Concurrent XBTs and historical hydrography are used to construct the
potential vorticity field for the upper layer
(surface to sigmat=27.0).
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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.
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The potential vorticity evinces a slight minimum south
of the velocity maximum that appears to be robust.
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We write Ertel (layer) Potential Vorticity for the upper layer as:
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PV = (f + un-uz[rhon/rhoz])/h,
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where f is the Coriolis parameter,
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u is downstream velocity,
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n is the normal direction to the right looking downstream,
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z is positive upwards,
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rho is density, and
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h is upper layer thickness.
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