Subsurface Equatorial Pacific is Key to ENSO Forecast

Fig. 1: Current equatorial Pacific Ocean subsurface temperature anomalies.

Fig. 2: April 2015 subsurface equatorial Pacific Ocean temperature anomalies prior to the intense 2015 El Nino.

Fig. 3: April 1997 subsurface equatorial Pacific Ocean temperature anomalies prior to the intense 1997 El Nino.

Discussion: The warming trend in the equatorial Pacific Ocean subsurface during the past 2-3 weeks is impressive (Fig. 1). The warming was triggered by the convection phase of the Madden Julian oscillation (MJO) easing or shutting down trade winds and allowing an attendant subsurface Kelvin Wave to push warm waters eastward to the northwest coast of South America. Except for shallow waters to the immediate east of the Dateline, the entire equatorial Pacific subsurface is warmer than normal.

Dynamic models (ECMWF, NCEP CFS V2, POAMA, and IMME) are forecasting an intense El Nino to develop after midyear potentially similar in strength to the historic 1997 and 2015 El Nino episodes. Note that during April 2015 (Fig. 2) and 1997 (Fig. 3) the subsurface equatorial Pacific warmth was robust, but all the warming was to the east of the Dateline. West of the Dateline, waters were generally cooler than normal in the subsurface equatorial region. Currently, despite the vigorous warming east of the Dateline, the core of warm subsurface environment remains west of the Dateline typical of when La Nina is present.

The core of the warmth west of the Dateline presents two possibilities for ENSO phase, most likely during the middle third of 2023. First, the warm core stays west of the Dateline and the recent warmth across the subsurface equatorial Pacific persists but cannot intensify leaving the most likely ENSO scenario ahead for MAY-SEP 2023 a weak El Nino. Second, the warm core west of the Dateline roars eastward inspired by additional transient MJO events. If so, a vigorous El Nino, similar to 2015 and 1997 emerges just after mid-year. Climate Impact Company assigns a 70% probability to option 1 and a 30% probability to option 2. ENSO statistical/analog forecasts have a tendency to offer a weaker El Nino ahead and are preferred for now.

Meanwhile, despite the fireworks in the subsurface, the Nino34 SSTA slowly edges toward El Nino currently at 0.3C (Fig. 4).

Fig. 4: Running 12-month analysis of upper ocean heat identifies the surge in recent warming.