Publication Date:
2022-05-25
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 September 1995
Description:
Evidence of enhanced oceanic convection over Maud Rise in the Weddell Sea indicates
that bottom topography may play a role in selecting the location and scale of deep
convecting oceanic chimneys below large scale atmospheric negative buoyancy forcing.
Topographic preconditioning of open ocean deep convection is studied using an idealized,
three-dimensional, primitive-equation model. A barotropic mean flow impinges
on an isolated Gaussian-shaped seamount in a stratified domain with uniform negative
surface buoyancy forcing. A region of topographically trapped flow forms over the topography.
When this "Taylor cap" is tall enough to interact with the surface mixed-layer,
the local isolation from mean horizontal advection forms a conduit into the deep water.
The convective penetration depth within this local region is significantly enhanced relative
to ambient levels away from the seamount and to similar runs performed without
bottom topography. The parameter dependencies for these preconditioning processes are
investigated.
With uniform background stratification, the doming of isopycnals does not play a
major role in the preconditioning process. However, when a surface intensified stratification
is included, domed isopycnals associated with the Taylor cap circulation can
also play a preconditioning role. In this case, the pycnocline is first ventilated over the
seamount, leading to rapid convective deepening into the weakly stratified deep water.
An analytical formula for one-dimensional, non-penetrative convection into an exponential
stratification profile is derived and compares well with results from the numerical
model.
Previous modeling studies have often parameterized the mehanism by which the horizontal
scale of oceanographic chimneys is set through the use of disk-shaped surface
forcing functions. Unlike in such experiments, topographically preconditioned chimneys
are not prone to breakup by the growth of baroclinic instabilities. Instead, convection
is generally shut down by horizontal fluxes of heat due to the mean flow across the
temperature gradients of the chimney walls. The presence of the mean flow, which is
neccessary in order for the topographic preconditioning to work, causes instabilities to
be advected downstream faster than they can grow locally. These results suggest that the role of baroclinic eddies in shutting down oceanographic convection is probably misrepresented
in studies which parameterize the preconditioning mechanism, particularly
if the preconditioning mechanism being parameterized is a topographic one.
Description:
Financial
support for the thesis research was provided by NOAA grant number NA16RC0073
and NSF grant number OCE90-04864. Additional financial support was provided by the
Office of Naval Research, Physical Oceanography Division under grant number N00014-
86-K-0751 as well as an AASERT fellowship, grant number N00014-89-J-1106.
Keywords:
Submarine topography
;
Convection
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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