The Structure of Small Scale Fronts in River Plumes
River plumes occur at the mouths of many rivers. O'Donnell (1993) provides a recent review. Examples include the Connecticut (Garvine, 1974), Frazer (Stronach,1977) and Mississippi Rivers (Wright and Coleman, 1971). Large estuaries also produce outflow plumes. The Delaware Bay (Munchow, 1992) and the Chesapeake Bay (Boicourt et al. 1987) are examples. These plumes are often found to be bounded by fronts, narrow zones of strong horizontal gradients in density and velocity. Garvine and Monk (1974) first documented the structure of the frontal zone in the plume of the Connecticut River. Subsequently, Luketina and Imberger (1987 and 1989) documented a similar phenomenon in the plume of Koombana Bay. In both studies the spatial resolution of the current and density fields were inadequate to properly resolve the structure of the front.
Recently, the combination of the instrument packages developed by O'Donnell (1997), termed SCUD and Trump et al. (1995), termed TOAD have been employed to resolve the structure of the front in the Connecticut River plume. In essence, SCUD is a rigid 3m long array of four electro-magnetic current meters and five conductivity-temperature sensors mounted vertically over the bow of a small boat. Ship speed and position was estimated using a differential GPS system and ship motion was measured by a flux gate compass and a tilt and roll sensor. TOAD is a 600 kHz broadband acoustic Doppler current profiler and conductivity-temperature sensor towed just below a surface float. Together, these instruments observe the water velocity in the interval 0.6 m below the surface and 1.5 m above the bottom in 10 m of water with a vertical resolution of 0.5 m. The density field in the top 3 m is measured. Typical ship speeds and sampling rates resulted in a horizontal resolution of between 2 and 5 m for SCUD and 10 and 25 m for TOAD. O'Donnell et al. (1998) provides details of the observations.
When the front was moving to the west at approximately 0.3 ms-1, two across-front transects revealed a horizontal convergence in the across-front velocity components at 0.6 m of 0.05-0.1 s-1. This was associated with a salt induced horizontal density gradient of 10-2 kg/m4. These patterns are consistent with existing gravity current theories and laboratory observations. An along-front transect with the towed instruments in the zone of maximum surface convergence showed that a downwelling of 0.2 ms-1 occurred at the front. Vertical velocities of this magnitude are consistent with a simple argument based on continuity and slow along-front variations.
Structures similar to those summarized have been reported in the outflow plume of the Mississippi River. Theoretical models that explicitly parameterize fronts (see O'Donnell, 1993) for a summary) suggest that they can influence the location and timing of mixing between the plume and the ambient fluid. These structures could, therefore, be important to the distribution of buoyancy on the Gulf of Mexico shelf.References
Boicourt, W.C., S.-Y. Chao et al., 1987. Physics and microbial ecology of a buoyant plume on the continental shelf. EOS, Trans. Amer. Geophys. Union 68: 666-668.
Garvine, R.W., 1974. Physical features of the Connecticut river outflow during high discharge. J. Geophys. Res. 79: 831-846.
Hickey, B.M., L.J. Pietrafesa, D.A. Jay, and W.C. Boicourt, 1998. TheColumbia river plume study: subtidal variability in the velocity and salinity fields. J. Geophys. Res. 103: 339-10, 368.
Munchow, A., 1992. The formation of a buoyancy driven coastal current. Ph.D. Dissertation, University of Delaware, Newark, DE. 205 pp.
O'Donnell, J., 1993. Surface Fronts in Estuaries: A Review. Estuaries 16: 12-39.
O'Donnell, J., 1997. Observations of near surface currents and hydrography in the Connecticut River plume with the SCUD array. J. Geophys. Res. 102: 25021-25033.
O'Donnell, J., G.O. Marmorino, and C.L. Trump, 1998. Convergence and downwelling at a river plume front. J. Phys. Oceanogr. 28: 1481-1495.
Trump, C.L., G.O. Marmorino, and J. O'Donnell, 1995. Broadband ADCP measurements of the Connecticut River plume front. Proc. I.E.E.E. Fifth Working Conf. on Current Measurement.
Wright, L.D. and J.M. Coleman, 1971. Effluent expansion and mixing in the presence of a salt wedge, Mississippi River delta. J. Geophys. Res. 76: 8649-8661.
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