Discussion: During 2018 one of the strongest year-long positive phase of the Pacific meridional modes (PMM) on record occurred. The index is measured with both sea surface temperature anomalies (SSTA) and wind anomalies. +PMM are characterized by trade winds blowing across the equator from cooler SSTA to the south toward warmer SSTA to the north (of the equator).
The result is anomalous warm SSTA in the tropics/subtropics of the eastern North Pacific Ocean as observed in 2018 (Fig. 1). The northern shift of anomalous warm SSTA also causes associated convection to shift north affecting climate. A close analog to 2018 is 1986 (Fig. 2-3).
1986 is an interesting ENSO year as El Nino Modoki developed late in the year as demonstrated by anomalous warm Nino34 SSTA compared to much cooler Nino12 SSTA. Interestingly, a very similar Nino SSTA structure is present in 2018 (Fig. 4).
Reasonable is the expectation that El Nino, forecast by all models for late 2018 into 2019 evolves as an El Nino Modoki given the persistence of surface and subsurface ocean warming near and just east of the Dateline (Fig. 5) in recent months (Fig. 5). Southern oscillation index (SOI) has been very El Nino-like the past 2 months implying the atmosphere is linking with the anomalous warm equatorial Pacific. However, the –SOI influence bringing a warmer Nino12 regime in early-to-middle September reversed last week and the waters off the northwest coast of South America suddenly cooled (Fig. 6).
While there’s certainly a case for El Nino Modoki for northern hemisphere winter 2018-19 there is also the likelihood that a conventional El Nino does eventually develop in 2019. A strong conventional El Nino developed in 1987. Validating the 1987 analog is the presence of widespread equatorial Pacific Ocean subsurface anomalous heat now (Fig. 7) more than likely to shift into the East Pacific next year.
Taking a look at 1986 as an analog year reveals striking similarities to 2018. The very wet pattern in the Great Plains during late summer and early autumn which is about to turn much wetter also occurred during SEP/OCT 1986 (Fig. 8). Was there a 1987 precipitation anomaly of interest caused by a strong El Nino? Yes! An intense drought affecting the Southeast U.S. during summer and autumn (Fig. 9). This scenario is quite possible in 2019 if the ENSO regime unfolds similarly to 1986-87 which is occurring (so far).
Fig. 1: One of the strongest +PMM annual regimes on record has caused warm SSTA to shift north of the equatorial zone demonstrated in the 90-day global SSTA observations.
Fig. 2: The SSTA component of +PMM in 2018 is similar (so far) to 1986.
Fig. 3: The wind component of +PMM in 2018 is similar (so far) to 1986.
Fig. 4: Late 1986 brought El Nino Modoki as the Nino34 (east-central Pacific) warmed past the El Nino threshold while off the northwest coast of South America (Nino12) was cooler. A similar scenario is developing in 2018.
Fig. 5: Upper ocean heat analysis by NOAA identifies persistent anomalous warmth near and just east of the Dateline reluctant to shift east typical of conventional El Nino.
Fig. 6: The Nino12 region off the northwest coast of South America warmed into El Nino threshold early-to-middle September implying conventional El Nino was on the way until the past 7-10 days when sudden cool reversal occurred.
Fig. 7: Widespread impressive subsurface anomalous warmth stretches across the equatorial Pacific Ocean.
Fig. 8: A potential analog for 2018 climate is 1986 due to the evolution of El Nino Modoki. During SEP/OCT 1986 excessive wet weather occurred in the Great Plains similar to 2018.
Fig. 9: A potential analog for 2018 climate is 1986 due to the evolution of El Nino Modoki. During SEP/OCT 1986 excessive wet weather occurred in the Great Plains similar to 2018.