Air-Sea Gas Exchange in Tidal Fronts

Burkard Baschek

PhD thesis, School of Earth and Ocean Sciences, University of Victoria, Canada, 2003.

Supervisors: Dr. D.M. Farmer, Dr. C. Garrett

Strong tidal currents in the Fraser Estuary, BC, Canada, cause intense flow-topography interaction around islands and over shallow sills. At Boundary Pass, a steep sill forms a pronounced barrier for flow of dense water from the Pacific Ocean into the Strait of Georgia. The processes at the sill control the renewal of deep and intermediate water in the Strait. The strong flood tidal flow forces dense water to flow over the sill. It then meets a fresh surface layer just downstream of the sill crest and subducts underneath the fresh water, setting up a hydraulic sill flow with an arrested upper layer. Vertical current speeds at the downstream side of the sill reach up to 0.75 ms-1, and intense detrainment of dense water from the lower into the upper layer causes a volume loss of 60% over a distance of 200 m.

Surface waves travelling into the convergence zone (tidal front) over the sill crest tend to steepen and break due to wave-current interaction. The breaking waves inject gas bubbles, which either rise back to the sea surface or dissolve completely, depending on their rise speed and the strength of the vertical currents. Bubbles injected close to the plunge point of the dense water mass are drawn down by the extreme currents to depths of up to 160 m, enhancing air-sea gas exchange.

The hydraulic flow, wave-current interaction, and gas bubble behaviour are described with simple models. They are used to interpret extensive ship-board measurements during two cruises in the Fraser estuary and help in the understanding of the physical processes involved in air-sea gas exchange in tidal fronts. The oxygen flux in the tidal front at Boundary Pass is compared with other oxygen sources in the Fraser Estuary and shows that tidal fronts may contribute significantly to the aeration of an estuary. The described processes may be also applicable to other coastal areas with strong tidal currents like Norway, Chile, or Japan, and may be important in convergence zones like deep convection regimes.