Influence of Two Spatially Distinct Types of Arctic Oscillation on El Niño

This study examines the cross-seasonal influence of the Arctic Oscillation (AO) on winter El Ni & ntilde;o, using atmospheric and oceanic reanalysis data sets. It has been revealed that the linkage between positive AO and subsequent El Ni & ntilde;o is closest during spring, indicating the seasonal dependent and selective impact of AO on ENSO. Composition analyses illustrate that positive spring AO is often associated with a dipole circulation anomaly over the North Pacific. The corresponding cyclonic circulation anomaly in the subtropical North Pacific initiates the Pacific Meridional Mode (PMM), which persists and evolves through the wind-evaporation-SST (WES) feedback from, triggering the westerly anomalies in the tropical western Pacific and subsequently triggering the onset of El Ni & ntilde;o. Further clustering analysis reveals that the positive spring AO events exhibit two distinct spatial patterns: Type1 with prominent positive SLP anomaly over the midlatitude North Pacific and Type2 with the positive SLP anomaly over the North Atlantic. Both types can trigger El Ni & ntilde;o by stimulating the subtropical cyclone anomaly in the North Pacific but through different mechanisms. In Type1 events, high-pressure anomalies in the midlatitude North Pacific trigger southward-propagating atmospheric waves, generating the cyclone anomalies in the subtropical North Pacific. Although in Type2 events, the AO-related circulation anomalies in the North Atlantic stimulate teleconnection wave trains that cross the Eurasian continent and propagate southward to the subtropical North Pacific, inducing the cyclone anomalies. However, Type1 events occur more frequently and are more closely linked to El Ni & ntilde;o, highlighting the significant role of air-sea coupling processes in the North Pacific in triggering El Ni & ntilde;o.