The El Niño–Southern Oscillation (ENSO) is the dominant mode of inter-annual climate variability on Earth. It is generally characterized as an evolution of relatively strong warm El Niño and relatively weak cold La Niña. ENSO episodes usually tend to last for 1-2 years and recur every 3-7 years. Most El Niño events tend to decay rapidly after its peak and turn into La Niña events, whereas many La Niña events persist through the following year and usually re-intensify in the second year. The long-lasting La Niña events following big El Niño events, like in 1982/1983, 1997/1998, and 2015/2016 as well as the 2010-2012 La Niña event, can be predicted up to 2 years in advance. Nevertheless, the second La Niña events, especially in case of the 2017/2018 La Niña event, are more difficult to predict as compared to the first La Niña events.
During the recent decades, both the tropical Indian and Atlantic Oceans have exhibited a rapid warming, implying that the warming trends in the other oceans may considerably contribute to the long-lasting easterly anomalies in the central equatorial Pacific.
Therefore, the research group of Professor Shuanglin Li that is working on the project of Antarctic climate change and its impact on East Asia summer climate under the CASEarth Poles project investigated the possible reasons for the occurrences of these second La Niña events. Based on the five long-lasting La Niña events that occurred after 1980, they found that the multi-year persistent easterly anomaly in the central equatorial Pacific is a key condition facilitating the development of second La Niña events, particular to the occurrence of the 2017/2018 La Niña event.
The occurrence of the second La Niña events, especially the 2017/2018 La Niña event, resulted from large warm sea surface temperature (SST) anomalies in the tropical Indian and Atlantic Oceans that acted to force the persistent easterly anomaly in the Pacific via modifying the Walker Circulations. About 24% of the variance of the Pacific dU/dt can be statistically explained by the tropical Indian Ocean and Atlantic SST anomalies.
Figure 1. Time series of (a) Niño 3.4 indices (unit: °C), (b) WWV anomalies (unit: m) in the equatorial Pacific (5°S to 5°N, 140°E to 80°W), and (c) zonal wind anomalies (unit: m/s) in the central equatorial Pacific (5°S to 5°N, 150°E to 150°W) for the five long-lasting La Niña events in 2016-2018 (red dotted lines), 2010-2012 (orange dotted lines), 2007-2009 (purple dotted lines), 1998-2000 (blue dotted lines), and 1983-1985 (brown dotted lines). Gray shadings indicate the peaks of the first and second La Niña events, respectively.
Figure 2. Schematic diagram for the impacts of the Indian Ocean and Atlantic SST on the Pacific climate. The Indian Ocean and Atlantic SSTs intensify the zonal wind in the equatorial Pacific by modulating the Walker circulation that connects the three tropical oceans and thus affects the ENSO through the Bjerknes feedback.
Reference: Zhang, C., Luo, J.-J., & Li, S. (2019). Impacts of tropical Indian and Atlantic Ocean warming on the occurrence of the 2017/2018 La Niña. Geophysical Research Letters, 46(6), 3435-3445. https://doi.org/10.1029/2019GL082280.