Tai
Jen-Hua
Tai
Jen-Hua
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ArticleValidation of the remotely sensed nighttime sea surface temperature in the shallow waters at the Dongsha Atoll(Terrestrial, Atmospheric and Oceanic Sciences, 2017-06) Pan, Xiaoju ; Wong, George T. F. ; DeCarlo, Thomas M. ; Tai, Jen-Hua ; Cohen, Anne L.Fine scale temperature structures, which are commonly found in the top few meters of shallow water columns, may result in deviations of the remotely sensed night-time sea surface temperatures (SST) by the MODIS-Aqua sensor (SSTsat) from the bulk sea surface temperatures (SSTbulk) that they purport to represent. The discrepancies between SSTsat and SSTbulk recorded by temperature loggers at eight stations with bottom depths of 2 - 20 m around the Dongsha Atoll (DSA) between June 2013 and May 2015 were examined. The SSTsat had an average cool bias error of -0.43 ± 0.59°C. The bias error was larger in the warmer (> 26°C) waters which were presumably more strongly stratified. The root mean square error (RMSE) between SSTsat and SSTbulk, ±0.73°C, was 25% larger than that reported in the open northern South China Sea. An operational calibration algorithm was developed to increase the accuracy in the estimation of SSTbulk from SSTsat. In addition to removing the cool bias error, this algorithm also reduced the RMSE to virtually the same level as that found in the open northern South China Sea. With the application of the algorithm, in June 2015, the average SST in the lagoon of the DSA was raised by about 0.5°C to 31.1 ± 0.4°C, and the area of lagoon with SSTbulk above 31°C, the median value of the physiological temperature threshold of reef organisms, was increased by 69% to about three quarters of the lagoon.
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ArticleSubtidal current structure and variability of the continental shelf and slope of the northern South China Sea(Terrestrial, Atmospheric and Oceanic Sciences, 2017-06) Tai, Jen-Hua ; Yang, Kai-Chieh ; Gawarkiewicz, Glen G.The spatial structures and temporal variations in subtidal currents in the northern South China Sea (SCS) are quantified in this study using 20-day to 9-month measurements from eight acoustic Doppler current profiler moorings deployed on the shelf and slope. The moored observations demonstrated that subtidal currents varied considerably with no persistent current feature on the shelf or slope. In winter, transient northeastward subtidal flows appeared sporadically but only when the wind stress was lower than 1 dyne cm-2. The potential transient along-shelf flow formation mechanisms during winter are attributed to the variation in cross-shelf and along-shelf sea levels. The variation in cross-shelf sea level is due to Ekman transports varying over a period of 100 h. The variation in cross-shelf sea level gradient induced by the wind generates an along-shelf geostrophic current. When the sea level gradient anomaly is negative (i.e., sea level anomaly increased seaward), a transient shelf current appears, whereas the opposite phenomenon occurs as the sea level anomaly decreases seaward. The variation in the along-shelf sea level is due to the along-shelf sea level setup or set down as a result of shelf water accumulation or reduction near Hainan Island over a period of approximately 400 h. Other possible factors affecting subtidal currents include fresh water inputs from the Taiwan Strait, typhoons and eddies in the northern SCS.
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ArticleSubmesoscale eddy and frontal instabilities in the Kuroshio interacting with a cape south of Taiwan(American Geophysical Union, 2020-04-23) Cheng, Yu‐Hsin ; Chang, Ming-Huei ; Ko, Dong S. ; Jan, Sen ; Andres, Magdalena ; Kirincich, Anthony R. ; Yang, Yiing-Jang ; Tai, Jen‐HuaThe processes underlying the strong Kuroshio encountering a cape at the southernmost tip of Taiwan are examined with satellite‐derived chlorophyll and temperature maps, a drifter trajectory, and realistic model simulations. The interaction spurs the formation of submesoscale cyclonic eddies that trap cold and high‐chlorophyll water and the formation of frontal waves between the free stream and the wake flow. An observed train of eddies, which have relative vorticity about one to four times the planetary vorticity (f), is shed from the recirculation that occurs in the immediate lee of the cape as a result of flow separation. These propagate downstream at a speed of 0.5–0.6 m s−1. Farther downstream, the corotation and merging of two or three adjacent eddies are common owing to the topography‐induced slowdown of eddy propagation farther downstream. It is found that the relative vorticity of a corotating system (1.2f) is 70% weaker than that of a single eddy due to the increase of eddy diameter from ~16 to ~33 km, in agreement with Kelvin's circulation theorem. The shedding period of the submesoscale eddies is strongly modulated by either diurnal or semidiurnal tidal flows, which typically reach 0.2–0.5 m s−1, whereas its intrinsic shedding period is insignificant. The frontal waves predominate in the horizontal free shear layer emitted from the cape, as well as a density front. Energetics analysis suggests that the wavy features result primarily from the growth of barotropic instability in the free shear layer, which may play a secondary process in the headland wake.