Effects of large-scale wind on the Kuroshio path south of Japan in a 60-year historical OGCM simulation
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Effects of large-scale wind on the Kuroshio path south of Japan in a 60-year historical OGCM simulation Hiroyuki Tsujino • Shiro Nishikawa • Kei Sakamoto Norihisa Usui • Hideyuki Nakano • Goro Yamanaka
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Received: 12 July 2012 / Accepted: 17 December 2012 Springer-Verlag Berlin Heidelberg 2013
Abstract The effects of large-scale wind forcing on the bimodality of the Kuroshio path south of Japan, the large meander (LM) and non-large meander (NLM), were studied by using a historical simulation (1948–2007) with a highresolution Ocean general circulation models (OGCM). The Kuroshio in this simulation spent much time in the NLM state, and reproduced several aspects of its long-term path variability for the first time in historical OGCM simulation, presumably because the eddy kinetic energy was kept at a moderate level. By using the simulated fields, the relationships between wind forcing (or Kuroshio transport) and path variation proposed by past studies were tested, and specific roles of eddies in those variations were investigated. The long-term variation of the simulated net Kuroshio transport south of Japan was largely explained by the linear baroclinic Rossby wave adjustment to wind forcing. In the simulated LM events, a triggering meander originated from the interaction of a wind-induced positive sea surface height (SSH) anomaly with the upstream Kuroshio and was enlarged by cyclonic eddies from the recirculation gyre. The cyclonic eddy of the trigger meander was followed by a
H. Tsujino (&) S. Nishikawa K. Sakamoto N. Usui H. Nakano G. Yamanaka Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan e-mail: [email protected] S. Nishikawa Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8568, Japan Present Address: S. Nishikawa Data Research Center for Marine-Earth Sciences, Japan Agency for Marine-Earth Science and Technology, Yokohama, Kanagawa 236-0001, Japan
sizable anticyclonic eddy on the upstream side. Subsequently, a weak (strong) Kuroshio favored the LM (NLM). The LM tended to be maintained when the Kuroshio transport off southern Japan was small, and increasing Kuroshio transport promoted decay of an existing LM. The supply of disturbances from upstream, which is related to the wind-induced SSH variability at low latitudes, contributed to the maintenance of an existing LM. Keywords Kuroshio Large meander OGCM Historical simulation North Pacific Ocean Long-term variability
1 Introduction The Kuroshio, the western boundary current of the winddriven subtropical gyre of the North Pacific Ocean, imposes profound effects on the climate and marine environment of the western North Pacific region. It carries a large amount of heat from low to middle latitudes, keeping the climate mild there. Its path exhibits a remarkable bimodality south of Japan (Fig. 1) with a multi-decadal time scale, affecting ship navigation and fisheries. Recent studies suggest that the variability of the Kuroshio has nontrivial impacts on aspects of
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