Effect of the Earth's rotation on the circulation in regions of freshwater influence

Abstract

Recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region. An analytical model including viscous effects and the Earth's rotation is proposed to examine the along‐channel flow pattern and to explain the differences. Model results show that the flow pattern is strongly dependent on the Ekman number E. With a large Ekman number (E > 1) the system is governed by gravitational circulation, and thus horizontal density gradients in along‐channel direction are important. The whole water column is in the Ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross‐channel direction. With an intermediate Ekman number (E ∼ 0.1) this jet inflow concentrates in the lower layer in which the viscosity still plays an important role. The flow in the upper layer is, on the other hand, determined by the geostrophic balance. The contribution by the geostrophic flow becomes larger so that the cross‐channel density gradients are important when die Ekman number is small (E < 0.01). Since the Ekman layer is clung to the bottom, the jet inflow exists only in the thin bottom layer. The hydrographic and acoustic Doppler current profiler surveys were conducted in Ise Bay, Japan. Both the density structure and the flow pattern were different from those observed in many drowned river valleys. A strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation. The estimated Ekman number is 0.07 in the bay and thus the observed pattern is consistent with the model result when E = 0.1.