Effect of Equatorial Long Waves on the North Equatorial Countercurrent

赤道長波對北赤道反流之影響

唐存勇(T. Y. Tang) ； 楊穎堅(Y. J. Yang) ； 吳明進(Miug-Chin Wu)

《Terrestrial, Atmospheric and Oceanic Sciences》 3卷2期 (1992/06) Pp. 199-223

摘要

本文利用一赤道長波理論模式，研究赤道長波對北赤道反流之影響。在此模式中海洋運動乃受一東西向風剪力所引起。由簡而繁的數種不同風場分佈，將應用於此模式中。由各風場所引起的直接受力長波及由邊界反射而產生的反射長波將被分別研究。當風一開始作用於海面，直接受力引起的長波分別爲一向西傳遞的羅士培波及一向東傳遞的開爾文波。模式結果顯示流速最大之北赤道反流位於北緯五度附近、最厚之斜温層位於北緯三度，此結果與實際觀測資料相吻合。因開爾文波之振幅隨緯度增加而成指數銳減，故模式中北赤道反流的發生與直接受力引起的羅士培波有關。又因羅士培波向西傳遞增長，故北赤道反流在西熱帶海洋有較高的流速。如考慮東、西邊界的影響，其所產生的一系列反射波，僅引起一系列振盪。當風剪力作用停止後，所有作用反轉，西向的羅士培波將所引發的北赤道反流消彌，且因反射波的振盪作用，在北赤道反流區會發生短暫的西向流。待基本的分析探討過後，一組較接近實際熱帶大西洋風場的模式風場將應用於此模式。其所得結果與位於北緯六度、西經二十八度實際觀測所得之流速資料相比較，結果顯示，二者相似。故我們認爲，赤道長波對北赤道反流有顯著的影響力，尤以直接受力所引起的羅士培波的影響力最爲重要。

並列摘要

An analytical, reduced-gravity, equatorial long wave model is employed to study the effect of equatorial long waves on the North Equatorial Countercurrent(NECC).This model is first forced by an abruptly switched-on-off zonal wind stress which is distributed uniformly in the zonal direction. As the wind is turned on, an eastward current with a maximum speed at 5°N and a trough of thermocline at 3°N are generated in the western basin. These features coincide with the observed distribution of NECC. The initial intensification of this eastward current is due to the forced first meridional mode Rossby wave. The duration of intensification is related to the wind fetch and Rossby wave speed. After the forced Rossby wave has passed, the eastward current is decelerated or accelerated by a series of reflected Rossby waves generated at the eastern boundary. As the steady state is reached, the easterly wind is then turned off. The forced first meridional mode Rossby wave, now generated by wind relaxation, decelerates the eastward current and even turns the current westward, due to overshooting.
Replacing the uniform wind with a linear wind, we find that the general features remain unchanged but the NECC now has larger amplitude and smaller zonal domain. The present linear wind has similar zonal distribution to the trade wind on the tropical Atlantic Ocean.
Finally, the linear wind stress distribution, with a representative of time variations observed along the equator, is applied to force the model. The resultant ocean response is compared with the velocity measurement at 6°N, 28°W. General agreement is found. The equatorial long waves, especially the forced Rossby waves, have significant impact upon the NECC.