Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving Model Hindcast*

Abstract

Low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidecadal hindcast by a high-resolution (0.1°), eddy-resolving, global ocean general circulation model for the Earth Simulator (OFES). In both the OFES hindcast and satellite altimeter observations, low-frequency sea surface height (SSH) variability in the North Pacific is high near the KE front. An empirical orthogonal function (EOF) analysis indicates that much of the SSH variability in the western North Pacific east of Japan is explained by two modes with meridional structures tightly trapped along the KE front. The first mode represents a southward shift and to a lesser degree, an acceleration of the KE jet associated with the 1976/77 shift in basin-scale winds. The second mode reflects quasi-decadal variations in the intensity of the KE jet. Both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations.A linear Rossby wave model forced by observed wind successfully reproduces the time series of the leading OFES modes but fails to explain why their meridional structure is concentrated on the KE front and inconsistent with the broadscale wind forcing. Further analysis suggests that KE variability may be decomposed into broad-and frontal-scale components in the meridional direction-the former following the linear Rossby wave solution and the latter closely resembling ocean intrinsic modes derived from an OFES run forced by climatological winds. The following scenario is suggested for low-frequency KE variability: basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic modes of the KE jet and reorganizing SSH variability in space.

Climatological mean features and interannual to decadal variability of ring formations in the Kuroshio Extension region

“…In this study, we consider the KE jet as a single jet, following previous studies (e.g., Qiu and Chen 2005;Taguchi et al 2007;Sasaki and Schneider 2011). Thus, we ignore the bifurcation branches of the KE jet (e.g., one branch around the Shatsky Rise shown in Mizuno and White 1983), and only focus on ring formation and absorption by the main part of the KE jet.…”

Climatological mean features and interannual to decadal variability of ring formations in the Kuroshio Extension region

“…It is well known that the KE jet undergoes prominent fluctuations on decadal timescales (e.g., Deser et al 1999;Schneider et al 2002;Taguchi et al 2007;Sasaki and Schneider 2011;Sasaki et al 2013). Qiu and Chen (2005b) showed that when the eddy activity in the KE region is high and low, the KE jet is weakening and strengthening, respectively, and referred to the former as a stable state and the latter as an unstable state.…”

“Hot Spots” in the climate system—new developments in the extratropical ocean–atmosphere interaction research: a short review and an introduction

“…In reality, however, oceanic Rossby waves forced by wind stress variations in the central Pacific propagate westward, while associated sea surface height anomalies become concentrated into the jet to effectively modify it (Sasaki and Schneider 2011). The wind variations thus act as a "pace maker" for the quasi-decadal KE variability by triggering its regime transition (Taguchi et al 2007). Meanwhile, modulated mesoscale eddy activity can lead to modulations in sub-mesoscale streamers and associated vertical motion (Sasaki et al 2014).…”