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SW Climate Outlook - ENSO Tracker - Aug 2017 | CLIMAS


SW Climate Outlook - ENSO Tracker - Aug 2017

Sunday, August 20, 2017

Oceanic and atmospheric indicators remain within the range of neutral (Figs. 1-2). Seasonal outlooks and forecasts generally agree that ENSO-neutral conditions are the most likely outcome through winter 2017-2018. On Aug. 10, the Japanese Meteorological Agency (JMA) identified a continuation of ENSO-neutral conditions with a 60-percent chance of El Niño conditions until winter 2017-2018. On Aug. 10, the NOAA Climate Prediction Center (CPC) observed that oceanic and atmospheric conditions remained within the range of ENSO-neutral conditions, and that “the majority of models favor ENSO-neutral for the remainder of 2017.” They identified an 85-percent chance of neutral conditions through September 2017, and a 55-percent chance through February 2018. On Aug. 15, the Australian Bureau of Meteorology ENSO tracker remained at neutral/inactive, highlighting that every indication (models and forecasts) suggested ENSO-neutral conditions through 2017. On July 20, the International Research Institute for Climate and Society (IRI) and CPC identified a high likelihood of ENSO neutral conditions for the rest of 2017 (Fig. 3). The North American Multi-Model Ensemble (NMME) is ENSO-neutral as of August 2017. The model spread indicates a range of outcomes for the rest of 2017 (Fig. 4), but the ensemble mean indicates ENSO-neutral as the most likely outcome.

Summary: As with last month, ENSO indicators remain well within the bounds of ENSO-neutral, and there is little to suggest any other outcome in winter 2017-2018. An interesting detail has emerged from a few of the forecast discussions, however: the appearance of a slight uptick in the likelihood of a La Niña event in 2017-2018, running counter to discussion of the last few months. What’s going on? In Fig. 3, subtracting the current forecast percentage (bars) from the seasonal climatological probability percentage (lines) flattens the plot and reveals the deviation from normal climatology these forecast percentages represent (Fig. 5). Under this formulation, neutral conditions are forecast well above their climatological average through winter 2017-2018, while both El Niño and La Niña conditions are below their climatological average. This corresponds with current forecasts discussed above, and given the expected uncertainty associated with longer-term forecasts, the forecast percentages converge on climatological averages by the Mar-Apr-May period of 2018.

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