Inhibitors of atmosphere/ocean coupling during the El Nino of 2014-15

Abstract:

In the spring of 2014, some members of the media expressed excitement about an impending large El Nino episode that may rival the 1997-98 extreme event. As summer and fall of 2014 arrived, however, the typical tropical atmospheric response to El Nino had not materialized, and a large El Nino episode did not occur in either the fall or winter of 2014-15. This study examines the reasons why the atmosphere did not respond in the expected manner, with a focus on a diagnosis of atmospheric stability. Analyses of profiles of equivalent potential temperature (θe) and saturation equivalent potential temperature (θes) in key regions of ocean/atmosphere coupling suggest that the spring of 2014 was characterized by unusually stable air despite oceanic conditions that were conducive for El Nino development. In a retrospective forecasting context for predictions made in March, April or May 2014, a multiple linear regression model that uses both eastern Pacific oceanic heat content and a measure of atmospheric stability as predictors predicted a low probability of an extreme El Nino in 2014-15, with probabilities substantially lower than if the regression model uses eastern Pacific oceanic heat content alone. This study places the conditions in the spring of 2014 in a historical context and contrasts them with the spring of 2015.