Air-Sea Gas Exchange in Tidal Fronts
Burkard Baschek1) and David Farmer2)
Institute of Ocean Sciences, Sidney, Canada
Fresh water from the Fraser River flows into the Pacific ocean (Figure 1) creating an estuarine circulation. Strong tidal currents cause intense flow-topography interaction around islands and over shallow sills forming tidal fronts (Figure 2). Wave-current interaction in the frontal areas enhances wave breaking and gas bubble injection.
In particular, the tidal front at the entrance to the Strait of Georgia (Boundary Pass) is important because of a) the subduction of oxygenated surface water to intermediate depth (Figures 3, 7) and b) the enhanced aeration of water by the extensive entrainment of gas bubbles (Figure 9).
|Figure 1: Map of southern British Columbia, Canada.|
|In fall 1999 and 2000 we acquired CTD, ADCP, echo sounder, oxygen, and resonator bubble measurements in this region.|
|Figure 2: ebb (red) and flood (green) tidal fronts in Haro Strait.||Figure 3: O2-measurements [ml/l] by D. Masson (IOS, Canada) showing oxygenated intermediate water in the Strait of Georgia.|
Hydraulic sill flow
At flood tide, dense water from the Pacific Ocean mixes with fresh surface water upstream of the sill crest. The newly formed water mass plunges to intermediate depth in the Strait of Georgia setting up a hydraulically controlled sill flow with an arrested upper layer (Figure 4, 7).
Current speeds reach up to 2.2 m/s in the horizontal and 0.75 m/s in the vertical. 60% of the lower layer volume is lost to the upper layer by detrainment.
The hydraulically controlled flow over the sill is modeled with a 1-layer reduced gravity model, which includes momentum transfer and detrainment. Friction is neglected.
|Figure 4: Echo sounder image of hydraulic sill flow. Arrows represent current speed and colors acoustic backscatter intensity.|
Surface gravity waves travel into the convergence zone of the tidal front, steepen due to the effect of opposing currents, break, and create gas bubbles which are then subducted by downwelling currents enhancing air-sea gas exchange (Figure 5).
|Figure 5: Wave-current interaction in tidal front causing wave breaking.|
|Figure 6: Example of wave travelling from left to right into a linearly increasing current. a) current speed, b) wave amplitude (model by Bretherton & Garrett ), c) dissipated wave energy, d) void fraction of injected bubbles.|
|Figure 7: Sketch of processes important for air-sea gas exchange at the tidal front at Boundary Pass: hydraulic sill flow, wave-current interaction, gas bubble dynamics.|
Gas bubble dynamics
Gas bubbles which are injected into tidal fronts are drawn down by strong vertical velocities of up to 75cm/s and dissolve (partly). A 1-D model by Thorpe  shows that the amount of dissolved gas increases with the strength of the downwelling currents. (Figure 8).
Gas bubbles which have been observed at 160m depth were traced back to the surface where they had a radius of > 2mm (Figure 9).
|Figure 8: Percentage of dissolved gas for O2, N2, CO2, Ar as a function of downwelling current speed. Injection depth: 0.1m (red), 1m (green), 2m (blue).|
The contribution of gas bubble entrainment in tidal fronts to the aeration of intermeditae water in the Strait of Georgia is of the order of 7% (Figure 10).
The investigated processes may play an important role in the mixing and aeration of the water masses in the Fraser Estuary and other estuarine circulation regimes with strong tidal currents.
|Figure 9: Acoustical backscatter of gas bubbles (color). The corresponding resonance radius is given by the red curve and the "path" of the injected bubbles in blue.|
|Figure 10: Sketch of the O2-sources and sinks in the Fraser Estuary: tidal fronts, advection, diffusion, biological production/ decomposition.|
L. Armi and D.M. Farmer(2002): Stratified flow over topography: Bifurcation fronts and transition to uncontrolled state. Proc. Roy. Soc. 458, A, 513-538.
B.Baschek (2003): Air-sea gas exchange in tidal fronts. PhD thesis, University of Victoria, Canada.
B. Baschek, D.M. Farmer, and C. Garrett, 2006: Tidal fronts and their role in air-sea gas exchange. J. Marine Res., Vol. 64, No. 4, pp. 483-515.
F.P. Bretherton and C.J.R. Garrett (1969): Wavetrains in inhomogeneous moving media. Proc. Roy. Soc., A. 302, pp. 529-554.
D.M Farmer, E.A. D'Asaro, M.V. Trevorrow, and G.T. Daikiri (1995): Three-dimensional structure in a tidal convergence front. Continental Shelf Res., Vol. 15, 13, pp. 1649-1673.
D.M. Farmer, S. Vagle, and A.D. Booth (1998): A free-flooding acoustical resonator for measurement of bubblesize distributions.
J. Atm. and Oceanic Tech., Vol. 15, pp. 1132-1146.
D.M. Farmer and L. Armi (1999): Stratified flow over topography: The role of small scale entrainment and mixing in flow establishment. Proc. Roy Soc., A 455, pp. 3221-3258.
S.A. Thorpe (1984): A model of the turbulent diffusion of bubbles below the sea surface.. J. Phys. Oceanogr., Vol. 14, pp. 841-854.