The Roles of Off-Equatorial Subsurface Cold-Water Incursions in Triggering the Second-Year Cooling of the La Niña Event in 2021

Abstract

During 2020–2022, a consecutive two-year La Niña event occurred in the tropical Pacific. This work analyzes the evolution of atmospheric and oceanic anomalies in the equatorial Pacific between 2020 and 2022 to describe and illustrate the processes that led to the second-year cooling of the La Niña event in 2021/22. After the first year of the La Niña event in 2020/21, neutral SST conditions resumed in the eastern equatorial Pacific prior to June 2021. However, in the first half of 2021, significant cold water persisted at the surface and thermocline depth in the off-equatorial areas, which were more noticeable in the tropical South Pacific. The turning point of the second-year cooling event occurred in June 2021 when the subsurface cold water from the off-equatorial regions intruded into the equatorial regions between approximately 115°–125° W. This interrupted the eastward propagation of warm water along the thermocline. Negative SST anomalies and positive SLP anomalies occupied the tropical southeast Pacific, which strengthened southeasterly wind stress anomalies from the tropical South Pacific, driving them northward across the equator. Subsequently, the subsurface ocean current that originated from the tropical southeast Pacific began to prominently propagate to the northwest. This transported cold water across the equator, which then mixed with the cold subsurface waters in the tropical north Pacific. Finally, negative ocean temperature anomalies gradually intensified from the subsurface to the surface throughout the tropical central-eastern Pacific, which initiated a local air–sea coupling process that led to the second-year cooling of the La Niña event through autumn 2021.