Why Is The Atmosphere Not Coupling With El Nino Warming?
Discussion: El Nino warming of the eastern equatorial Pacific Ocean and all forecast models indicate El Nino is with us now or will be shortly. But! The atmosphere is unaware of the El Nino warming. The atmosphere is not coupling with the El Nino oceanic warming which is needed to drive convection in the East Pacific tropics across the warm El Nino waters which in-turn release heat poleward and cause the jet stream orientation to change and deliver a mild Pacific maritime climate to the U.S. for winter.
Fig. 1: Multivariate ENSO index identifies very weak reaction of the atmosphere to the El Nino warming of the East Pacific. The Nino34 (region) of the East-central equatorial Pacific is where ENSO phase is evaluated.
One possible reason for the lack of conventional El Nino signatures in equatorial convection and attendant sea level pressure/sea surface temperature (anomaly) patterns may be the persistence of ocean warming near the Dateline the past 6+ months and still present now. This is one reason why the Hawaiian waters experienced so many rare hurricanes this year. While technically not an El Nino Modoki the climate pattern certainly has a Modoki look most obvious in North America.
Fig. 2: El Nino warming has reached the equatorial East Pacific typical of conventional El Nino. But the heat budget produced by El Nino warming is most robust to the west toward the Dateline.
El Nino Modoki means the warming of the eastern equatorial Pacific, typical of conventional El Nino is biased west – toward the Dateline. El Nino Modoki is easily measured by comparing sea surface temperature anomalies in the eastern equatorial Pacific versus the area near the Dateline and if the Dateline is significantly warmer a Modoki is present. But now the warming in the equatorial region is similar while the surrounding area is much warmer toward the Dateline providing a larger ocean heat budget which is affecting the atmosphere.
Fig. 3: Contributing to the second most expansive North America snow cover on record (for Nov. 1st) is the El Nino signature in the tropical Pacific as the farther western tropical heat budget favors warming in western North America while downstream the polar vortex spreads snow cover into the U.S.
Effects on the atmosphere are abundant. One example of ENSO warming closer to the Dateline is the tendency to cause western North America warmth and downstream activation of the polar vortex. During OCT/NOV 2018 as the cold season arrives the pattern described has emerged and produced near record snow cover across North America. Snow cover biases the U.S. climate colder! Sharp cold is emerging over the U.S. for middle November.