Dominant Forcing Regions of Decadal Variations in the Kuroshio Extension Revealed by a Linear Rossby Wave Model.

The Kuroshio Extension (KE) is known to undergo large decadal variations, especially after the 1976/77 climate regime shift. Due to strong ocean‐atmosphere interactions over the KE, the region may be a key region for basin‐scale Pacific decadal climate variability. Although baroclinic Rossby waves are suggested to influence the KE decadal variability, it remains unclear where and how decadal atmospheric forcing anomalies that induce the Rossby waves are generated. Using a linear Rossby wave model forced by reanalysis surface wind stress, the relative contribution of wind stress at each longitude is quantified for the first time. It is shown that the contribution is largest at 160°W, and 40% of the total variance in the decadal KE variability is explained by the wind forcing around 170°–150°W. Further analyses suggest that the atmospheric forcing anomalies are generated by the El Niño/Southern Oscillation Modoki through an atmospheric teleconnection after the regime shift. Plain Language Summary: The Kuroshio Extension (KE) is an intense eastward ocean flow in the western North Pacific formed after the Kuroshio separates from the Japanese coast. Satellite altimeter records and reanalysis products constrained by observational data have revealed that the KE strongly fluctuates with the decadal timescale between a stable and an unstable state, characterized by positive and negative sea surface height (SSH) anomalies to the south of the KE axis, respectively. A westward propagating oceanic wave with an SSH anomaly is considered as a primary cause of the phase reversal of the KE decadal variability. However, it has not been quantified to what extent wind forcing at each longitude contributes to the KE decadal variability through the ocean wave. In our study, we find that the largest contribution of wind forcing to the KE decadal variability is at 160°W, and 40% of the total variance in the KE variability is owing to wind forcing around 170°–150°W by simulation of westward propagating oceanic waves forced by reanalysis surface wind stress. Furthermore, it is suggested that the surface wind anomalies at 170°–150°W are remotely forced by a mode of climate variability in the tropical Pacific via an atmospheric bridge. Key Points: Wind forcing at 150°–170°W explains 40% of the total variance in Kuroshio Extension (KE) decadal variability via Rossby wavesThe dominant forcing at 150°–170°W is due to an atmospheric teleconnection of El Niño/Southern Oscillation (ENSO) ModokiThe statistically significant relation between KE decadal variability and ENSO Modoki is found only after the 1976/77 climate regime shift [ABSTRACT FROM AUTHOR]