10:00 AM - 10:15 AM
[ACG47-05] Coastal trapped wave response to boundary current variations
Keywords:Coastal trapped wave, Boundary current, Bottom Ekman pumping/suction, Forced–dissipative system, Eigenmode expansion, Kuroshio
Coastal trapped waves (CTWs) play a crucial role in coastal variability over long distances. A primary energy source for CTWs is wind stress, as indicated by numerous studies. On the other hand, coastal responses that could result from boundary current variations are also found far from the source regions. For example, Kida et al. (2020) found an increasing trend in the sea level and the volume transport of the Tsushima Current, which could be related to the northward shift of the Kuroshio axis south of Japan, suggesting that CTWs are generated by variations in the boundary currents such as the Kuroshio. However, the generation mechanism, amplitude, and propagation distance of such CTWs are unclear.
In this study, we model the generation, propagation, and dissipation processes of CTWs forced by Ekman pumping/suction over a sloping bottom in a manner similar to the Matsuno–Gill problem, to derive a semi-analytical solution for the coastal sea level and velocity anomalies. The validity of this solution is confirmed by idealized numerical experiments where localized Ekman pumping/suction is placed on a uniform continental shelf and slope. The solution shows that the spatial extent of the response is governed by the propagation speed and the decay time scale of the lowest-mode CTW. Furthermore, when the Ekman pumping/suction at the bottom is of similar magnitude to that at the surface due to wind stress, the former tends to produce a stronger response.
Finally, a numerical experiment assuming idealized bottom Ekman pumping/suction associated with the northward shift of the Kuroshio axis south of the Kii Peninsula shows that the generated CTWs propagate clockwise around the Japanese archipelago up to the Noto Peninsula while being gradually dissipated by the complex bottom and coastal topography. The major part of the CTWs, however, propagates southwestward along the continental slope of the East China Sea off the Goto Islands, with a relatively small fraction entering the Sea of Japan through the Tsushima Strait. Nevertheless, it can still cause part of the sea level and velocity anomalies along the wide area of the west coast of Japan obtained by Kida et al. (2020).
In this study, we model the generation, propagation, and dissipation processes of CTWs forced by Ekman pumping/suction over a sloping bottom in a manner similar to the Matsuno–Gill problem, to derive a semi-analytical solution for the coastal sea level and velocity anomalies. The validity of this solution is confirmed by idealized numerical experiments where localized Ekman pumping/suction is placed on a uniform continental shelf and slope. The solution shows that the spatial extent of the response is governed by the propagation speed and the decay time scale of the lowest-mode CTW. Furthermore, when the Ekman pumping/suction at the bottom is of similar magnitude to that at the surface due to wind stress, the former tends to produce a stronger response.
Finally, a numerical experiment assuming idealized bottom Ekman pumping/suction associated with the northward shift of the Kuroshio axis south of the Kii Peninsula shows that the generated CTWs propagate clockwise around the Japanese archipelago up to the Noto Peninsula while being gradually dissipated by the complex bottom and coastal topography. The major part of the CTWs, however, propagates southwestward along the continental slope of the East China Sea off the Goto Islands, with a relatively small fraction entering the Sea of Japan through the Tsushima Strait. Nevertheless, it can still cause part of the sea level and velocity anomalies along the wide area of the west coast of Japan obtained by Kida et al. (2020).