Concurrent XBTs and historical hydrography are used to construct the
potential vorticity field for the upper layer
(surface to sigma_{t}=27.0).

The doubleexponential 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 + u_{n}u_{z}[rho_{n}/rho_{z}])/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.
