由2003-2005年在南灣近岸海底實測海溫資料，經72小時低通過濾之後，顯示夏季(5-10月)有約1-4週明顯的溫降與回溫現象，溫降幅度約2-5℃。利用小波轉換(Wavelet)分析水溫資料，顯示高頻的溫降與潮汐(尤其在大潮期間)有密切的關係。根據衛星浮標漂流軌跡統計的結果(Centurioni et al., 2004)，顯示夏季黑潮流軸較冬季大幅退出呂宋海峽，同時南海北部環流向東伸展。一般而言，典型南海水在120公尺以下的水溫比黑潮水大約低4℃，若逢大潮期間，呂宋海峽內強烈的內潮現象持續將較冷的南海次表層水往海面湧升，加上垂直混合效應，使得南灣近岸底層水溫下降；大潮過後，內潮的湧升現象減緩，南灣水溫也隨之逐漸回升。由蘭嶼風速風向資料與QSCAT區域風場資料顯示，2004年敏督莉颱風引進的西南氣流，加強了長週期溫降事件的溫降與回復時間以及溫降幅度。 The near bottom temperature off the western coast of the Nan Wan Bay showed that there are about for 1-4 weeks temperature drop-recovery events in each summer of 2003-2005. The temperature difference in each event ranges from 2 to 5oC. Sea level data at Nan Wan and Lanyu, wind data at Lanyu and collected from QSCAT, and typhoon data in 2003-2005 were used to facilitata the explanation of the summertime temperature drop-recovery event. The scenario is that, in summer, the intruded Kuroshio retreats from the northern South China Sea to the entrance of the Luzon strait ; the South China Sea water expands toward the Luzon strait where is immediately adjacent to the Nan Wan Bay. Historical hydrographic data suggest that the temperature of South China Sea subsurface water is lower than that of Kuroshio intruded water by 4oC. How the subsurface cold water intrudes to the shallow shelf off the western Bay is attributed to the vigorous internal tides generated in the Luzon strait particularly during spring tide. The internal tides pump the subsurface cold water continuously to the surface, which may propagate toward the coast following the bottom topography, resulting in the 1-4 weeks temperature drop-recovery in the inner shelf of the western Bay in summer.