Distinctive impact of spring AO on the succedent winter El Nino event: sensitivity to AO's North Pacific component

Previous studies indicated that positive spring Arctic Oscillation (+ AO) has a marked impact on the succedent winter El Nino. However, + AO cannot always lead to an El Nino. We compare the observations with two groups: when + AO is followed by an El Nino (denoted as AO-EL) and when + AO cannot result in an El Nino (denoted as AO-NEL). Large difference is originally found in the North Pacific component of the + AO between the two groups. North Pacific center of the + AO is much stronger and extends more southward for AO-EL than AO-NEL years. Specifically, for the AO-EL years, a strong anticyclonic anomaly appears over mid-latitude North Pacific in spring, accompanied by significant cyclonic anomaly, sea surface temperature (SST) warming, and increase in the atmospheric heating over subtropical North Pacific. Increase in SST and atmospheric heating over subtropical North Pacific lead to westerly wind anomalies over the tropical central and western Pacific via positive air-sea interaction, and further impacts succedent winter El Nino. Furthermore, the southwesterly wind anomalies over subtropical North Pacific provide negative wind stress curl over equatorial central Pacific and lead to subsurface warming there, which also has a contribution to the following winter El Nino occurrence. For the AO-NEL years, North Pacific component of the + AO is much weaker, which cannot induce clear westerly wind anomalies over the tropical central and western Pacific, and thus has little impact on the following winter El Nino. In addition, the Atlantic Multidecadal Oscillation has an effect on the North Pacific center of the spring AO. The observed results can be well verified in a long simulation of a coupled climate model.