Triple‐Dip La Niñas in 1998–2001 and 2020–2023: Impact of Mean State Changes

Abstract

This study compares the evolution of atmospheric and oceanic anomalies as well as predictions for the two most recent triple‐dip La Niña events in 1998–2001 and 2020–2023. Subsurface cooling in the equatorial Pacific was stronger and more persistent during 1998–2001. In contrast, surface easterly winds were stronger during 2020–2023 as was the east‐west sea surface temperature (SST) contrast along the equator. We argue that in the absence of appreciable equatorial Pacific heat discharge, persistent and strong surface trade winds and a strengthened mean zonal SST contrast across the tropical Pacific contributed to the development of the 2020–2023 triple‐dip La Niña. In terms of the subsurface layer heat budget, the growth and maintenance of unusually cold SSTs during the triple‐dip La Niña in 1998–2001 were mainly the result of ocean vertical entrainment and diffusion, as well as meridional advection, associated with enhanced equatorial upwelling; while for the triple‐dip La Niña in 2020–2023, zonal advection was the largest contributor. The two events were mostly well predicted by multi‐model averages at 1–8 months lead times. We hypothesize that mean state change with enhanced zonal SST contrast and trade winds over the last several decades altered the physical processes associated with the growth and maintenance of the most recent La Niña, affecting its predictability. Successful prediction in real‐time of the 2020–2023 event more than half a year in advance was surprising because there was little memory in oceanic heat content which is often considered a key predictor.

Dominant contribution of atmospheric nonlinearities to ENSO asymmetry and extreme El Niño events

“…Extreme El Niño events strongly enhance the overall El Niño/Southern Oscillation (ENSO) asymmetry 7 , 8 , with their large amplitude in the eastern Pacific indeed intensifying and shifting the El Niño signals eastward relative to those of La Niña 9 . The ENSO phase transition also displays asymmetry 10 , often transitioning from extreme El Niño to La Niña, while La Niña events can persist for multiple years, as exemplified by triple-dip La Niña 11 from mid-2020 to early 2023. ENSO amplitude, spatial pattern and duration asymmetries are closely tied to its diversity 12 .…”

On the Divergent Evolution of ENSO After the Coastal El Niños in 2017 and 2023

“…For the coastal El Niño in 2017, SSTA warming in the Niño1+2 region began in late 2016 and was larger than 0.5°C during January-April 2017 with a peak value of 1.8°C in March 2017 (the black line in Figure 2a). The Niño3.4 index (the shading in Figure 2a) was negative in July-December 2016, consistent with the La Niña conditions, Similarly, SSTA warmed up quickly in the Niño1+2 region beginning in late 2022 after the triple-dip La Niña during 2020-2023 (Hasan et al, 2022;Jiang et al, 2023;Li et al, 2022Li et al, , 2023bZhang et al, 2022) (the black line in Figure 2b). The peak of the coastal El Niño appeared in August 2023 with the value of 3.30°C for the Niño1+2 index, exceeding 3 standard deviations (Figure S1a in Supporting Information S1).…”

“…Similarly, SSTA warmed up quickly in the Niño1+2 region beginning in late 2022 after the triple‐dip La Niña during 2020–2023 (Hasan et al., 2022; Jiang et al., 2023; Li et al., 2022, 2023b; Zhang et al., 2022) (the black line in Figure 2b). The peak of the coastal El Niño appeared in August 2023 with the value of 3.30°C for the Niño1+2 index, exceeding 3 standard deviations (Figure S1a in Supporting Information ).…”

Multi‐Time Scale Variations in Atlantic Niño and a Relative Atlantic Niño Index

“…(2020) suggested that an enhanced zonal contrast along the tropical Pacific with appreciable warming in the west and slight cooling in the east may affect the ENSO evolution and prediction. Such impact was evident in the triple‐dip La Niña during 2020–2023 (Li, Hu, et al., 2023).…”