Coastal tomographic mapping of nonlinear tidal currents and residual currents

摘要

Depth-averaged current data, which were obtained by coastal acoustic tomography (CAT) July 12-13, 2009 in Zhitouyang Bay on the western side of the East China Sea, are used to estimate the semidiurnal tidal current (M-2) as well as its first two overtide currents (M-4 and M-6). Spatial mean amplitude ratios M-2:M-4:M-6 in the bay are 1.00:0.15:0.11. The shallow-water equations are used to analyze the generation mechanisms of M-4 and M-6, In the deep area, where water depths are larger than 60 m, M-4 velocity amplitudes measured by CAT agree well with those predicted by the advection terms in the shallow water equations, indicating that M-4 in the deep area is predominantly generated by the advection terms. M-6 measured by CAT and M-6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20 m, indicating that friction mechanisms are predominant for generating M-6 in the shallow area. In addition, dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents. This is the first ever nonlinear tidal current study by CAT.