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

Warming of the Tropical North Atlantic (TNA) in boreal spring and early summer (April-June) following El Nino 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 Nino in boreal winter does not guarantee a warm TNA in the following April-June (AMJ): for sixteen observed El Nino events that occurred during 1950-2005, the TNA ( AMJ) remained neutral in six of them. A careful examination of the sixteen El Nino events leads to a hypothesis that if an El Nino 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 Nino-induced warming of TNA. Therefore, if an El Nino 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 Nino continues beyond JFM, the El Nino-induced warming of TNA in AMJ can be greatly reduced by Atlantic internal variability, and vice versa.