Hydrodynamic Characteristics And Its Response To Wind In The Qinhuangdao Coastal Area From Late Summer To Early Autumn

Qinhuangdao coastal area is the critical area connecting Bohai Bay with Liaodong Bay in the Bohai Sea, and also a significant passage through which the substance and energy is exchanged among Liaodong Bay, the central of Bohai Sea and Bohai Bay. It is very important to understand change regularity and mechanism of hydrodynamic processes in Qinhuangdao coastal area for intensively studying on long-term transport and exchange of substance and energy, diffusion and distribution of contaminants, its selfpurification ability and ecological enviroment restoration. Based on sea currents, sea levels and bottom temperatures data that obtained by four sets of seabed based observation systems settled in Qinhuangdao coastal area in September 2013 and numerical wind fields and high-resolution Sea Surface Temperature(SST) data in the corresponding period, we analyzed the characteristics of tidal and residual currents by means of Classic Tidal Harmonic Analysis and compared with the tidal and residual currents in the east coast of Liaodong Bay for studying on distribution of tidal currents and circulation form of the whole Liaodong Bay. On this basis, we then turned to discuss how the hydrodynamic elements response to local wind. From the perspective of energy, we studied the energy input from wind fields to residual currents and waves. Also the propagation way of energy in vertical by means of wavelet analysis.The results of tidal currents characteristics show that the tidal currents in Qinhuangdao coastal area are regular semidiurnal ones, and four stations from W1 to W4 are all in the shallow water areas with significant shallow water tidal constituents. M2 tidal current constituent is dominated, and O1 tidal current constituent is the next largest constituent. The motion type of M2 tidal current constituent is rectilinear, with the maximum tidal velocities between 20 ~ 29.6 cm/s in the direction of WSW-ENE. The maximum velocities decrease progressively with depth below 5 m. The directions of maximum velocities only have a little change with depth, and the occurring time of maximum velocities is ahead with depth. O1 tidal current constituent is anti-clockwise flows, with the ellipticities increasing with depth below 10 m. The maximum velocities are between 3.6 ~ 8.2 cm/s. The distribution of maximum velocities at station W1 and W4 in the nearshore region show a trend of decreasing with the whole depth, while at station W2 and W3 decreasing progressively with depth below 5 m. The maximum velocities at staions from W1 to W3 occur earliest in the middle layer and latest in the bottom layer, whist at staion W4 occur earliest in the bottom and latest in the surface layer.The results of residual currents characteristics show that residual currents in the sea area are weak during seasonal transition from summer to autumn, with velocities between 0.3 ~ 2.5 cm/s. The residual currents happen to bifurcate in the surface and middle layer, and the location of bifurcation are approximately 39°40’N that near station W1 and W3 in Qinhuangdao central area, while show prejudiced north flow in the bottom layer. Residual currents in vertical at station W1 and W4 rotate clockwise with the increasing of the depth under the middle layer that shows the Ekman spiral structure.Comparative studies with the eastern coastal area of Liaodong Bay show that the tidal currents in the Qinhuangdao coastal area are half of the tidal currents in the east of Liaodong Bay. There is a clockwise weak circulation system in the bottom of Liaodong Bay in autumn that the residual current magnitude is only one-tenth of the tidal current with 5 cm/s at most.The results of ocean response to winds are shown as follow: The residual currents above middle layer are forced response on local wind field, while largely influenced by current systems in the central of Bohai Sea from the middle layer to the bottom layer during the first process. The residual currents which influenced by the clockwise circulation in the centre of Bohai Sea driven by northeasterly winds behave as north-east flows against the winds during the second process. The sea levels response forcedly to the cross-shore winds under the circumstances of the wind speed increasing slowly, whilst sea levels increase or decrease with wind-driven Ekman advection effects under the wind fields influence of rapidly increasing and long duration. The primary reason for the decreasing of bottom temperatures during the first process is the joint effects of offshore Ekman caused by southwesterly alongshore winds and offshore winds, whist on-shore Ekman effect which driven by enhanced northeasterly alongshore winds generate cold water compensation from the central of Bohai Sea can finally lead to obvious decreasing of bottom temperatures during the second process. The SST indicates the significant cooling phenomenon occurring in the Qinhuangdao coastal area that behaves as a cold water tongue stretching from the northeast to the sea areas surrounding observation stations after sustained northeasterly winds impact on sea surface from Sept. 23 to Sept. 24.The studies on energy input from winds to Qinhuangdao coastal area are shown as follow: Compared with tidal currents, winds blowing on Qinhuangdao coastal area provides main energy demanded by ocean internal mixing. Sea surface wind fields put the energy into ocean by nearly semidiurnal period flows with period of 10 to 16 hours and internal wave frequency flows with period of 4 days during the first process, while in the second process the energy transfers to ocean by nearly inertial motions. No matter which process it is, the sea surface wind fields input energies to ocean through the flows with wide frequency bands from the surface to the bottom. The energies of inertial motions mainly come from sea surface wind fields, and there exists certain time-lagged effects in the process which sea surface winds input energy to the inertial motions, and the energies usually reach to maximum after the gale blowed down for 6 to 12 hours. Waves are mainly controlled by sea surface winds. If wind speed increases faster, then wave energy will become greater. Moreover, waves are more likely to get sufficient growth and then apperar to amplify energy rapidly when sustainable winds affecting on some direction, but it is little possible for rapid accumulation of wave energy during the days that wind speed doesn’t change a lot or changes slowly.