Current structure and behavior of the river plume in Suo-Nada

The behavior of a river plume in Suo-Nada, Japan, has been studied using a primitive equation numerical model, the Princeton Ocean Model. Special attention has been paid to the current structure and behavior of the anticyclonic eddy (bulge) induced by high freshwater inflow changing on a timescale of one week. First, the freshwater is supplied from a river to a rectangular basin with a simple topography. When the river discharge subsides after reaching its peak value, the bulge propagates upstream (i.e., opposite to the direction of the Kelvin wave propagation). Next, the freshwater is supplied from eight major rivers to the basin with realistic topography. The less saline water mass in the southern part of Suo-Nada propagates to the west (i.e., upstream) after the river discharge subsides. This is consistent with an observed phenomenon, viz., that the less saline water mass appears in the western part of Suo-Nada, suggesting that the upstream propagation of the bulge is possible in the real ocean. Finally, the cause of the upstream propagation is considered. Onshore currents appear in the bottom layer beneath the bulge, propagating upstream. They produce an anticyclonic barotropic eddy due to the conservation of potential vorticity. The current component associated with the eddy crosses normally to the isohaline in the upper layer, and therefore transports the bulge upstream. No other current component (such as surface current velocity minus vertically-averaged value) is responsible for the upstream propagation of the bulge.