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[AOS15-01] Intrusion of lower water into a coastal bay driven by a tropical storm
Keywords:typhoon, inertial oscillation, internal gravity waves, bottom slope
Water circulations and exchanges in coastal bays are important process for fishery as well as for oceanography. In Otsuchi Bay, located in the Sanriku ria coast on the eastern coast of northeastern Japan facing the North Pacific, we are monitoring water properties such as temperature (Tanaka et al. 2017).
A significant temperature change was recorded in August 2016 after the center of typhoon Lionrock (the 16th typhoon in 2016) passed by just near Otsuchi Bay. The change was dominant at 25 m depth, which is close to the bottom there. The temperature dropped abruptly by about 8ºC and undulated for following several days. Meanwhile, the temperature at 5 m depth had a small time change. These suggest that the water motion is trapped by the bottom.
In order to understand the dynamic process, a numerical experiment is made to represent the effect of the tropical storm on the ocean. The numerical model is the Princeton Ocean Model (POM) with horizontal resolution of 1/10º × 1/12º and 52 vertical levels. The area lies in 30º–45ºN and 135º–150ºE with a realistic topography but the maximum depth is limited to 1500 m. Initially, the ocean has a horizontally uniform vertical stratification representing summer condition and has no motion. It is driven by surface wind stress and atmospheric pressure imitating typhoon Lionrock as in the manner of Hong and Yoon (2003).
In the numerical result, the storm drives upper motions similar to the inertial oscillation described in Gill (1982). In the lower layer below 200 m depth, the velocity has opposite direction to the upper velocity. This flow pattern can be explained by the internal gravity lee waves with near-inertial period and vertical structure of the first baroclinic mode that is driven by a moving storm. In the open ocean, the temperature or density change is dominated by the mechanical mixing by the wind forcing. The contribution of the vertical motion in the lee waves and that of the Ekman vertical motion seem both minor.
The temperature change at the corresponding point in the numerical model is compared to the observation in Otsuchi Bay. They show quite similar time changes despite the forcing in the model is simple. Near the coast, vertical displacement of the thremocline is significantly larger than that in the open ocean. It is suggested that the vertical displacement of the thermocline is generated by the lower flow in the baroclinic near-inertial waves going up and down along the bottom slope, which is similar to the case of the internal tide generation.
References
Hong CH and Yoon JH, 2003. JGR, 108.
Gill AE, 1982. Atmosphere-ocean dynamics, Chap 9.11.
Tanaka K et al., 2017. JO, 73.
A significant temperature change was recorded in August 2016 after the center of typhoon Lionrock (the 16th typhoon in 2016) passed by just near Otsuchi Bay. The change was dominant at 25 m depth, which is close to the bottom there. The temperature dropped abruptly by about 8ºC and undulated for following several days. Meanwhile, the temperature at 5 m depth had a small time change. These suggest that the water motion is trapped by the bottom.
In order to understand the dynamic process, a numerical experiment is made to represent the effect of the tropical storm on the ocean. The numerical model is the Princeton Ocean Model (POM) with horizontal resolution of 1/10º × 1/12º and 52 vertical levels. The area lies in 30º–45ºN and 135º–150ºE with a realistic topography but the maximum depth is limited to 1500 m. Initially, the ocean has a horizontally uniform vertical stratification representing summer condition and has no motion. It is driven by surface wind stress and atmospheric pressure imitating typhoon Lionrock as in the manner of Hong and Yoon (2003).
In the numerical result, the storm drives upper motions similar to the inertial oscillation described in Gill (1982). In the lower layer below 200 m depth, the velocity has opposite direction to the upper velocity. This flow pattern can be explained by the internal gravity lee waves with near-inertial period and vertical structure of the first baroclinic mode that is driven by a moving storm. In the open ocean, the temperature or density change is dominated by the mechanical mixing by the wind forcing. The contribution of the vertical motion in the lee waves and that of the Ekman vertical motion seem both minor.
The temperature change at the corresponding point in the numerical model is compared to the observation in Otsuchi Bay. They show quite similar time changes despite the forcing in the model is simple. Near the coast, vertical displacement of the thremocline is significantly larger than that in the open ocean. It is suggested that the vertical displacement of the thermocline is generated by the lower flow in the baroclinic near-inertial waves going up and down along the bottom slope, which is similar to the case of the internal tide generation.
References
Hong CH and Yoon JH, 2003. JGR, 108.
Gill AE, 1982. Atmosphere-ocean dynamics, Chap 9.11.
Tanaka K et al., 2017. JO, 73.