| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 96 === Upper ocean response to tropical cyclones is studied, using a three-dimensional primitive equation model that has higher vertical resolution in the mixed layer and uses level-2 turbulence closure scheme to estimate vertical mixing. In this study, the model is applied to investigate ocean responses to typhoons passing over open oceans and near the continental shelf region with or without strong background currents.
The model results showed that in the open ocean, without the interference of topography and background flows, the ocean cooling is primarily influenced by the wind frequency of the passing storm. When the wind frequency is lower than the local inertial frequency, upwelling is the dominant cooling process. When the wind resonates with the current at the inertial frequency, the mixing process becomes important. In resonant cases, momentum penetrates to the thermocline and forces the water to move. The flow is then coupled with that in the mixed layer, so the inertial oscillation of the upper ocean is persistent, which results in long-lasting cooling by mixing. For non-resonant cases, the vertical transfer scale is limited and cooling decays in 5 inertial periods. However, the vertical processes may become secondary when the typhoon moves to a region with both complicated topography and strong background flows, such as the area on the shelf of the East China Sea, near northeastern Taiwan. Both observations and model results indicate that the strong northeast wind in this area accompanied by a typhoon of certain path could significantly alter the circulations on the shelf. An intrusion event may be triggered through a similar mechanism as induced by the winter monsoon but at faster pace. Therefore, the cooling in this area is due to the onshore transport of the Kuroshio’s subsurface water onto the continental shelf, rather than entrainment mixing. If the background current is insignificant, the storm-induced circulation is then influenced by the regional geometry and topography and some warming areas may be produced. As shown in the event of a typhoon moving across the continental shelf of the northern South China Sea, the storm had built up a southward current east of Hainan Island before its center arrived. This flow feature is supportive of the downwelling structure later induced by the passage of the storm, and the warming effect that it creates remains after the storm impacts. Such warming will lead to an increase of upper ocean heat content, which may assist to uphold the storm’s intensity upon landing. In addition, there were topographically trapped waves, which were generated at 250 m depth, thus the cold anomaly quickly propagated southward along the shelf. Through this mechanism, the southern South China Sea, though remote from the storm’s wind field, was cooled in the storm event.
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