On Again/Off Again El Nino

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An indicator of the atmospheric connection to El Nino warming of the equatorial East Pacific is the southern oscillation index. After a robust EL Nino signature in FEB/MAR the evolving El Nino climate fades as strong +SOI emerges.

On again/off again El Nino is weakening

Discussion: As Bureau of Meteorology/Australia has stated, we are entering the prediction barrier time of year for ENSO forecasts as most El Nino and La Nina events tends to develop in a somewhat obvious manner during late year while northern hemisphere spring offers the widest range of possibilities therefore most difficult forecast. Currently, weak El Nino remains intact but the Nino SSTA regions have been trending a little cooler the past 2 weeks (Fig. 1).

Fig. 1: The 12-week Nino SSTA region observation trend identifies presence of El Nino but the east-central Pacific (N4, N34) cools the past 2 weeks.

The coupling of the atmosphere and warming equatorial East Pacific FINALLY occurred in February as a long duration El Nino-like period of negative phase southern oscillation (SOI) index emerged (Fig. 2). However, the SOI has flipped positive in April identifying the El Nino climate of FEB/MAR has subsided (and ended). A returning –SOI pattern usually driven by a westerly wind burst (WWB) in the tropical Pacific or transient Madden Julian oscillation (MJO) is required to cause a meaningful El Nino-like SOI to return and that regime is not seen through the next 2-3 weeks (at least).

Fig. 2: The southern oscillation index was very El Nino-like negative for February into March but is now flipping positive indicating whatever El Nino climate we had is dissipating.

Possibly the most revealing chart is the upper ocean heat analysis in the equatorial Pacific Ocean to the east of the Dateline (Fig. 3). Indicated is a second resurgent peak of subsurface warming early last month but once again, similar to late last autumn the upper ocean heat is diminishing. Will the borderline El Nino of late last autumn shifting to neutral ENSO during winter happen again this (late) spring?

Weak El Nino has limited summertime influence on climate except in the tropics. If El Nino weakens the tropical cyclone season in the North Atlantic basin will be stronger than initial forecasts indicate.

Fig. 3: Upper ocean heat analysis east of the Dateline during the past 1 year. Indicated is plenty of upper ocean heat to sustain an El Nino with a second just-finished peak. However, despite the ocean warming the atmosphere has struggled to respond with a full-blown El Nino episode.