Horizontal and vertical structure of velocity, potential vorticity and energy in the Gulf Stream
Citable URI
https://hdl.handle.net/1912/3150Location
Gulf StreamDOI
10.1575/1912/3150Keyword
Ocean currentsAbstract
From October, 1982 to October, 1983 a current meter mooring reaching
from the bottom into the thermocline was deployed for the first time in
the Gulf Stream at 68°W. The temperatures, pressures, and velocities at
the uppermost instrument indicate the Gulf Stream moved back and forth
across the mooring site, so that the entire Stream was sampled in time;
hence the data may be used to examine horizontal as well as vertical
structure of the Stream. The two key points to the success of the
analysis are: 1)the welldefined relationship between temperature and
crossstream distance in the thermocline, enabling the use of the former
as a horizontal coordinate; and 2)a dailychanging definition of Gulf
Stream flow direction based on the shear between the thermocline and
2000 m depth. Timeseries of dailyrotated velocities may be used to
calculate empirical orthogonal functions for the long and crossstream
vertical structures, which are decoupled and are respectively baroclinic
and barotropic. Using the inferred horizontal coordinate one can
estimate mass, momentum and kinetic energy fluxes for four individual
events when the entire Stream swept by the mooring. The results agree
well with historical data. Bryden's (1980) method has been used to
calculate vertical velocities from the temperature equation; the
resulting timeseries of w are visually coherent throughout the water
column and their vertical amplitude structure is reminiscent of that for
a twolayer system. The rms vertical velocities are large (0(.05 cm/s)),
and these as well as other estimates have been used to explore the
validity of the quasigeostrophic approximation at the mooring site. The
Rossby number for the thermocline flow is about 0.3, and for the deep
flow is ≤ 0.1.
The entire data set may also be used to construct a horizontal and
vertical profile of velocity in the Gulf Stream, from which a
crosssection of the mean potential vorticity can be produced. The
latter shares many common feature with crosssections from past work for
a nearby site, as well as analogous data from a threelayer numerical
model, thus suggesting that they are robust features of Gulf Streamlike
currents. These features are, in particular, a strong jump from low to
high values crossing the Stream from south to north; and a change in the
sign of the potential vorticity gradient on isothermal surfaces for
T > 12°C.
To complement the analysis of the observational data, a set of
diagnostic calculations has been performed on an eddyresolving qeneral
circulation model, to provide a complete picture of the kinetic energy
budgets of the free jet and its environs. It is found that the
downstream convergence of kinetic energy in the decelerating jet is
balanced primarily by an ageostrophic flow against the pressure gradient,
which in turn implies some conversion of kinetic to available potential
energy in the region. Energetic analysis of the observations as well as
the numerical data suggests barotropic and baroclinic instabilities may
be equally important to the kinetic energy budgets in the Stream.
Because there is but one mooring, the dynamics governing the
fluctuations remain elusive. Nonetheless, a kinematic framework is
proposed, which is consistent with the data and accounts for a variety of
unusual features that arise in the original analysis (for example,
distinct asymmetries in the four Gulf Stream crossings, and the rather
large vertical velocities). It is sugqested that the data we are now
capable of collecting is proffering fundamentally new attributes of the
Gulf Stream, which must be included and accounted for in future
theoretical work.
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 February 1985
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Suggested Citation
Thesis: Hall, Melinda M., "Horizontal and vertical structure of velocity, potential vorticity and energy in the Gulf Stream", 198502, DOI:10.1575/1912/3150, https://hdl.handle.net/1912/3150Related items
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