Dynamics of freshwater plumes : observations and numerical modeling of the wind-forced response and alongshore freshwater transport
Citable URI
https://hdl.handle.net/1912/4784Location
Gulf of MaineDOI
10.1575/1912/4784Keyword
Oceanic mixing; Hydrography; Ocean circulationAbstract
A freshwater plume often forms when a river or an estuary discharges water onto
the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean
and usually leave a striking signature in the coastal hydrography. The present study
combines both hydrographic data and idealized numerical simulations to examine
how ambient currents and winds influence the transport and mixing of plume waters.
The first portion of the thesis considers the alongshore transport of freshwater
using idealized numerical simulations. In the absence of any ambient current, the
downstream coastal current only carries a fraction of the discharged fresh water; the
remaining fraction recirculates in a continually growing "bulge" of fresh water in the
vicinity of the river mouth. The fraction of fresh water transported in the coastal
current is dependent on the source conditions at the river mouth. The presence of an
ambient current augments the transport in the plume so that its freshwater transport
matches the freshwater source. For any ambient current in the same direction as the
geostrophic coastal current, the plume will evolve to a steady-state width. A key
result is that an external forcing agent is required in order for the entire freshwater
volume discharged by a river to be transported as a coastal current.
The next section of the thesis addresses the wind-induced advection of a river
plume, using hydrographic data collected in the western Gulf of Maine. The observations
suggest that the plume's cross-shore structure varies markedly as a function
of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling
favorable winds spread the plume offshore, at times widening it to over 50
km in offshore extent, while downwelling favorable winds narrow the plume width
to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory.
Estimates of the terms in the alongshore momentum equation calculated from
moored current meter arrays also indicate an approximate Ekman balance within
the plume. A significant correlation between alongshore currents and alongshore
wind stress suggests that interfacial drag may be important.
The final section of the thesis is an investigation of the advection and mixing
of a surface-trapped river plume in the presence of an upwelling favorable wind
stress, using a three-dimensional model in a simple, rectangular domain. Model
simulations demonstrate that the plume thins and is advected offshore by the crossshore
Ekman transport. The thinned plume is susceptible to significant mixing
due to the vertically sheared horizontal currents. The first order plume response is
explained by Ekman dynamics and a Richardson number mixing criterion. Under
a sustained wind event, the plume evolves to a quasi-steady, uniform thickness.
The rate of mixing slowly decreases for longer times as the stratification in the
plume weakens, but mixing persists under a sustained upwelling wind until the
plume is destroyed. Mixing is most intense at the seaward plume front due to an
Ekman straining mechanism in which the advection of cross-shore salinity gradients
balances vertical mixing. The mean mixing rate observed in the plume is consistent
with the mixing power law suggested by previous studies of I-D mixing, in spite of
the two-dimensional dynamics driving the mixing in the plume.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998
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Suggested Citation
Thesis: Fong, Derek A., "Dynamics of freshwater plumes : observations and numerical modeling of the wind-forced response and alongshore freshwater transport", 1998-06, DOI:10.1575/1912/4784, https://hdl.handle.net/1912/4784Related items
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