Tropical atmospheric response of Atlantic Niños to changes in the ocean background state

Abstract

Since the 1970s, Atlantic Ninos during boreal summer have been linked to Pacific La Ninas the following winter. Earlier studies have explained the appearance of the Atlantic-Pacific teleconnection with changing Atlantic Nino configurations. Here we find that the non-stationarity of this teleconnection can also be explained by changes in the ocean background state, without changing the Atlantic Nino configuration. Experiments with different atmospheric general circulation models are performed where the same Atlantic Nino pattern is prescribed to different global ocean background states. The 1975-1985 global mean sea surface temperature forces a Walker Circulation response and low-level convergence over the Maritime Continent, increasing the chance of triggering a La Nina-like event in the Pacific. These results suggest that ENSO-predictions could be improved in certain periods by considering tropical Atlantic variability.

The Atlantic Nino is the main climate variability phenomenon in the equatorial Atlantic and has a strong influence on local and remote climate. Since the 1970s, warm Atlantic Nino events in June-August have been linked to cool La Nina events developing in the equatorial Pacific the following December-February. The appearance of this Atlantic-Pacific link has been explained by changes in the temperature pattern of Atlantic Ninos but, so far, no study has analyzed the role of mean state changes. In this study we demonstrate the contribution of changes in the mean background surface temperatures of the global oceans to the appearance of this Atlantic-Pacific link. These results imply that under certain global ocean background conditions, we should use information about the Atlantic Nino to improve seasonal forecasts of El Nino and La Nina events.

An Atlantic Nino in summer can produce atmospheric conditions of a La Nina-like event in the Pacific under certain ocean background statesThe ocean background state is of similar importance as the Atlantic Nino pattern for modifying the Atlantic Nino-Pacific teleconnectionAn Atlantic Nino intensifies easterly wind anomalies over the western equatorial Pacific when the Pacific has a warmer background state