|
The Gulf Stream has a very well-defined
structure that remains largely invariant as it meanders about.
|
-
The downstream velocity can be
characterized quite accurately as two back-to-back exponentials with
scale-widths quite comparable to the radius of deformation set by the
depth of the corresponding pycnocline.
-
This double-exponential
structure with an almost discontinuous shear in the center of the
current characterizes the velocity field at all sampled depths, 52 to
252 m.
-
It seems
likely that this stiffness is a general characteristic of other
separated western boundary currents such as the Kuroshio Extension
(Hall, 1989) and Agulhas Retroflection current (Lutjeharms and
Ansorge, 2001).
|
The transition from one side of the velocity maximum to the other
takes place over a scale that is an order of magnitude less than the
width of the Gulf Stream itself, about 6 km to either side.
This blurring of the velocity maximum, which according to the exponential
fit which might be described
statistically as a consequence of submesoscale mixing.
-
The traditional
mixing length argument says that
Kh ~ < v'2> 1/2
l where v' represents
perturbations normal to the mean flow in stream coordinates
and l is the mixing length.
We do not have a direct measure of l so
we use the mean (~6 km) of the two scale-widths in the second terms
of the equation for mean flow in stream coordinates.
This gives us Kh 1000 m2s-1.
-
In another approach the momentum deficit can be
thought of as a balance between the forcing that seeks to establish a
purely exponential profile on one hand, and its loss through
submesoscale diffusion on the other resulting in the observed
velocity profile.
We compute the loss as the difference
between a purely exponential profile and the one determined
from the observations.
This loss takes place through lateral mixing to
both sides (hence the factor 2) ~2<v'2>1/2 l =
2Kh.
Integration
yields
and estimate of ~3000 m2s-1
for the peak as a whole.
-
In summary, the smoothing of the velocity peak
can be interpreted as due to a submesoscale diffusivity of about
1-3x103 m2s-1.
-
This can be compared to mesoscale isopycnal
diffusivity obtained from the SYNOP float trajectory data which is
about an order of magnitude larger at
O(10-30) x103 m2s-1 (Zhang et al., 2001).
|
|
At this location the u-v
covariances to both sides of the Gulf Stream suggest a conversion of
kinetic energy from the eddy to mean flow. We interpret this as a
geometric result of the downstream decrease in meandering approaching
the Oleander line. It appears that patterns of in- and outflow and
energetics can be quite site specific, reflecting, we think,
preferred states or patterns of the meandering of the Gulf Stream.
|
|
Discussion and Conclusions
