Joint impacts of winter North Pacific Oscillation and early spring Aleutian Low intensity on the following winter ENSO

This study examines the joint impacts of the winter North Pacific Oscillation (NPO) and early spring Aleutian Low intensity (ALI) on the following winter El Niño-Southern Oscillation (ENSO). When the winter NPO and early spring ALI have the opposite sign (i.e., positive winter NPO was followed by weakened AL in early spring, and vice versa), pronounced sea surface temperature (SST) anomalies develop in the tropical central and eastern Pacific in the following winter. By contrast, SST anomalies are small in the tropical central and eastern Pacific for the same-sign NPO-ALI years. For the opposite-sign NPO-ALI years, SST and precipitation anomalies in the subtropical central North Pacific in late spring are considerably enhanced due to constructive superposition of the anomalies induced by the winter NPO and early spring ALI. This leads to marked cyclonic and low-level zonal wind anomalies over the tropical western central Pacific via the Gill-type atmospheric response, which further exert notable impacts on the following winter ENSO. In addition, the associated surface wind stress curl anomalies over the tropical central Pacific lead to subsurface sea water temperature anomalies via downward/upward Ekman Pumping, which also have an impact on the subsequent winter ENSO occurrence. For the same-sign years, SST and precipitation anomalies in the subtropical North central Pacific in late spring generated by the winter NPO and early spring ALI cancel out each other. This results in weak low-level wind anomalies in the tropical western central Pacific and thus has weak impacts on the following winter ENSO. The prediction skill of ENSO is also enhanced when both the winter NPO and early spring ALI are considered. The observed joint impacts of the NPO and ALI on ENSO and the underlying process can be reproduced in historical simulations of most of the CMIP6 models.