2014年夏季東沙環礁表層水體溫度日變異性成因

Abstract

本研究以2014夏季6月2日至6月14日間東沙環礁海表層溫變為例，以實測資料配合三維的海洋數值模式(Regional Ocean Modeling System, ROMS)探討此區溫變機制。模式作用力包含真實的海表淨熱通量與淨淡水通量、潮汐、風應力、大洋環流，並以實際觀測進行驗證，包括海面高度變化、流場、溫度場。 首先針對東沙環礁海表層的溫度變化做傅立葉分析，結果顯示環礁海表層溫度有明顯日變異性。對於日周期溫變訊號而言，潮汐與海表淨熱通量此二短周期作用力為關鍵的影響因素。本研究使用熱收支方程進一步了解此二作用力影響形式，並分析熱收支子項佔比，揭開溫變之控制機制。 熱收支子項分析結果顯示由海表淨熱通量主導的垂直熱通量項為海表層溫變的主導項，次要項則為平流項。統計上顯示垂直熱通量項貢獻8～9成的溫變，說明海表層溫變主要控制機制為垂直一維加熱。 次要的平流項整體時序統計雖僅有1～2成的溫變貢獻，但在某些時段可達3～4成，因此亦不可忽略平流項的影響力。本研究以東北瀉湖區為例，揭示平流項重要性隨時間變化的原因。 研究結果顯示前期(6/2～6/8)平流項之溫變重要性隨時間遞減，原因為M2與O1分潮與此地主要分潮K1隨時間產生相位偏移導致潮流變弱。在此地東西側溫差日變化強度大致相同之情況下，潮流變弱，則平流項也逐漸減小，其對溫變的重要性亦隨之下降。 後期(6/10～6/14)平流項之溫變重要性隨時間逐漸回升，雖潮流於後期亦逐漸增強，但單考慮潮流無法解釋此變化。分析結果顯示，海表淨熱通量與水深的相對相位亦對水平熱傳輸有顯著影響: 當兩者為反相位時，海表淨熱通量加熱與冷卻效果明顯，造成溫變遽增，進而使得平流項隨時間遞增。 綜合上述，東沙環礁水平熱傳輸的時變性主要受海表淨熱通量與潮汐(K1,M2,O1三個分潮)間之相對相位所控制。

We use a 3D hydrodynamic model (ROMS) to investigate the mechanism of daily near surface temperature variation at Dongsha Atoll during 2nd ~14th of June, 2014. Forcings in the model include surface heat flux(Q), evaporation/precipitation, tides , wind stress and open ocean current. The numerical model has been first validated by comparing model-derived sea surface elevation, depth-averaged velocity and near surface temperature against observations. Reasonable agreement with observations was found, indicating that the numerical model can represent the main features of temperature variations at Dongsha. Analyses of heat budget, integrated over the upper 4 m of the water column, show that over the entire study period the vertical heat flux divergence(VHF) accounted for 80~90 percent of the daily variation, while the rest was contributed by horizontal advection(Adv). VHF was dominated by the surface flux Q. Hence, the near surface temperature variation of Dongsha was primarily governed by one dimensional vertical balance between tendency and Q. Although the horizontal advection was of secondary importance, its contribution to the heat budget showed clear temporal variation, with the magnitude of Adv-to-tendency ratio changing between 10 to 40 percent. To examine this temporal variation, we further analyze the phase relation between tidal current and temperature gradient, using the northeastern lagoon as a case study. The main results are : During the earlier period (6/2～6/8), the temperature gradient is relatively steady, but Adv decreased with time. The decrease was due mainly to the weakening of tidal currents as a result of the phase difference between diurnal (K1&Q1) and semidiurnal (M2) tides. By contrast, during the later period (6/10～6/14), both Adv and tidal current increased with time. However, tidal current alone cannot explain the Adv increase. It is shown that the out-of-phase between surface flux Q and water level led to greater temperature changes. The changes in temperature gradient thus contributed significantly to the Adv variations. Overall, the temporal variation of horizontal heat transport at the Dongsha Atoll was largely controlled by phase relation between daily heat flux and tides (K1, M2, and O1).