The Tropical Atlantic's Asymmetric Impact on the El Niño-Southern Oscillation

Using observations and Atlantic forced coupled model simulations, we show evidence for an asymmetry in the link between beginning of year tropical Atlantic sea surface temperature anomalies (SSTAs) and end of the year El Nino-Southern Oscillation events. We find a greater tendency for warm Atlantic SSTAs to lead to a La Nina than for cold anomalies to lead to El Nino. The model experiments showed that the Atlantic had a greater chance to force the tropical Pacific if the Pacific was initially in a neutral state. In the model, a warm Atlantic from March-May was able to produce an atmospheric response leading to easterly wind anomalies in the western Pacific. This in turn induces a subsurface oceanic response, leading to La Nina at the end of the year. The atmospheric response does not occur for a cold Atlantic, leading to no impacts in the Pacific. It has been found that the tropical Atlantic can have an impact on an El Nino or La Nina events that form in the tropical Pacific at the end of the year. We show using observations and targeted model experiments that a warmer than normal Atlantic from March-May can lead to a La Nina, but a colder than normal Atlantic does not lead to an El Nino. The model shows a strong atmospheric response to warm Atlantic surface temperature, leading to wind changes in the Pacific that aid La Nina formation. This strong atmospheric response does not occur for cold Atlantic temperatures leading to no changes in the Pacific. The model also shows that the Atlantic has a greater chance of impacting the Pacific at the end of the year if the Pacific is already in near average conditions at the beginning of the year. These results may improve forecasts of La Nina events and may help in understanding past changes in the Atlantic-Pacific relationship. Observed tropical Atlantic forced pacemaker experiments produce La Nina events, but not El Nino events, in unison with observations A warm Atlantic had a greater chance of leading to a La Nina event if the tropical Pacific was initially in neutral conditions There was a tropical-wide response to a warm Atlantic in the model, affecting western Pacific wind. This was not seen for a cold Atlantic