An Eastward Shift of the North Pacific Oscillation After the Mid-1990s and Its Relationship With ENSO

Abstract

The North Pacific Oscillation (NPO), which is characterized by a north-south dipole-like pattern of sea level pressure (SLP) in the North Pacific, is an atmospheric circulation that is a key to understanding tropical-extratropical interactions in the Pacific. We show that the center of the southern lobe SLP in the NPO during boreal winter (December-January-February) is shifted to the east after the mid-1990s compared to before the mid-1990s. This leads to the change in the relationship between the NPO and El Nino and the Southern Oscillation (ENSO). The NPO is closely associated with the convective forcing in the tropical Pacific during boreal winter before the mid-1990s. After the mid-1990s, in contrast, the simultaneous relationship of the NPO and ENSO during boreal winter becomes weak. However, an eastward shift of the NPO's southern lobe SLP during boreal winter causes a close relationship with the ENSO in the winter of the following year through atmosphere-ocean coupled processes after the mid-1990s. These results indicate that atmospheric circulation in the North Pacific characterized by the NPO becomes more influential in the tropical Pacific with a lagged time after the mid-1990s, likely due to the eastward shift in the NPO's structure. We also briefly discuss which processes cause an eastward shift in the NPO's southern lobe SLP. Plain Language Summary Understanding the North Pacific Oscillation (NPO), which is characterized by a north-south dipole-like pattern of sea level pressure (SLP) in the North Pacific, is a key to understanding tropical-extratropical interactions in the Pacific. This study examines the decadal changes in the NPO's spatial structure and its consequent change in the relationship with El Nino and the Southern Oscillation (ENSO) for 1979-2016. It is found that the center of the southern lobe SLP in the NPO during boreal winter (December-January-February) is shifted to the east after the mid-1990s compared to before the mid-1990s. This leads to the change in the relationship between the NPO and ENSO. An eastward shift of the NPO's southern lobe SLP during boreal winter causes a close relationship with the ENSO in the winter of the following year through atmosphere-ocean coupled processes after the mid-1990s.