La Nina to Hang On!

Highlight: La Nina hanging around for a while. Weakens JUN/JUL but could return SEP/OCT. Very supportive of a big year in the North Atlantic tropics.

Discussion: In December 2021, a robust convective phase of the Madden Julian oscillation (MJO) shifted into the equatorial East Pacific (Fig. 1). The MJO caused trade winds associated with La Nina to break down and suppressed the up-welling of cool water process that maintains La Nina. This set of circumstances allowed the eastward progression of anomalous warm water born in the equatorial West Pacific to also shift eastward. The subsurface warming known as a Kelvin Wave gradually shifted through the eastern equatorial Pacific during JAN/FEB 2022 finally reaching the northwest coast of South America in March. During April, the Kelvin Wave has slowly eroded and a resurgent cool subsurface in the equatorial East Pacific has returned. The Kelvin Wave weakened the intensity of La Nina during quarter 1 of 2022. However, now that the Kelvin Wave has faded, the Nino SSTA regions are cooler and La Nina has reorganized. The next Kelvin Wave has formed in the subsurface equatorial West Pacific and shifted eastward to about 160W (Fig. 2). Further eastward progress by the Kelvin Wave occurs if the MJO is (also) transiting the eastern equatorial Pacific. At this time, MJO has become weak and there is uncertainty as to whether redevelopment and eastward progression across the eastern equatorial Pacific will occur. La Nina will gain some strength if the MJO fails but lose strength and possibly dissipate if the MJO reactivates and allows passage of another kelvin Wave through the East Pacific. Currently, the southern oscillation index (SOI) is very positive indicating the trade winds are strong and sea level pressure (SLP) in the East Pacific tropics above normal, each diagnostic is very supportive of La Nina. Additionally, the multivariate ENSO index (MEI) observed during March was -1.3 which is a moderate to strong signature (Fig. 3). The global climate pattern is firmly in La Nina mode and very resistant to any change. Consequently, the Kelvin Wave in the East-central equatorial Pacific Ocean may shift eastward slowly but weaken the next 1-2 months and assuming a reached destination off the northwest coast of South America, the attendant Kelvin Wave is similar to the FEB/MAR event which is not strong enough to kill-off La Nina. The Nino34 SSTA, a measurement of the oceanic component of ENSO phase is likely slow to fade (Fig. 4) to neutral as indicated by most models while MEI is locked-in a La Nina regime through the middle third of 2022. In late 2022 ENSO phase is very questionable. However, development of El Nino is unlikely while return of La Nina for late this year has a slightly higher risk.

Fig. 1: Tracking upper ocean heat in the equatorial Pacific Ocean helps to identify ENSO phase and any disruptions related to MJO and attendant Kelvin Waves.

Fig. 2: The early April equatorial Pacific Ocean subsurface temperature anomalies which identify a fading Kelvin Wave in the far eastern equatorial Pacific and a new Kelvin Wave shifting just past the Dateline.

Fig. 3: Multivariate ENSO index strengthened in La Nina phase last month.

Fig. 4: A collection of all dynamic and statistical ENSO forecast models is trending toward delayed ending of La Nina in JUN/JUL and a possible return of weak La Nina in SEP/OCT.