Eddy forcing of the mean circulation in the western North Atlantic
Citable URI
https://hdl.handle.net/1912/3145Location
Western North AtlantiDOI
10.1575/1912/3145Keyword
Eddies; Ocean currentsAbstract
This thesis addresses several aspects of the problem of determining the
effect of the lowfrequency eddy variability on the mean circulation of the
Western North Atlantic. A framework for this study is first established by
scale analysis of the eddy and mean terms in the mean momentum, vorticity, and
heat balances in three regions of the Western North Atlantic  the northern
recirculation, the southern recirculation, and the midocean. The data from
the last decade of field experiments suggest somewhat different conclusions
from the earlier analysis of Harrison (1980). In the momentum balance we confirm
that the eddy terms are negligible compared to the lowest order mean geostrophic
balance. The eddy term may be an 0(1) term in the vorticity balance
only in the northern recirculation region where the mean flow is anisotropic.
In the mean heat balance, if the mean temperature advection is scaled using the
thermal wind relation, then the eddy heat flux is negligible in the midocean,
but it may be important in the recirculation areas. For all the balances the
eddy terms are comparable to or an order of magnitude larger than the mean
advective terms. We conclude from the scale analysis that the eddy field is
most likely to be important in the Gulf Stream recirculation region.
These balances are subsequently examined in more detail using data from
the Local Dynamics Experiment (LDE). Several inconsistencies are first shown
in McWilliams' (1983) model for the mean dynamical balances in the LDE. The
sampling uncertainties do not allow us to draw conclusions about the longterm
dynamical balances. However, it is shown that if we assume that the linear
vorticity balance holds between the surface and the thermocline for a finite
record, then the vertical velocity induced by the eddy heat flux divergence is
nonzero.
The local effect of the mesoscale eddy field on the mean potential vorticity
distribution of the Gulf Stream recirculation region is determined from
the quasigeostrophic eddy potential vorticity flux. This flux is calculated
by finite difference of current and temperature time series from the Local
Dynamics Experiment. This longterm array of moorings is the only experimental
data from which the complete eddy flux can be calculated. The total eddy
flux is dominated by the term due to the time variation in the thickness of
isopycnal layers. This thickness flux is an order of magnitude larger than
the relative vorticity flux. The total flux is statistically significant and
directed 217° T to the southwest with a magnitude of 1.57 x 10 5 cm/2s.
The direction of the eddy flux with respect to the mean large scale
potential vorticity gradient from hydrographic data indicates that eddies in
this region tend to reduce the mean potential vorticity gradient. The results
are qualitatively consistent with numerical model results and with other data
from the Gulf Stream recirculation region. We find that the strength of the
eddy transfer in the enstrophy cascade is comparable to the source terms in
the mean enstrophy balance. The Austauch coefficient for potential vorticity
mixing is estimated to be 0(107cm2/sec). An order of magnitude estimate of
the enstrophy dissipation due only to the internal wave field shows that other
processes must be important in enstrophy dissipation.
The measured eddy potential vorticity fluxes are compared to the linear
stability model of Gill, Green, and Simmons (1974). An earlier study (Hogg,
1984) has shown agreement between the empirical orthogonal modes of the data
and the predicted wavenumbers, growth rates, and phase speeds of the most unstable
waves. However, we show substantial disagreement in a comparison of
the higher moments the eddy heat and potential vorticity fluxes. Because
the critical layer of the model is located near the surface, the model predicts
that most of the eddy potential vorticity and eddy heat flux should
occur within about 300 meters of the surface. The data show much greater deep
eddy heat flux than predicted by the model. It is suggested that the unstable
modes in the ocean have a longer vertical scale because of the reduction in
the buoyancy frequency near the surface.
The evidence for in situ instability is also examined in the decay
region of the Gulf Stream from an array of current and temperature recorders.
Although there is vertical phase propagation in the empirical orthogonal modes
for some of the variables at some of the moorings, there is not much evidence
for a strong ongoing process of wave generation.
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 August 1984
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