| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 87 === Historical data in Nan Wan indicate that sudden temperature drops occur daily during spring tides. A typical drop can be 8~9 ℃ in four hours. Subsequent relaxation takes about the same amount of time.
From 1995 on, a series of tide gages, CTD, current meters, temperature sensors and ADCP has been deployed to infer mechanisms leading to sudden temperature drops. Data in the spring tidal period indicate the development of a cyclonic eddy in the western half of the basin as the tide is ebbing from higher high water to lower low water. Later, an anticyclonic eddy develops from lower high water to higher high water. The anticyclone first shows up in the eastern side of the basin, expanding in time to the entire basin. In general, the cyclone on the west side is weaker and smaller than the subsequent expanding anticyclone. Both eddies are sufficiently cyclostrophic and therefore induce upwelling. A three-dimensional numerical simulation using realistic basin geometry and realistic tides lends support to observations.
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