RIASSUNTO
ABSTRACT
A role of internal tide in estuary-shelf water exchange is studied in Gokasho Bay, which is a drowned river valley estuary facing continental shelf of the Pacific Ocean. Thermistor chains and current meters were moored in and out of the bay for 15 days in the four seasons. The internal tide is predominant on the shelf throughout the stratified period. This internal tide propagates into Gokasho Bay as a progressive internal wave and acts as a major source of the fluctuations in temperature, salinity and velocity in the bay. Water exchange between the estuary and shelf waters is driven by the internal tide rather than the surface tide.
INTRODUCTION
There are a lot of semi-enclosed and small ( several kilometers in length ) drowned river valley estuaries along the southern coast of Japan. Because these bays have small tidal prisms (the volume between high and low water levels), it is presumed that the water exchange between the bay and the shelf waters is inert. Nevertheless the water quality in these bays have been kept relatively well until now, despite prosperous aquaculture. Hagino (1993) studied water exchange in Nomi Bay and indicated that the water exchange was not driven by the surface tide but mainly by the internal tide during the stratification period. The internal tide having a period of 12 hours is prevalent and causes the two-layer flow structure during the stratification period in the bay. When the stratification disappears in autumn, the internal tide also ceases and the magnitude of current velocity reduces remarkably, which results in hypoxia in the benthic layer. The internal tide is a common phenomenon on the continental shelf waters ( Nagashima, 1989; Pineda, 1995; Rosenfeld, 1990; Summers and Emery, 1963; Winant, 1974 ), and it may control estuarine-shelf water exchange.