How sensitive are the Pacific–tropical North Atlantic teleconnections to the position and intensity of El Niño-related warming?

Abstract

The atmospheric teleconnections associated with the Eastern Pacific El Nino and El Nino Modoki events onto the tropical Atlantic Ocean are investigated. The Eastern Pacific El Ninos drive significant warming of the tropical North Atlantic basin during boreal spring after its peak via the atmospheric bridge and tropospheric temperature mechanisms. However, the tropical Atlantic does not show a robust response to El Nino Modoki events. Here our results suggest that the preconditioning of the tropical North Atlantic sea surface temperature (SST) anomalies in boreal winter plays an important role in the following season, not only during Eastern Pacific El Ninos but also during El Nino Modoki events. Additionally, we examine three other factors that could explain potential differences in the tropical Atlantic teleconnections from El Nino Modoki and Eastern Pacific El Nino events: (1) The distant location of the maximum SST warming in the Pacific; (2) The weak warming associated with this pattern; and (3) The SST pattern including a cooling in the eastern Pacific. Using numerical experiments forced with idealised SST in the equatorial Pacific, we show that the location of the El Nino Modoki SST warming during its mature phase could be favourable for exciting atmospheric teleconnections in boreal winter but not in the following spring season due to the seasonal shift of the Inter-Tropical Convergence Zone that modulates deep convection over the anomalous SST. This demonstrates the importance of the mean seasonal atmospheric circulation in modulating the remote teleconnections from the central-western Pacific warming in the model. However, it is suggested here that the cooling in the eastern Pacific associated with El Nino Modoki counteracts the atmospheric response driven by the central western Pacific warming, generating a consequent weaker connection to the tropical Atlantic compared to the stronger link during Eastern Pacific El Nino events. Finally we show that the modeled Pacific–tropical Atlantic teleconnections to an eastern Pacific warming depends strongly on the underlying seasonal cycle of SST.