El Niño-Southern Oscillation can influence the Tropical North Atlantic (TNA), leading to anomalous sea surface temperatures (SST) at a lag of several months. Several mechanisms have been proposed to explain this teleconnection. These mechanisms include both tropical and extratropical pathways, contributing to anomalous trade winds and static stability over the TNA region. The TNA SST response to ENSO has been suggested to be nonlinear. Yet the overall linearity of the ENSO-TNA teleconnection via the two pathways remains unclear. Here we use reanalysis data to confirm that the SST anomaly (SSTA) in the TNA is nonlinear with respect to the strength of the SST forcing in the tropical Pacific, as further increases in El Niño magnitudes cease to create further increases of the TNA SSTA. We further show that the tropical pathway is more linear than the extratropical pathway by sub-dividing the inter-basin connection into extratropical and tropical pathways. This is confirmed by a climate model participating in the CMIP5. The extratropical pathway is modulated by the North Atlantic Oscillation (NAO) and the location of the SSTA in the Pacific, but this modulation insufficiently explains the nonlinearity in TNA SSTA. As neither extratropical nor tropical pathways can explain the nonlinearity, this suggests that external factors are at play. Further analysis shows that the TNA SSTA is highly influenced by the preconditioning of the tropical Atlantic SST. This preconditioning is found to be associated with the NAO through SST-tripole patterns.

Abstract. Equatorial Pacific sea surface temperature anomalies (SSTAs) associated with El Niño–Southern Oscillation (ENSO) can influence the North Atlantic European (NAE) region. ENSO tends to be negatively correlated with the North Atlantic Oscillation in winter, while this connection is less clear in boreal spring and summer when the ENSO teleconnection encounters altered background conditions (i.e., a weaker subtropical jet), which can modulate the signal on the way to the NAE region. One such region that modulates the ENSO teleconnection to the NAE region is the tropical North Atlantic (TNA). While several mechanisms exist for this modulation, we center our analysis on the Caribbean region and the Walker cells. In order to isolate the relevant mechanism, we force an idealized atmospheric circulation model with three different seasonally varying sea surface temperature patterns that represent an ENSO event with or without the influence of the TNA, focusing on the decaying phase of ENSO in boreal spring and summer. We find that in boreal spring, the TNA modulates the ENSO teleconnection to the NAE primarily through a propagating Rossby wave train, while in summer, the TNA's influence tends to strengthen the ENSO influence over the NAE sector. Overall, this study offers a deeper understanding of the inter-basin interactions through the Walker cell following an ENSO event and the central role of tropical Atlantic SSTAs in modulating the teleconnection to the NAE region in boreal spring and summer.

Abstract. El Niño–Southern Oscillation (ENSO) is a major source for teleconnections, including towards the tropical North Atlantic (TNA) region, whereby TNA sea surface temperatures (SSTs) are positively correlated with ENSO in boreal spring following an ENSO event. However, the Pacific–Atlantic connection can be impacted by different ENSO characteristics, such as the amplitude, location, and timing of Pacific SST anomalies (SSTAs). Indeed, the TNA SSTAs may respond nonlinearly to strong and extreme El Niño events. However, observational data for the number of extreme ENSO events remain limited, restricting our ability to investigate the influence of observed extreme ENSO events. To overcome this issue and to further evaluate the nonlinearity of the TNA SSTA response, two coupled climate models are used, namely the Community Earth System Model version 1 – Whole Atmosphere Community Climate Model (CESM-WACCM) and the Flexible Ocean and Climate Infrastructure version 1 (FOCI). In both models the TNA SSTAs respond linearly to ENSO during extreme El Niño events but nonlinearly to extreme La Niña events for CESM-WACCM. We investigate differences by using indices for all major mechanisms that connect ENSO to the TNA and compare them with reanalysis. CESM-WACCM and FOCI overall represent the teleconnection well, including that the tropical and extratropical pathways are similar to observations. Our results also show that a large portion of the nonlinearity during La Niña is explained by the interaction between Pacific SSTAs and the overlying upper-level divergence.

Abstract. El Niño–Southern Oscillation (ENSO) in the tropical Pacific is known to have remote effects on the sea surface temperatures (SSTs) in the tropical North Atlantic (TNA). Here, the TNA SSTs are positively correlated with ENSO in boreal spring following an ENSO event. The ENSO-TNA teleconnection is also not straightforward as the response of TNA SSTs may be nonlinear to the strength of ENSO (i.e., how strongly the Atlantic reacts to an increase in strength of the Pacific signal), especially during extreme ENSO events. However, the number of extreme ENSO events in observational data remains limited, restricting our ability to investigate the influence of observed extreme ENSO events. To overcome this issue and to further evaluate the nonlinearity of the TNA SSTA response, two coupled climate models are used, namely the Community Earth System Model version 1-– Whole Atmosphere Community Climate Mode (CESM-WACCM) and the Flexible Ocean and Climate Infrastructure version 1 (FOCI). Our results show that the TNA responds linearly to extreme El Niño events in both models but nonlinearly to extreme La Niña events for CESM-WACCM. A large portion of the nonlinearity during La Niña is explained by the interaction between Pacific SSTAs and the overlying troposphere. Overall, our study shows that CESM-WACCM and FOCI are capable of reproducing the ENSO-TNA teleconnection and expands on key differences between climate models and reanalysis.

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