After the strong 2015/16 El Niño event, cold conditions prevailed in the tropical Pacific with the second-year cooling of the 2017/18 La Niña event. Many coupled models failed to predict the cold SST anomalies (SSTAs) in 2017. By using the ERA5 and GODAS (Global Ocean Data Assimilation System) products, atmospheric and oceanic factors were examined that could have been responsible for the second-year cooling, including surface wind and the subsurface thermal state. A time sequence is described to demonstrate how the cold SSTAs were produced in the central-eastern equatorial Pacific in late 2017. Since July 2017, easterly anomalies strengthened in the central Pacific; in the meantime, wind stress divergence anomalies emerged in the far eastern region, which strengthened during the following months and propagated westward, contributing to the development of the second-year cooling in 2017. At the subsurface, weak negative temperature anomalies were accompanied by upwelling in the eastern equatorial Pacific, which provided the cold water source for the sea surface. Thereafter, both the cold anomalies and upwelling were enhanced and extended westward in the centraleastern equatorial Pacific. These changes were associated with the seasonally weakened EUC (the Equatorial Undercurrent) and strengthened SEC (the South Equatorial Current), which favored more cold waters being accumulated in the central-equatorial Pacific. Then, the subsurface cold waters stretched upward with the convergence of the horizontal currents and eventually outcropped to the surface. The subsurface-induced SSTAs acted to induce local coupled air-sea interactions, which generated atmospheric-oceanic anomalies developing and evolving into the second-year cooling in the fall of 2017.

中文翻译：

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风应力和地下冷水在 2017/18 拉尼娜事件第二年降温中的作用

2015/16 年强厄尔尼诺事件后，热带太平洋地区普遍寒冷，2017/18 年拉尼娜事件第二年降温。许多耦合模型未能预测 2017 年的冷海温异常（SSTA）。通过使用 ERA5 和 GODAS（全球海洋数据同化系统）产品，检查了可能导致第二年冷却的大气和海洋因素，包括地表风和地下热状态。描述了一个时间序列来说明 2017 年末赤道中东部太平洋冷 SSTA 是如何产生的。自 2017 年 7 月以来，中太平洋东风异常加强；与此同时，远东地区出现风应力发散异常，并在随后几个月加强并向西传播，2017 年第二年变冷的发展。在次表层，弱负温度异常伴随着赤道东太平洋的上升流，为海面提供了冷水源。此后，赤道中东部地区冷异常和上升流均增强并向西延伸。这些变化与季节性减弱的 EUC（赤道暗流）和加强的 SEC（南赤道海流）有关，这有利于在赤道中太平洋积聚更多冷水。然后，地下冷水随着水平流的汇聚向上伸展，最终露出水面。地下诱导的 SSTA 作用于诱导局部耦合的海气相互作用，