Decay mechanism of the 2004/05 Kuroshio large meander

Abstract

[1] The decay mechanism of the 2004/05 Kuroshio large meander (LM) is examined using a data assimilation/prediction system. Assimilation results reveal detailed variations of the Kuroshio path during the LM period. The Kuroshio takes the typical LM in September 2004 to January 2005. The LM then moves eastward and starts crossing the Izu Ridge (IR) in late January 2005. A minor path transition occurs again in June 2005. As a result, the meandering path straddles the IR, and subsequently the meander amplitude decreases rapidly, resulting in the disappearance of the LM. Prediction results suggest that these path transitions are caused by disturbances propagating along the Kuroshio. The assimilation results also indicate that the LM weakens after the first transition in late January 2005 and thus the transition triggers the subsequent LM decay. We propose three possible mechanisms for the weakening of the LM. (1) Extraction of high potential vorticity (PV) water from the LM by disturbances coming from the upstream of the Kuroshio. (2) Generation and propagation of the costal trapped waves, which are induced as a result of interaction of cold eddy of the LM and the IR, and extract potential energy from the LM. (3) Outflow of high-PV water of the LM to the downstream passing through a southern gate of the IR south of Hachijo-jima, which works after the meandering path straddles the IR. The mechanisms 2 and 3 are closely related to the existence of the IR, suggesting that the LM decay is prescribed by the IR.