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

Abstract

Abstract El Niño 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 Niño with moderate intensity. Nearly half of the moderate El Niño 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 Niño and the NTA. The differences in the El Niño–related NTA SSTAs are due to the different extratropical teleconnection between El Niño 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, modulates the El Niño–generated PNA pattern. Therefore, El Niños 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 Niño plays a crucial role in modulating the connection between the moderate El Niño and the NTA SSTAs. This observational evidence about the uncertainty in the El Niño–NTA connection and the role of the AL in modulating this connection 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 Niño and the AL must be considered to fully understand the NTA SSTA variability. Significance Statement El Niño events often lead to significant warming over the north tropical Atlantic (NTA). We find that the strong El Niño almost always is accompanied by the strong NTA sea surface temperature anomalies (SSTAs), whereas the moderate El Niño can be followed by either the strong or weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA SSTAs. Further analysis demonstrates that El Niño, 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 Niño–NTA connection.