Multivariate ENSO Index (Analog) Suggests La Nina Continues Well Into 2021

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ENSO discussion: Last week ENSO remained in full-throttle mature phase of La Nina. In recent days some surface warming has developed in the far eastern equatorial East Pacific off the northwest coast of South America. La Nina appears to have started weakening.

The multivariate ENSO index (MEI) for NOV/DEC 2020 was -1.2. This value represents moderate intensity La Nina. The MEI is an excellent ENSO diagnostic as the index measures the reaction of the atmosphere to the cooling waters of the equatorial East Pacific.

During the mature phase of La Nina, typically occurring in DEC/JAN for La Nina events since the middle 1990’s a mature phase La Nina MEI of <-1.0 was typically followed by sustained La Nina whereas MEI >-1.0 were more likely to produce La Nina dissipation and possibly El Nino later in the year (Fig. 1).

Currently, a collection of ALL dynamic and statistical ENSO phase forecast models indicate La Nina ends by northern hemisphere spring and is followed by neutral phase (Fig. 2). Some models indicate weak El Nino ahead for later this year.

Currently, the global oceans are quite warm and the La Nina signature in the equatorial East Pacific appears to be weakening (Fig. 3). The global SSTA pattern forecast for May 2021 by ECMWF (also) indicates dissipation of La Nina (Fig. 4).

The one supporting diagnostic for La Nina continuing as indicated by the MEI-based analog is the persistent cool water in the subsurface equatorial Pacific (Fig. 5). The subsurface water east of the Dateline needs to turn warmer as a leading indicator that La Nina will weaken.

A La Nina climate supports drought in the West U.S. and developing in the Southeast States while spring is wet in the Ohio Valley. As observed last year La Nina also supports a very active tropical cyclone season. For now, the ENSO phase forecast for the middle third of 2021 is a low confidence projection.

Fig. 1: Using multivariate ENSO index (MEI) a comparison of La Nina events since 1995 is reviewed. The December 2020 MEI was -1.2. Note that mature phase La Nina (typically occurring in DEC/JAN) when MEI was <-1.0 have a tendency to continue into the following year. MEI >-1.0 during mature phase are more likely to fade and potentially transition toward weak El Nino.

Fig. 2: A collection of all ENSO phase forecasts from both dynamic and statistical models provided by the International Research Institute for Climate and Society indicate La Nina ends, neutral ENSO develops and El Nino is possible later this year.

Fig. 3: Current global SSTA analysis reveals a warm ocean! In the equatorial East Pacific La Nina continues but looks weaker in recent days,

Fig. 4: The ECMWF global SSTA forecast for May 2021 dissipates La Nina although cool SSTA shifts to the south of the equator. Note the warm mid-latitude Pacific and Atlantic which will support anomalous hot summer 2021.

Fig. 5: The subsurface equatorial East Pacific is cooler than normal and offers plenty of fuel to sustain La Nina. The cool subsurface needs to turn warmer for dynamic/statistical models to verify an ending of La Nina forecast otherwise the MEI-based analog will be correct.