A Historical Perspective of the La Nina Event in 2020/2021

El Nino-Southern Oscillation is the strongest interannual variability in the tropical oceans and the major source of global climate predictability. In this work, we examine the evolution of oceanic and atmospheric anomalies in the tropical Pacific during 2020/2021 La Nina and compare it with the historical strong La Nina events since 1982, identify the contributions of different time scale components, and assess the predictions and the impact on extra-tropical climate. 2020/2021 La Nina emerged in August 2020 and dissipated in May 2021. 2020/2021 La Nina was uniquely preceded by a borderline El Nino instead of an El Nino and a weak equatorial-heat discharge process. That resulted in the weakest event among the strong La Ninas since 1982, although there were strong upwelling Kelvin wave activities. Moreover, compared with other strong La Nina events, the surface easterly wind anomalies and the warm pool extended further eastward in 2020/2021 La Nina, linking to a relatively weaker dipole-like pattern of the subsurface ocean temperature anomalies. The strength of all the strong La Nina events is determined by the in-phase amplification of all time scale variations. Their decay in the boreal spring and early summer is mainly controlled by the intra seasonal-inter seasonal variation. 2020/2021 La Nina was successfully predicted, however, the North American climate anomalies did not match the typical La Nina response, leading to low prediction skill in the extra-tropics during its mature phase.

Automated summary

This study examines the evolution and impact of the 2020/2021 La Nina event and compares it with historical events. It found that the La Nina event during this period was weaker than expected and showed some differences compared to other events, indicating that the in-phase amplification of time scale variations is an important factor determining the intensity of La Nina events.