Southeastward Extension of the Aleutian Low Favors Long Decaying El Niño Events

Abstract

Abstract El Niño can be categorized into long and short decaying cases in terms of the lengths of decaying phases. We find that the difference between long and short decaying El Niño emerges as early as the decaying spring, rather than the decaying summer mentioned in previous studies. Observational evidence suggests that this difference corresponds to the different positions of the spring Aleutian low (AL). Further analysis using 15 Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations suggests that the AL that extends southeastward into the northeastern subtropical Pacific contributes to the long decaying El Niño. The northeastern subtropical Pacific AL, with an anomalous southwesterly on the southeast edge, is associated with warming in situ, which further induces cyclonic circulation anomalies over the eastern tropical Pacific as Gill’s response. The anomalous westerly at the equator along the southern side of the cyclonic circulation anomalies is related to positive SST anomalies over the central and eastern tropical Pacific and therefore the persistence of El Niño in the decaying spring. On the other hand, the AL restricted to the North Pacific contributes to anticyclonic circulation anomalies over the central Pacific and favors the decline of El Niño through the seasonal footprint mechanism. Moreover, the models that reproduce many more long (short) decaying El Niño cases tend to simulate the AL southeastward extension (restriction over the North Pacific) during the El Niño decaying spring. The results imply that the AL position in the El Niño decaying spring could be a precursor to the length of El Niño decaying phase. Significance Statement El Niño events can be classified into long and short decaying cases with regard to the different lengths of decaying phases. We find that the difference between the two types of El Niño appears as early as the decaying spring, rather than the decaying summer noticed in previous studies. We highlight that this difference is due to the different positions of the spring Aleutian low (AL). Particularly, the AL that extends southeastward into the northeastern subtropical Pacific is associated with warming over the central and eastern tropical Pacific and contributes to the persistence of El Niño in the decaying spring. The results indicate that locations of the spring AL should be paid attention to when considering the decaying phase of El Niño.