast, southeasterly wind anomalies in the 1997/98 El Niño were partly restrained by strong anomalously negative sea level pressure and northwesterlies in the northeast flank of the related anomalous cyclone in the subtropical South Pacific. In addition, the strong southeasterly wind and modest SST anomalies in the 2015/16 El Niño may also have been partly related to decadal climate variability.

摘 要

尽管2015/16和1997/98均为超强厄尔尼诺,但2015/16年厄尔尼诺远东太平洋SST暖异常却比1997/98年厄尔尼诺弱很多。探究其差异背后的原因有助于增加我们对厄尔尼诺多样性的理解和认识。通过诊断分析和模式敏感性试验,本研究表明较弱的2015/16年远东太平洋SST暖异常与较强的东南太平洋东南风异常密切相关。混合层热量收支诊断方程表明东南风异常通过增强洋流的垂直上翻,西向运动以及表面蒸发来减缓SST暖异常的增加。较强的东南风异常不仅与2014年底赤道中东太平洋SST暖异常有关,还与赤道外东北太平洋SST暖异常以及东南太平洋SST冷异常有关。另外,2015/16年超强厄尔尼诺中较强的东南风异常以及较弱的SST暖异常还可能与年代际的气候变率有关。