Subsurface Equatorial Oceans Offer Clues to the Climate Ahead

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Discussion: Anticipated is evolution of an El Nino episode over the next 1-2 months in the eastern equatorial Pacific. However, the vast anomalous warmth across most of the equatorial Pacific Ocean in which the warmest anomalies are located in the East Pacific (usually foreshadowing El Nino ahead) are not reaching the surface leaving ENSO in neutral phase. The anomalous warmth across the entire equatorial subsurface is rare and may mean something other than a standard warm ENSO episode ahead.

One possibility is El Nino Modoki which occurs when the anomalous warmth of an East Pacific El Nino is biased toward the Dateline rather than the northwest coast of South America as a standard El Nino produces. One of the strongest El Nino Modoki episodes on record, evolving during mid-to-late 2002 and peaking during that (northern hemisphere) winter also produced widespread anomalous warmth across the equatorial Pacific prior to onset of El Nino (Fig. 1).

Fig. 1:  ECMWF subsurface global oceans temperature anomaly analysis for June 2018.

Normally, the subsurface equatorial East Pacific is very warm while the subsurface West Pacific is near normal or cooler than normal when an El Nino develops. The importance of this potential different El Nino developing is extremely important. Recent El Nino Modoki episodes have correlated with cold winter seasons in both the eastern U.S. and Europe (versus a more typical mild climate that a standard El Nino produces).

Meanwhile the upper ocean heat in the central/east-central Atlantic equatorial region is cooler than normal (and trending cooler). The cool upper ocean heat signature implies limited energy for hurricane development in the main development region for North Atlantic hurricanes in-between the Caribbean Sea and northwest coast of Africa.

However, the warmer than normal Gulf of Mexico and waters off the U.S. East Coast are the most likely regions to produce and sustain tropical cyclones for the core of the season which is ahead (Aug. 1 to Sep. 30).

Summary: Subsurface equatorial ocean temperatures (and their trend) can foreshadow surface temperature anomalies which correlate to prevailing climate conditions. In the equatorial Pacific the vast subsurface warmth is not dissimilar to June 2002 when El Nino that eventually formed produced a warm bias toward the Dateline in the East Pacific tropics correlating a cold winter in the eastern U.S. (after a mild winter was anticipated). El Nino Modoki is possible for 2018-19. In the North Atlantic tropics upper ocean heat to produce and sustain hurricanes is in low supply.