利用海氣浮標探討西北太平洋上層海水溫度之日變化

Abstract

海洋表層數公尺的溫度日變化對於海氣之間的水氣通量與熱通量變化有著重大的影響。本研究使用位於西北太平洋的兩組海氣象浮標進行為期5個月連續現場觀測，將其所得的海洋與大氣資料進行分析，觀測結果顯示海洋表層數公尺的溫度變化主要受控於每日的日照變化，也就是全日熱力潮(diurnal thermal tide, DTT)，由日照加熱所產生的日溫層平均可達5-10公尺深，並使1公尺深溫度(T_1m)平均升溫約0.4 °C，但是與熱通量形成約4小時相位差，彼此之間的相位差會受強風所造成的紊流混合而縮短。表層海洋的溫度觀測結果可被區分為(1)對流穩定狀態(~20 %)，表示充足日照(> 400 W m-2)且風速微弱(< 8 m s-1)的加熱階段；(2)對流不穩定狀態(~5 %)，為表層海水溫度翻轉事件，歸因於海洋被大氣冷卻或是有降雨發生；以及(3)均勻混合狀態(~75 %)發生在強風時期(> 10 m s-1)。同時我們將Himawari-8衛星遙測海表溫度與兩組海氣象浮標觀測T_1m進行比較，彼此之間分別有0.15 °C與-0.02 °C的差值，從衛星遙測顯示最主要的海表溫日變化(> 1 °C)發生在夏季期間的副熱帶高壓帶，而海氣象浮標位於副熱帶高壓帶的西邊界顯示與衛星相近的觀測結果。Price-Weller-Pinkel (PWP)一維混合模式所模擬的溫度剖面、日溫層深度以及日照與海溫的相位差均與浮標觀測結果相吻合。

The diurnal variation (DV) of temperature in the top few meters of the ocean can largely affect the air-sea heat fluxes and moisture by changing the sea surface temperature (SST) in the air-sea interactions. In the present study, the diurnal warming over the northwestern Pacific (NWP) is examined using in situ observations from two buoys simultaneously measuring oceanic and meteorological data for five months. The observed temperature in the upper few meters are strongly influenced by the insolation varying at a predominant diurnal period, reflecting diurnal thermal tide (DTT). The diurnal warm layer caused by diurnal heating can reach 5-10 m depth by warming the upper 1-m temperature (T1m) ~0.4 °C, but with a time lag of ~4 hours. The lagging time can be shortened due to the enhanced turbulent mixing under the high winds. As a result, the observed upper thermal structure over five months can be categorized as (1) the convectively stable regime (~20 %) during the warming stage, when the insolation is sufficient (> 400 W m-2) and the wind is weak (< 8 m s-1), (2) the convectively unstable regime (~5 %), when the temperature overturns due to the atmospheric cooling or rainfall and (3) the well-mixed regime (~75 %) occurring during stronger winds (> 10 m s-1). The Himawari-8 SST data over the NWP region is used to compare with the measured T1m with the mean difference of 0.15 °C and -0.02 °C respectively. The most significant DV SST (> 1 °C) measured by the satellite Himawari-8 occurs during the summer in the subtropical high pressure belt. The measured T1m on the buoy, located at the western end of the band, is similar with the satellite observations. The thermal structure, DV SST, and time lag revealed in the observations are well consistent with the Price-Weller-Pinkel (PWP) mixed layer model results.