| 摘要: | 越南外海的夏季湧升流是南海最重要的海洋學特徵之一。基於分析38年（1982年至2019年）的海表溫度（SST）衛星影像，此湧升流中心的位置被分為三個子區域：一為南部沿岸湧升區（SCU；北緯12.5°以南）、二為北部沿岸湧升區（NCU；北緯12.5°以北）、三為離岸湧升區（OU；東經110°以東）。本研究利用以衛星海表溫度建立之湧升流指數進一步量化這三個子區域的湧升流强度變化，並分别探討風場（包括風應力及風應力旋度）和海流對各湧升區的影響。分析結果指出，沿岸風應力誘發的離岸艾克曼傳輸可以產生有利於沿岸湧升流的條件，但並非控制沿岸湧升流年際變化的唯一因素。在越南南部海岸附近，伴隨越南東部雙渦漩結構的擴大，風應力旋度偶極的增強，導致越南外海東向噴流的增強和南移，以及與氣旋環流相關的南向沿岸流的增強，上述動力過程使得SCU增強，但抑制了NCU的發展。良好發展的雙渦漩結構是引發OU的重要條件，而OU的變化對於東向噴流的變化更為敏感。此外，本研究亦使用三维HYCOM海洋再分析資料探討SCU、NCU和OU的溫度、鹽度和水平速度的垂直結構，並利用Ω方程式重建越南附近海域之垂直速度。氣候平均結果顯示了伴隨湧升流產生之等溫線、等鹽線及溫躍層的向岸抬升、以及冷却和鹽化等效應，這些特徵可歸因於西方邊界流（東向噴流之上游）、離岸傳輸、12-16ºN之南向沿岸流、以及東向噴流等。在強SCU湧升流事件時期，風驅之艾克曼離岸輸送增強並伴隨增強的西方邊界流。在強NCU湧升流事件時期，風驅之艾克曼離岸輸送亦增強，但南向沿岸流消失，並由北向之西方邊界流取而代之。在強OU湧升流時期則有東向噴流北側之氣旋渦增強的現象。在Ω方程重建之垂直速度方面， 在SCU、NCU和OU等區域有顯著的向上垂直速度。就平均而言，SCU、NCU和OU之最大向上垂直速度分别為2.7公尺/天、1.7公尺/天和0.6公尺/天。在強湧升流事件時期，向上垂直速度則分別增加到3.1公尺/天、2.7公尺/天和1.4公尺/天。;Summertime upwelling off the southern Vietnamese coast is one of the most essential oceanographic features in the South China Sea. Based on analyzing the 38-year (1982–2019) sea surface temperature (SST) images, locations of summertime Vietnamese upwelling centers are found to be classified into three sub-regions: the Southern Coastal Upwelling (SCU; south of 12.5°N), the Northern Coastal Upwelling (NCU; north of 12.5°N), and the Offshore Upwelling (OU; east of 110°E). Variations of upwelling intensities in the three sub-regions are further quantified via an adaptive SST-based upwelling index, and possible processes relevant to wind field (including wind stress and its curl) and currents are, respectively, proposed. The analyses show that the local along-shore wind stress, inducing offshore Ekman transport, can produce the coastal upwelling-favorable condition. Besides, the enhancement of wind stress curl dipole off the southern Vietnamese coast, which accompanies the scaling up of the double-gyre structure east of Vietnam and results in the reinforcement and southward shift of eastward-flowing jet, and the exaggeration of the cyclonic gyre-associated southward along-shore current are responsible for the intensification of upwelling in the SCU, but suppress the development of upwelling in the NCU. The well-developed double-gyre structure serves as the essential condition for the OU occurrence and the OU is much more sensitive to the change of cyclonic gyre to the north of the jet. By means of the HYCOM product in the period of 1994-2015, the vertical structures of temperature, salinity, and horizontal velocity in the SCU, NCU, and OU regions are further explored and the vertical velocity field is reconstructed by the omega equation. Under the climatological condition, the isotherms, isohalines, and thermocline layers in the studied region experience an uplifting towards the coast, and a shoaling towards the north in the offshore area. These characteristics are attributed to the presence of 1) the western boundary current (the upstream of eastward-flowing jet), which sets isoline lifted toward the coast and is the western flank of the anti-cyclonic gyre in the southern basin of SCS, 2) the cross-shelf transport, which is driven by Ekman transport, 3) the southward coastal current in 12-16ºN, which belongs to the western flank of cyclonic gyre to the north of eastward-flowing jet and sets the isolines lifted toward the open ocean, and 4) the eastward-flowing jet. Under the strong upwelling condition, which is identified via the SSTUI, pronounced uplifting of isolines and intensified cooling and salting effects are observed in each subregion. During the strong SCU condition, the wind-driven cross-shelf transport is enhanced, accompanying the intensifications of the southward current along the coast and the eastward-flowing jet. In the strong NCU condition, cross-shelf transport is enhanced as well, while the southward coastal current disappears with a northward extension of the western boundary current. In the strong OU condition, the strengthening of a cyclonic eddy is observed to the north of the jet. By using the omega equation, well-defined upwellings in the SCU, NCU, and OU regions are characterized by strong upward velocities. On average, the maximum upward velocities in SCU, NCU, and OU are estimated at 2.7 m/d, 1.7 m/d, and 0.6 m/d, respectively. Enhanced vertical velocities during stronger upwelling conditions are estimated at 3.1 m/d, 2.7 m/d, and 1.4 m/d, respectively. |
