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Daily near surface temperature variation mechanism
of Dongsha Atoll during summer 2014
Dongsha atoll,summer,near surface ocean,daily temperature variation,heat budget,surface heat flux,tidal current,horizontal advection,
|Publication Year :||2019|
|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。
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).
|Appears in Collections:||海洋研究所|
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