Relationship of Internal Waves with Tide and Wind Drift Effects and Propagation of Internal Kelvin Waves in Brackish Lake Nakaumi

Abstract

An internal wave with an amplitude of about 2 m driven by tidal and wind drift effects is frequently observed in Lake Nakaumi. Highly saline water of the lower layer creeps up toward the lake coast driven by the internal waves, greatly affecting the general water quality and biological environment not only of Lake Nakaumi but also of the Ohashi River and Lake Shinji because of the upstream flow of highly saline water. In this paper, the effects of tides and wind drift on the internal waves were evaluated with the results of observations made with neutral buoys.
Tides appear to clearly affect the internal oscillations only at the observation point near the Nakaura water gate ; the internal oscillation near the gate occurs with a phase lag of 1 hr after a water level change. The effect of wind drift on internal oscillation is clearly apparent near the upwind and downwind coasts. The halocline in the eastern littoral region of the downwind coast descends about 2.5 hr after a westerly wind starts to blow, while the halocline in the western littoral region of the upwind side rises about 3 hrs after the wind comes up.
These phenomena agree well with the relationship between wind drift and inclination of the halocline in a two-layer lake model. The effect of wind drift on internal oscillation is greater than tidal effects, except for the observation point near the Nakaura water gate, because a dominant wind usually blows toward the east in Lake Nakaumi during summer. Cross correlation of observed displacement of the halocline between neighboring points and internal phase speeds calculated from density stratification conditions in Lake Nakaumi suggest that an internal Kelvin wave propagates counterclockwise along the coast.