Abstract
Early in 2014 several forecast systems were suggesting a strong 1997/98-like El Niño event for the following northern hemisphere winter 2014/15. However the eventual outcome was a modest warming. In contrast, winter 2015/16 saw one of the strongest El Niño events on record. Here we assess the ability of two operational seasonal prediction systems to forecast these events, using the forecast ensembles to try to understand the reasons underlying the very different development and outcomes for these two years. We test three hypotheses. First we find that the continuation of neutral ENSO conditions in 2014 is associated with the maintenance of the observed cold southeast Pacific sea surface temperature anomaly; secondly that, in our forecasts at least, warm west equatorial Pacific sea surface temperature anomalies do not appear to hinder El Niño development; and finally that stronger westerly wind burst activity in 2015 compared to 2014 is a key difference between the two years. Interestingly, in these years at least, this interannual variability in wind burst activity is predictable. ECMWF System 4 tends to produce more westerly wind bursts than Met Office GloSea5 and this likely contributes to the larger SST anomalies predicted in this model in both years.
Highlights
In 2014 several forecast systems were suggesting a strong 1997/98-like El Niño event for the following northern hemisphere winter 2014/15
The ensemble means for both G5 (Fig. 1a) and S4 (Fig. 1b) forecasts are greater than observations throughout the forecast, and both show a steady increase in sea surface temperature (SST) anomaly with October means of 0.8 °C and 1.3 °C respectively, neither capturing the observed cooler July-August period
As numbers of westerly wind bursts (WWBs) are observed to be dependent on El Niño Southern Oscillation (ENSO) state[27], we focus on WWBs in the first two months after initialisation (May-June), before forecast SST in individual ensemble members has diverged too far from the initial state
Summary
In 2014 several forecast systems were suggesting a strong 1997/98-like El Niño event for the following northern hemisphere winter 2014/15. First we find that the continuation of neutral ENSO conditions in 2014 is associated with the maintenance of the observed cold southeast Pacific sea surface temperature anomaly; secondly that, in our forecasts at least, warm west equatorial Pacific sea surface temperature anomalies do not appear to hinder El Niño development; and that stronger westerly wind burst activity in 2015 compared to 2014 is a key difference between the two years. In these years at least, this interannual variability in wind burst activity is predictable.
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