Boundary Layer Forcing of a Semidiurnal, Cross-Channel Seiche

Abstract

The outer Strait of Juan de Fuca is a stratified, tidal channel about 100 km long, 20 km wide, and 200 m deep. Tidal currents of O(0.5 m s?1) occur at both diurnal and semidiurnal periods and there is a pronounced spring?neap variation in the stratification due to changes in tidal mixing. Vertical isotherm excursions of O(50 m) have previously been observed along the northern side of the channel that appear to be phase locked to the tidal currents. Analyses of recent ADCP and thermistor chain data confirm the isotherm excursions and find that they are accompanied by distinctive baroclinic structures in the horizontal currents that persist across spring?neap cycles, and from year to year. The authors find that the phase of the semidiurnal signal does not vary along the channel, as would be expected of an along-channel internal tide. Instead, comparison of the semidiurnal measurements with two-dimensional analytical and numerical models indicate that much of the baroclinic structure can be explained as a cross-channel, internal seiche that is locally driven by reversing Ekman forcing in the bottom boundary layer. Near the bottom, the vertical motions are kinematic, resulting primarily from cross-channel flow on the side slopes. In the middepths, the seiche appears to explain part but not all of the vertical motions. The seiche appears to persist and to maintain a similar mode shape across large, O(2), changes in stratification. In Juan de Fuca, it is expected that these motions affect the overall salt balance of the system by enhancing mixing between the upper and lower layers of the estuary.