Meridional Position Changes of the Sea Surface Temperature Anomalies in the North Pacific

Abstract

AbstractChanges in the meridional position of the sea surface temperature (SST) anomalies (SSTAs) associated with the interannual component (PC1-I) of the principal component 1 (PC1) of the first leading mode of the North Pacific SST (referred to here as PC1-I-related SSTAs) are investigated using reanalysis products and climate model output. It is found that the PC1-I-related SSTAs (or PC1-I anomalies) significantly shift southward at a rate of 1.04° latitude per decade and have moved southward by 4.4° since the 1960s. Our further analysis indicates that the southward shift of the PC1-I-related SSTAs is due to changes in ENSO teleconnections. Compared to the 1950–75 period (PRE era), the meridional width of the ENSO-induced tropical positive geopotential height (GH) anomaly is narrower during the 1991–2016 period (POST era), inducing a southward shift of the subtropical westerly anomaly over the North Pacific through geostrophic wind relations. This southward shift of the westerly anomaly favors the southward shift of the ENSO-induced negative GH anomaly (cyclonic circulation anomaly) over the North Pacific by positive vorticity forcing of the zonal wind shear. The southward-shifting GH anomaly associated with ENSO further forces the PC1-I anomaly to shift southward. Furthermore, the contraction of the ENSO-induced tropical positive GH anomaly is related to the contraction of the meridional width of ENSO. The modeling results support that the decrease in the ENSO meridional width favors the contraction of the ENSO-induced tropical positive GH anomaly and the southward shift of ENSO teleconnections over the North Pacific, contributing to the southward shift of the PC1-I anomaly.