On the Delayed Coupling Between Ocean and Atmosphere in Recent Weak El Nino Episodes

The recent borderline El Nino events of 2014/2015 and 2018/2019 provided operational centers with unique challenges because of the apparent absence of typical coupling between the tropical atmosphere and ocean before onset. The mismatch between atmosphere and ocean raises questions about its causes and predictability. Here we analyze observational data since 1979 to show that a sea surface temperature pattern characterized by an anomalous gradient in the western and central equatorial Pacific played a critical role in inhibiting the expected onset of central tropical Pacific deep convection during these events. This sea surface temperature pattern, which produces an atmospheric response that opposes the response to elevated eastern Pacific sea surface temperatures, has become more prevalent over the past 40 years.