Why do some El Niños have no impact on tropical North Atlantic SST?



[1] Warming of the Tropical North Atlantic (TNA) in boreal spring and early summer (April–June) following El Niño peaks in boreal winter is a well-known phenomenon that involves formation of the so-called atmospheric bridge (or teleconnection) from the Pacific. However, the existence of an El Niño in boreal winter does not guarantee a warm TNA in the following April–June (AMJ): for sixteen observed El Niño events that occurred during 1950–2005, the TNA (AMJ) remained neutral in six of them. A careful examination of the sixteen El Niño events leads to a hypothesis that if an El Niño ends before April, the TNA remains neutral. Here, we test this working hypothesis by performing multiple sets of ensemble model experiments using the NCAR atmospheric general circulation model coupled to a slab mixed layer ocean model. Analysis of the model experiments indicates that January–March (JFM) are the crucial months for the El Niño-induced warming of TNA. Therefore, if an El Niño does not continue throughout JFM, the atmospheric bridge connecting the tropical Pacific to the TNA is not persistent enough to force the TNA, thus the TNA remains neutral. Finally, our model experiments indicate even if an El Niño continues beyond JFM, the El Niño-induced warming of TNA in AMJ can be greatly reduced by Atlantic internal variability, and vice versa.