10:45 AM - 11:00 AM
[AOS19-07] Dynamics of water exchange processes at a deep submarine canyon off the western coast of the Sakhaline which causes the origin of the cold water belt along the Soya Warm Current
★Invited Papers
Keywords:water exchange across the shelf break, continental shelf wave, submarine canyon, cold water belt along the Soya Warm Current
A cold water belt (CWB) is observed off the Soya Warm Current, the former of which appears from the west coast of the Sakhaline Island. Observations of the CWB (Kuma et al., 2016; Iida et al., 2018) indicates that the CWB, characterized by a cold and saline water (temperature 7℃, and salinity 34.0), is derived from the lower part of the thermocline in the Japan Sea basin. This implies that the cold and saline water in the Japan Sea basin should overcome the shelf break, which is not easy in general, and be transported to the vicinity of the coast of Sakhaline. The aim of this study is to elucidate mechanisms of water exchange between the Japan Sea basin (continental slope) and the continental shelf.
We first analyzed the output of a general circulation model OFES with the 1/30° resolution. It was found that the water over the continental slope enters the continental slope at a deep submarine canyon off the Kholmsk city on the west coast of the Sakhaline. Further, a barotropic model using Princeton Ocean Model (POM) with a realistic topography shows the exchange at the canyon in a similar manner to OFES, even though water exchange would not readily occur at the shelf break because of a potential vorticity (PV) barrier. We thus conducted numerical experiments using idealized topographic features mimicing the canyon off the Kholmsk in order to understand the barotropic exchange processes. The throughflow across the Soya Strait occurs owing to the adjustment of sea level difference between the Japan Sea and the Sea of Okhotsk by long shelf waves. It was found that the shelf waves are scattered into the second and third mode waves at the canyon, which have large transport over the continental slope; the scattered flow over the continental slope directed southward, and enters the continental shelf at the canyon. Further, nonlinearity is important in the vicinity of the canyon during the scattering process. Specifically: 1) nonlinearity produces a wave guide across the shelf break overcoming the PV barrier, by bringing low PV water from the continental slope to the continental shelf; 2) High-order mode waves (higher than 4th mode) are retarded from propagation because of the background flow caused by 1st -3rd modes, making the flow over the continental slope steady. The nonlinear steady flow over the slope upwells at the canyon and connects to the flow over the shelf, and consequently flows through the strait.
Finally. hydrographic data during 1960-1985, archived at the Japan Hydrographic Data Center, was analyzed. Potential temperature and salinity from July to October was used. It was confirmed that the upwelling occurs at the canyon off the Kholmsk.
We first analyzed the output of a general circulation model OFES with the 1/30° resolution. It was found that the water over the continental slope enters the continental slope at a deep submarine canyon off the Kholmsk city on the west coast of the Sakhaline. Further, a barotropic model using Princeton Ocean Model (POM) with a realistic topography shows the exchange at the canyon in a similar manner to OFES, even though water exchange would not readily occur at the shelf break because of a potential vorticity (PV) barrier. We thus conducted numerical experiments using idealized topographic features mimicing the canyon off the Kholmsk in order to understand the barotropic exchange processes. The throughflow across the Soya Strait occurs owing to the adjustment of sea level difference between the Japan Sea and the Sea of Okhotsk by long shelf waves. It was found that the shelf waves are scattered into the second and third mode waves at the canyon, which have large transport over the continental slope; the scattered flow over the continental slope directed southward, and enters the continental shelf at the canyon. Further, nonlinearity is important in the vicinity of the canyon during the scattering process. Specifically: 1) nonlinearity produces a wave guide across the shelf break overcoming the PV barrier, by bringing low PV water from the continental slope to the continental shelf; 2) High-order mode waves (higher than 4th mode) are retarded from propagation because of the background flow caused by 1st -3rd modes, making the flow over the continental slope steady. The nonlinear steady flow over the slope upwells at the canyon and connects to the flow over the shelf, and consequently flows through the strait.
Finally. hydrographic data during 1960-1985, archived at the Japan Hydrographic Data Center, was analyzed. Potential temperature and salinity from July to October was used. It was confirmed that the upwelling occurs at the canyon off the Kholmsk.