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


Potential Vorticity Structure

Concurrent XBTs and historical hydrography are used to construct the potential vorticity field for the upper layer (surface to sigmat=27.0).
The double-exponential velocity pattern is consistent with a uniformity of potential vorticity between the Gulf Stream and recirculating gyres to either side, but not across the velocity maximum where it undergoes a nearly factor 5 change in ~20 km.
The potential vorticity evinces a slight minimum south of the velocity maximum that appears to be robust.
We write Ertel (layer) Potential Vorticity for the upper layer as:
PV = (f + un-uz[rhon/rhoz])/h,
where f is the Coriolis parameter,
u is downstream velocity,
n is the normal direction to the right looking downstream,
z is positive upwards,
rho is density, and
h is upper layer thickness.

  • Trends in lateral shear and layer thickness compensate leading to a uniform PVs to either side but a sharp PV transition at the core of the Gulf Stream.
  • Including the vertical shear contribution to relative vorticity shifts the PV structure northward about 10 km and slightly broadens the PV transition across the core of the Gulf Stream
  • On the cyclonic side PV remains remarkable uniform
  • PV on the anticyclonic side shows a minimum in PV. that is 13% less than its far-field value. Watts (1983) found a similar PV minimum.
Figure 13. Construct of mean potential vorticity for the upper layer using vertically averaged velocity and pycnocline depth. The potential vorticity shows a sharp increase from south to north across the velocity maximum. Note the weak minimum on the anticyclonic side. The insert box shows the normalized (to f) relative vorticity terms. Lateral shear term reaches +0.5 on the cyclonic side and -0.3 south fo the velocity maximum. Vertical shear term is large in a narrow band near the velocity maximum.

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