Predicting El Niño in 2014 and 2015

Area of Science:

• Climate Science
• Oceanography
• Meteorology

Background:

• Seasonal forecast systems predicted a strong El Niño event for winter 2014/15, but a modest warming occurred.
• Winter 2015/16 experienced one of the strongest El Niño events on record, contrasting with the previous year.

Purpose of the Study:

• To assess the forecasting ability of two operational seasonal prediction systems for the 2014/15 and 2015/16 El Niño events.
• To understand the reasons behind the differing development and outcomes of these two El Niño events using forecast ensembles.

Main Methods:

• Testing three hypotheses regarding ENSO (El Niño-Southern Oscillation) conditions, sea surface temperature anomalies, and wind burst activity.
• Analyzing forecast ensembles from ECMWF System 4 and Met Office GloSea5.

Main Results:

• Neutral ENSO conditions in 2014 were linked to a persistent cold southeast Pacific sea surface temperature anomaly.
• Warm west equatorial Pacific sea surface temperature anomalies did not impede El Niño development in the forecasts.
• Stronger westerly wind burst activity in 2015 was identified as a key differentiator from 2014, and this variability was predictable.

Conclusions:

• The continuation of neutral ENSO conditions and cold southeast Pacific anomalies contributed to the weaker 2014/15 event.
• Increased westerly wind burst activity was a critical factor in the development of the strong 2015/16 El Niño.
• ECMWF System 4's tendency to predict more westerly wind bursts than GloSea5 may explain its prediction of larger SST anomalies.