Dec 2016 La Niña Tracker

From the December issue of the CLIMAS Southwest Climate Outlook

Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) continue to indicate a weak La Niña event that is likely to last through mid-winter at least and perhaps into early spring (Figs. 1-2). The borderline weak status of the event, along with some discrepancy between the forecast agencies discussed here, means a more rapid transition to ENSO-neutral conditions cannot be ruled out. As with last month, there is some hedging in the forecasts and outlooks that likely stems from ongoing uncertainty as to whether the event can maintain even weak La Niña strength through winter 2017 (December–February). Fluctuations in forecasts and models are due to the limited coordination between oceanic and atmospheric conditions described in previous outlooks, “masking…by intra-seasonal activity” (as described by the CPC on Dec 8), and the difficulty in categorizing borderline conditions into a binary choice between weak La Niña and ENSO-neutral.

A closer look at the forecasts and seasonal outlooks continues to provide some insight into the range of expectations for a La Niña event this winter. On December 6, the Australian Bureau of Meteorology ended its La Niña Watch, declaring that La Niña is no longer likely in the coming months. They identified “very weak La Niña-like patterns,” including cool sea surface temperature (SST) anomalies, but the cluster of conditions did not meet the bureau’s threshold for designating an official La Niña event. On December 8, the NOAA Climate Prediction Center (CPC) maintained its La Niña advisory for an ongoing weak La Niña event, based on oceanic and atmospheric conditions during November, and forecast a transition to ENSO-neutral conditions during January–March 2017. On December 9, the Japanese Meteorological Agency identified the ongoing presence of La Niña conditions in the equatorial Pacific and forecast equal chances (50 percent) of this La Niña lasting through winter 2017 or a return to ENSO-neutral conditions. The agency also identified a 70 percent chance of ENSO-neutral conditions by spring 2017. On December 15, the International Research Institute for Climate and Society (IRI) and CPC forecasts identified a borderline weak La Niña event that was “just barely going,” and forecast a likely rapid decline to ENSO-neutral conditions in winter 2017 (Fig. 3). The North American Multi-Model Ensemble (NMME) characterizes the current model spread and highlights the variability looking forward to 2017. The NMME mean already has risen above the La Niña threshold in the current run and is forecast to remain ENSO-neutral through the first half of 2017 (Fig. 4).

According to most forecasts, a weak La Niña remains in the cards for the Southwest during winter 2017, which is more likely than not to bring warmer- and drier-than-average conditions to the region over the cool season. Even if the event decays more rapidly than currently forecast and conditions tack back toward neutral conditions, borderline La Niña conditions still could affect temperature and precipitation patterns through the winter. Southwestern winters are relatively dry already; La Niña years tend to cluster on the dry end of the distribution (Fig. 5), so even a weak event could shift that seasonal pattern to an even drier state. Given the baseline climate of the Southwest, a weak La Niña event might not stand out, but snowpack and water supply could be affected going into the spring and summer, so this is something to keep an eye on. Seasonal forecasts likely were already incorporating the influence of La Niña into monthly and seasonal forecasts, and for as long as this La Niña event persists, even in a weak form, we can expect these forecasts will continue to suggest warmer- and drier-than-average conditions (Figs. 6a–b, 7).

Figure Credits:

• Figure 1 - Australian Bureau of Meteorology
• Figure 2 - NOAA - National Climatic Data Center
• Figure 3 - International Research Institute for Climate and Society
• Figure 4 - NOAA - Climate Prediction Center
• Figure 5 - Climate Science Applications Program
• Figures 6a-6b - NOAA - Climate Prediction Center
• Figure 7 - International Research Institute for Climate and Society