SW Climate Outlook - ENSO Tracker - Sept 2017
Oceanic and atmospheric indicators remain within the range of neutral but have shifted more towards La Niña conditions in the past month (Figs. 1-2). Seasonal outlooks and forecasts reflect these changes, and most now see La Niña conditions as the more likely outcome for fall 2017, with ENSO-neutral conditions mostly still favored for winter. On Sept. 11, the Japanese Meteorological Agency (JMA) forecast that ENSO-neutral conditions had a 60-percent chance of persisting through winter 2018, but that the chance of La Niña had increased to 40 percent. On Sept. 12, the Australian Bureau of Meteorology ENSO tracker remained at neutral/inactive; they noted that the tropical Pacific is on a cooling trajectory (supporting potential La Niña formation) but that other indicators remained within the range of ENSO-neutral. On Sept. 14, the NOAA Climate Prediction Center (CPC) observed that oceanic and atmospheric conditions were shifting towards La Niña, with a 55- to 60-percent chance of a La Niña event in winter 2017-2018. On Sept. 21, the International Research Institute for Climate and Society (IRI) and CPC identified cool oceanic conditions and that “La Niña odds edge out neutral” over winter 2017-2018. The North American Multi-Model Ensemble (NMME) is ENSO-neutral as of September 2017 (Fig. 4), with a majority of the models predicting ENSO neutral or weak La Niña this winter.
Summary: In a slight shift from the last few months’ forecasts favoring ENSO-neutral conditions for this fall and winter, seasonal outlooks have turned more bullish on a weak La Niña event this fall. While many of the ENSO indicators remain within the range of neutral, oceanic indicators in particular have moved towards La Niña. What has caused the slight uptick in the likelihood of a 2017-2018 La Niña event that we noted in last month’s forecasts to become more pronounced? One answer is related to the way that expert forecasters respond to model outputs. The CPC/IRI outlook on Sept. 14 stated that “a majority of the models in the IRI/CPC suite of Niño-3.4 predictions favor ENSO-neutral through the Northern Hemisphere 2017-18 winter,” but they saw something in the NCEP Climate Forecast System (CFSv2) and North American Multi-Model Ensemble (NMME) that indicated a more rapid swing to La Niña conditions, and this influenced the overall assessment of an increasing chance of a weak La Niña this fall. It remains to be seen whether the winter will play out with ENSO-neutral or weak La Niña conditions. Given the warmer- and drier-than-average conditions associated with La Niña, this will be a closely watched phenomena going into the cool season of the Southwest, but neither scenario points toward particularly abundant winter precipitation.
Online Resources - Image Credits