Application of singular vector analysis to the Kuroshio large meander

Abstract

Singular vector analysis is applied to the formation of the Kuroshio large meander south of Japan. We use a simulation run of an ocean general circulation model (OGCM), in which a large meander occurs, as the background state. Singular vectors are calculated by the Lanczos method in which the tangent linear and adjoint models of the OGCM are used. We apply an incremental correction approach for setting up the initial perturbation and averaging for the final norm in order to get modes reflecting nonlinear physics by filtering out high‐frequency variations. The perturbation extracted as the leading right singular vector affects the formation of the large meander most if it is set to the background state two months before. When the perturbation is added, it enhances an anticyclonic eddy southeast of Kyushu in the upper layer, which induces downwelling north of it and develops an anticyclonic eddy in the deep layer. The anticyclonic eddy produces a cold anomaly at the downstream edge, and produces baroclinic instability there, resulting in further development of the large meander. In contrast, the anticyclonic eddy in the deep layer is weakened when the perturbation is subtracted, the development of the cold anomaly in the deep layer is delayed, and consequently the meander is not so well developed. Thus singular vector analysis reveals the role of the anticyclonic eddy approaching southeast of Kyushu in the upper layer, and indicates the importance of the anticyclonic eddy in the deep layer.