Near-surface velocity and PV structure of the Gulf Stream
Abstract >> Introduction >> Data >> Velocity Structure >> Potential Vorticity Structure >> Eddy Exchange Processes >> Discussion and Conclusions


Discussion and Conclusions

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.

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