Near-average sea surface temperatures (SST) were evident in the east-central Pacific Ocean during most of September, though SST anomalies increased during the past couple of weeks [Fig. 1]. In the last week, the SST indices in the westernmost Niño-4 and Niño-3.4 regions were +1.0°C and +0.5°C, respectively, and the indices in the easternmost Niño-3 and Niño-1+2 regions remained near-to-below average (+0.3°C and -0.6°C respectively; [Fig. 2]). The subsurface temperature anomalies (averaged across 180°-100°W) increased during the month [Fig. 3] partially because a downwelling oceanic Kelvin wave expanded eastward [Fig. 4]. This wave was triggered by low-level westerly wind anomalies across the western and central equatorial Pacific Ocean. At upper-levels, easterly wind anomalies prevailed over much of the Pacific during September. Also, the region of suppressed convection over Indonesia intensified and expanded to the Date Line [Fig. 5]. Despite the recent warming, the overall oceanic and atmospheric system remained consistent with ENSO-neutral.
The majority of models in the IRI/CPC plume [Fig. 6] continue to favor ENSO-neutral (Niño-3.4 index between -0.5°C and +0.5°C) through the Northern Hemisphere spring. Many dynamical forecast models, including the NCEP CFSv2, suggest Niño-3.4 SST index values will remain near +0.5°C during the next month or so before decreasing, but remaining above zero. Consequently, forecasters believe the recent oceanic warmth reflects sub-seasonal variability and is not indicative of an evolution toward El Niño. However, chances for El Niño remain between approximately 25-30% through the winter and spring. In summary, ENSO-neutral is favored during the Northern Hemisphere fall 2019 (~85% chance), continuing through spring 2020 (55-60% chance; click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).