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°C with a maximum observed value of 4.7°C, and the SST drop rate was 0.3-0.5°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°C on average and dropped at a rate of 0.3-0.5°C hr −1 . The observational record indicated that the water cooled as much as 4.7°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.

Effects of feeding on the physiological performance of the stony coral Pocillopora acuta

“…Fox et al 51 demonstrated that in regions with relatively high primary production (as gauged by chlorophyll-a levels), corals are consistently more heterotrophic. The highest chlorophyll-a values reach 0.8 mg m −3 , with the monthly average between 0.17 and 0.37 mg m −3 (annual mean = 0.25 mg m −3 ) in Nanwan Bay 52 . Such high variation is due to upwelling of cold seawater and nutrients, the latter of which stimulate phytoplankton growth and thereby leads to increased chlorophyll-a concentrations at our study site 53 .…”

High‐Resolution Numerical Modeling of Barotropic Global Ocean Tides for Satellite Gravimetry

“…This includes periodic variations in ocean currents registered by moored instruments or acoustic tomography (Dushaw et al., 1997; Luyten & Stommel, 1991; Ray, 2001) as well as by induced secondary magnetic fields (Maus & Kuvshinov, 2004; Saynisch et al., 2018), sea surface height changes measured from tide gauges and satellite altimetry (Doodson, 1928; Schrama & Ray, 1994), and global bottom pressure variations from pelagic pressure recorders and gravimetric satellite missions (Wiese et al., 2016). More recently, even tiny variations in sea surface temperature (Hsu et al., 2020) and tropical precipitation observations (Kohyama & Wallace, 2016) were related to ocean tidal dynamics.…”

Impacts of heat stress and storm events on the benthic communities of Kenting National Park (Taiwan)

“…Satellite-based Sea Surface Temperature (SSTs) estimations can be crucial for large-scale studies or locations that are difficult to access ( Van Hooidonk & Huber, 2009a ; Van Hooidonk & Huber, 2009b ; Burt et al, 2019 ). However, the specific environmental context of certain areas—such as KNP, where tide-associated upwelling and recurrent typhoons induced important smaller scale seawater temperature variations ( Keshavmurthy et al, 2019 ; Hsu et al, 2020 ; Lee et al, 2020 )—are not likely to be identified by satellite data; this may have misguided estimates of the capacity of reefs to resist large-scale heat-stresses.…”

“…In the bay of Nanwan, a tidal upwelling can cause further sudden temperature drops as the topography of the bay induces the formation of cyclonic eddies. These eddies move from west to east throughout the bay, during the ebb tidal current, mixing the cooler deeper seawater layers with the warmer shallow ones ( Lee et al, 1997 ; Lee et al, 1999 ; Hsu et al, 2020 ). The upwelling within Nanwan has been shown to play an important role in the physiology of marine organisms in the area ( Chen et al, 2005 ; Keshavmurthy et al, 2019 ), especially during summer periods, when the upwelling can cause daily variations in seawater temperatures to increase by up to 2.5 °C ( Hsu et al, 2020 ).…”

“…These eddies move from west to east throughout the bay, during the ebb tidal current, mixing the cooler deeper seawater layers with the warmer shallow ones ( Lee et al, 1997 ; Lee et al, 1999 ; Hsu et al, 2020 ). The upwelling within Nanwan has been shown to play an important role in the physiology of marine organisms in the area ( Chen et al, 2005 ; Keshavmurthy et al, 2019 ), especially during summer periods, when the upwelling can cause daily variations in seawater temperatures to increase by up to 2.5 °C ( Hsu et al, 2020 ). In addition, some sites in Nanwan have been further affected by warmer waters (as much as 2 to 3 °C higher than other sites in summer) from the nearby outlet of the nuclear power plant since its opening in 1987 ( Huang, Hung & Fan, 1991 ; Carballo-Bolaños et al, 2019 ).…”