Climate Summary from the September issue of the CLIMAS Southwest Climate Outlook
Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) remain in the range of neutral conditions (Figs. 1-2). In the last month, seasonal forecasts and models further reduced their certainty of a weak La Niña event forming in late 2016 or early 2017. The current projections find ENSO-neutral conditions to be the most likely outcome for this fall and winter, although the chance of a weak La Niña event cannot be ruled out. As past outlooks have noted, there was ongoing uncertainty regarding the prospects for La Niña, especially as it appeared to be having difficulty organizing, with limited coordination between ocean and atmosphere.
A closer look at the various forecasts and seasonal outlooks provides insight into the range of expectations for this La Niña event. On September 8, the NOAA Climate Prediction Center (CPC) highlighted ENSO-neutral conditions in both the ocean and atmosphere, and given the persistent neutral conditions, it shifted its forecast from slightly favoring La Niña to a 55–60 percent chance of ENSO-neutral conditions during fall and winter 2016-17. On September 9, the Japanese Meteorological Agency identified La Niña conditions as being in place, and projected a 70 percent chance that they would remain through winter 2017. On September 13, the Australian Bureau of Meteorology maintained its La Niña watch, but noted the lack of connection between ocean and atmosphere and the generally neutral conditions. The agency identified a limited chance of a “late and weak La Niña” emerging at some point, and highlighted that the borderline weak La Niña conditions may lead to weather patterns more in line with La Niña even if the event is not designated as such. On September 14, the International Research Institute for Climate and Society (IRI) and CPC forecasts described oceanic and atmospheric conditions that were borderline La Niña, but that models indicated a likely retreat into ENSO-netural status during winter 2016-2017 (Fig. 3). The North American multi-model ensemble characterizes the current model spread and highlights the variability looking forward to 2017. In the last month, the ensemble mean returns to neutral conditions over fall and winter 2016-17, whereas previously it had remained close to weak La Niña during this timeframe (Fig. 4).
The Southwest remains in a holding pattern regarding La Niña, even while the current forecast suggests it is now increasingly likely the region will experience an ENSO-neutral winter. Forecasters were likely already integrating the influence of La Niña into long-term precipitation forecasts and seasonal outlooks (Figs. 5a-5b), and it will be interesting to see if and how these forecasts change given the current ENSO-neutral forecast.
It is important to note that while a weak La Niña event would be more likely than not to bring drier-than-average conditions to the Southwest over the cool season, there is no basis to think that ENSO-neutral conditions will bring increased moisture over the winter, as the range of precipitation values for ENSO-neutral cool seasons ranges from very wet to very dry (Fig. 6). This is particularly the case if oceanic and atmospheric conditions are borderline weak La Niña yet they exert some influence on seasonal weather patterns even if not officially designated as a La Niña event.