Tide-Induced Periodic Sea Surface Temperature Drops in the Coral Reef Area of Nanwan Bay, Southern Taiwan

Abstract

Tidal current-induced cyclonic eddies cause cold-water upwelling and periodic sea surface temperature (SST) drops around the coral reef area in Nanwan Bay, which is located at the southern tip of Taiwan. This study used Himawari-8 satellite data and tide gauge and coastal ocean dynamics application radar (CODAR) data to analyze the characteristics of the SST drops and cyclonic eddy propagation and used an oceanic general circulation model (OGCM) to simulate the tidal current flowing process. According to the CODAR data analysis, the mixed primary semidiurnal tide had an average current velocity of 0.3-0.4 m s(-1) throughout the bay, and the average life cycle of a cyclonic eddy is 6.6 hr, with a propagation speed of 0.35 m s(-1). The SST drop during the spring tide period was greater than that during the neap tide period, and the SST dropped in both summer and winter. The average daily SST drop in the summer reached 2 degrees C with a maximum observed value of 4.7 degrees C, and the SST drop rate was 0.3-0.5 degrees C hr(-1). The annual mean chlorophyll-a concentration was 0.25 mg m(-3). This study explored the special properties of the Nanwan Bay coral reef area from the perspective of ocean physics to allow ecologists to facilitate the implementation of long-term conservation and monitoring programs. Plain Language Summary In recent years, the increased global sea temperatures have caused severe coral bleaching; however, Nanwan Bay has suffered a comparatively smaller impact. To understand why, we analyzed satellite and radar data. We found that Nanwan Bay experienced an almost daily sea surface temperature (SST) drop during the summer and that this periodic variation was caused by a cyclonic eddy upwelling cold water to the surface. In the summer, the SST drop was 2 degrees C on average and dropped at a rate of 0.3-0.5 degrees C hr(-1). The observational record indicated that the water cooled as much as 4.7 degrees C in 1 day. These eddies were caused by the unique topography in this area of the bay; they survived an average of 6.6 hr and moved at a rate of 0.35 m s(-1). The formed eddies disappeared in the bay or flowed southward out of the bay. With this study, we hope to help marine ecologists understand the Nanwan Bay environment from a geophysical perspective so that they have sufficient background knowledge when investigating coral biology and coral reef ecology in the future.