Roles of Wind Stress and Subsurface Cold Water in the Second-Year Cooling of the 2017/18 La Niña Event

After the strong 2015/16 El Nino event, cold conditions prevailed in the tropical Pacific with the second-year cooling of the 2017/18 La Nina 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.