Role of the Aleutian Low in Modulating the Connection between the Moderate El Nino and the Following Spring North Tropical Atlantic SST Anomalies

El Nifio events are likely to be followed by significant positive sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA). We find that this is not always true for El Nifio with moderate intensity. Nearly half of the moderate El Nifio cases are followed by the strong NTA SSTAs, but others are followed by the weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Nifio and the NTA. The differences in the El Nifio-related NTA SSTAs are due to the different extratropical teleconnection between El Nifio and the NTA, manifesting as a Pacific-North American (PNA) pattern. Further analysis suggests that the deepened Aleutian low (AL) induces the negative North Pacific SSTA, which is associated with a wave train propagating eastward, and, mod-ulates the El Nifio-generated PNA pattern. Therefore, El Nifios that are accompanied by the strong AL induce a strong PNA pattern that, in turn, leads to the strong NTA SSTAs. The strength of the AL coinciding with El Nifio plays a crucial role in modulating the connection between the moderate El Nifio and the NTA SSTAs. This observa-tional evidence about the uncertainty in the El Nifio-NTA connection and the role of the AL in modulating this connec-tion is further supported by the model simulations that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). These results indicate that the combined effects of El Nifio and the AL must be considered to fully un-derstand the NTA SSTA variability.SIGNIFICANCE STATEMENT: El Nifio events often lead to significant warming over the north tropical Atlantic (NTA). We find that the strong El Nifio almost always is accompanied by the strong NTA sea surface temperature anomalies (SSTAs), whereas the moderate El Nifio can be followed by either the strong or weak NTA SSTAs, indicat-ing the uncertainty in the connection between the moderate El Nifio and the NTA SSTAs. Further analysis demon-strates that El Nifio, which is accompanied by the strong (weak) Aleutian low leads to the strong (weak) NTA SSTAs, suggesting that the role of the Aleutian low must be considered to fully understand the El Nifio-NTA connection.