SWCO Aug 2016 - La Niña Tracker
Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) remain in the range of neutral conditions (Figs. 1-2). Seasonal forecasts and models identify the most likely scenario being a weak La Niña event forming sometime in late summer or fall 2016 and lasting through winter 2017. Some uncertainty exists regarding the specific timing of this event, as the equatorial sea surface temperature (SST) anomalies have not yet dropped into La Niña range and there is a lack of coordination between ocean and atmosphere (and in particular the lack of enhanced trade winds).
A closer look at the various forecasts and seasonal outlooks provides insight into the range of expectations for this La Niña event. On August 10, the Japanese Meteorological Agency saw below-normal equatorial convective activity and enhanced trade winds as indicative of the start of La Niña-favorable conditions, even while the SST anomalies were slow to swing more negative (cool) after the sustained warm period associated with El Niño conditions. The agency forecast a 70 percent chance of a La Niña event developing, but not until fall 2016. On August 11, the NOAA Climate Prediction Center (CPC) highlighted ENSO-neutral conditions in both ocean and atmosphere and continued to focus on tension between statistical and dynamical models, the former predicting a later onset and weaker event than the latter. The CPC forecast remains at a 55–60 percent chance of a weak La Niña event starting sometime between August and October 2016. On August 16, the Australian Bureau of Meteorology maintained its La Niña watch, albeit with slightly reduced confidence compared to the previous month, hedging slightly by stating “if La Niña does develop it is likely be weak.” The bureau maintained its forecast probability at a 50 percent chance of a La Niña event developing, noting that is approximately twice the normal chance of a La Niña forming. On August 18, the International Research Institute for Climate and Society (IRI) and CPC forecasts indicated the probability of a “borderline/weak” La Niña forming was just less than 60 percent (Fig. 3). The IRI forecasters noted that until the atmosphere cooperates (i.e., the enhanced trade winds show up), the ENSO-neutral holding pattern will remain. The North American multi-model ensemble characterizes the current model spread and highlights the variability looking forward to 2017, but the ensemble mean hovers close to weak La Niña status for fall and winter of the coming year (Fig. 4).
The Southwest is in a holding pattern regarding La Niña but will likely see the effects of a weak La Niña this winter. Forecasters are likely already integrating the influence of the drier-than-average signal associated with La Niña into long-term precipitation and temperature forecasts and seasonal outlooks (Fig. 5a-b), and researchers are preparing to track both the timing and intensity of this event in relation to precipitation, temperature, snowpack, and water supply over the coming year.
Given the dry climate of the Southwest, a weak La Niña event does not necessarily deviate far from the seasonal climatology of the region, but while ENSO-neutral winters have a wider range of precipitation values observed over winter, La Niña winters skew more dry. Even a weak La Niña event is likely to suppress winter precipitation totals in the Southwest, which is unwelcome news given the underperformance of last year’s El Niño winter and the longer-term effects of multi-year drought.