Impacts of winter NPO on subsequent winter ENSO: sensitivity to the definition of NPO index

This study investigates the linkage between boreal winter North Pacific Oscillation (NPO) and subsequent winter El Nio-Southern Oscillation (ENSO) based on seven different NPO indices. Results show that the influence of winter NPO on the subsequent winter El Nio is sensitive to how the NPO is defined. A significant NPO-El Nio connection is obtained when the NPO-related anomalous cyclone over the subtropical North Pacific extends to near-equatorial regions. The anomalous cyclone induces warm sea surface temperature (SST) anomalies through modulating surface heat fluxes. These warm SST anomalies are able to maintain into the following spring and summer through an air-sea coupled process and in turn induce significant westerly wind anomalies over the tropical western Pacific. In contrast, the NPO-El Nio relationship is unclear when the NPO-related anomalous cyclone over the subtropical North Pacific is confined to off-equatorial regions and cannot induce significant warm SST anomalies over the subtropical North Pacific. The present study suggests that definitions of NPO should be taken into account when using NPO to predict ENSO. In particular, we recommend defining the NPO index based on the empirical orthogonal function technique over appropriate region that does not extend too far north.