Special ENSO Update: Risk of El Nino Modoki Later 2018

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Discussion: Currently, oceanic warming of the equatorial Pacific Ocean near the Dateline to the east-central equatorial Pacific (in the Nino34 region) is occurring (Fig. 1). Trade winds have decreased in this area to allow substantial subsurface anomalous warmth to emerge at surface level and an El Nino signature (SSTA >0.5C) in the Nino4/Nino34 regions is likely over the next 1-2 months.

However, trade winds have not decreased in the far eastern equatorial Pacific Ocean off the northwest coast of South America (Fig. 2). In this zone the Nino12 SSTA region remains quite cool (-1.0C). The Pacific meridional model (PMM) is in moderate positive phase which indicates wind flow is across the equator from the southern hemisphere toward the northern hemisphere. In this scenario sea surface temperature anomalies (SSTA) tend to be cool in the northeast South Pacific and warm in the southeast North Pacific and this scenario is certainly present now (Fig. 3).

The NCEP CFS V2 global SSTA forecasts for SEP/OCT/NOV 2018 indicate El Nino warming near the Dateline to east-central equatorial East Pacific (Fig. 4). However, the El Nino warming is very weak in the far eastern equatorial Pacific Ocean while the far western equatorial Pacific is neutral. These conditions represent a weak El Nino Modoki which is forecast by the Japan Agency for Marine-Earth Science and Technology (Fig. 5) and agreed upon by Climate Impact Company. An El Nino Modoki will cause a different kind of (El Nino) global climate for later 2018.

Fig. 1: The Nino SSTA regions indicate sudden warming toward El Nino qualifications of the Nino4, Nino34 and Nino 3 regions (central/east-central equatorial Pacific Ocean). However, the Nino12 zone (off the northwest coast of South America) is La Nina cool.

Fig. 2: Trade winds remain active in the far eastern equatorial Pacific Ocean while easing near the Dateline and east-central Equatorial Pacific.

Fig. 3: Global SSTA analysis reveals steady El Nino warming in the central/east-central equatorial Pacific (but not off the northwest coast of South America). Cool SSTA in the northeast South Pacific and warm SSTA in the southeast North Pacific is characteristic of the positive phase of the Pacific Meridional Mode which is sustaining trade winds off the northwest coast of South America.

Fig. 4: The NCEP CFS V2 global SSTA forecast for SEP/OCT/NOV 2018 identifies a full-blown El Nino episode centered on the equatorial Pacific just east of the Dateline. The El Nino-like warming off the northwest coast of South America is very weak (and could be weaker than indicated). The far western equatorial Pacific SSTA is also near normal. Indicated is a weak El Nino Modoki signature, stronger if the warm core shifts slightly farther west to near the Dateline.

Fig. 5: Weak El Nino Modoki is indicated for the last third of 2018 continuing into northern hemisphere spring.