长江河口北支上口不规则周期潮流的动力机制

摘要

数值模拟和动量分析长江河口北支上口枯季大潮期间1d内出现“四涨四落”不规则周期涨落潮流现象.长江河口北支上口1日内两次涨潮流和两次落潮流为常规涨落潮流,受外海半日潮流控制,两次涨潮流和落潮流为非常规涨落潮流.北支上口非常规的涨潮流处于南支落潮的末期,范围小,流速弱,历时约2h;表层主要是垂向黏滞项和水平扩散项与正压项间的作用,南向的垂向黏滞项起着决定的作用,底层则是斜压项与正压项间的作用；北支上口非常规涨潮流是北风、盐度锋面产生的南向斜压压强梯度力和南支末期落潮流的牵引作用共同造成的；径流抑制非常规涨潮流的产生,持续时间随径流量的增加呈指数递减,当径流量达到22 300m3/s时,非常规涨潮流现象消失.北支上口非常规落潮流处于南支涨潮流的初期,由于在北支上口南支的涨潮流早于北支涨潮流,导致南支水体进入北支,形成北支上口第二次落潮流,范围较大,流速较强,历时约2.5h,从表层至底层主要是垂向黏滞项与非线性平流项和正压项之间的作用.本文揭示了北支上口1d内出现“四涨四落”不规则周期涨落潮流的动力过程和机制.%To analyze the cause of irregular period tidal currents called "four floods and four ebbs" at the mouth of the upper North Branch in Changjiang River estuary during spring tides in dry season, the paper simulated the dynamic mechanism by means of numerical simulation and momentum analysis. The resultsrn showed that two flood currents and two ebb currents controlled by semidiurnal tide were regular period tidal currents, while the other two flood currents and two ebb currents were irregular in one day. The irregular flood currents happened with little velocity and short duration of about 2 h in a small range, meanwhile,the ebb currents in the South Branch were in the late stage. At the surface layer, the southward vertical diffusion played a decisive role, as well as the advection and the surface elevation gradient. While the baroclinic pressure gradient, which generated by the salinity front, reacted with the surface elevation gradient at the bottom layer. It indicated that the irregular flood currents at the mouth of the upper North Branch were mainly caused by the north wind stress, southward baroclinic pressure gradient forcing and the tractive action of the late ebb currents in the South Branch. The duration decreased exponentially with river discharge and disappeared when the value was greater than 22 300 m3/s. The irregular ebb currents occurred with large velocity and duration of 2. 5 h in a larger area. The water mass in the South Branch spilled over into the North Branch for the earlier flood currents and formed the irregular ebb currents at the upper North Branch. The vertical diffusion, as well as the advection, did the work with the surface elevation gradient forcing from surface to bottom layers. It showed the dynamic mechanism and process of the irregular flood and ebb currents as stated above.