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



Between 1980 and 1983 the Pegasus program took bimonthly sections of temperature and velocity across the Gulf Stream at a site ~250 kilometers downstream from Cape Hatteras. The program sought to determine, from direct measurements of velocity, the contributions by the Gulf Stream to the mean and annual variations in poleward transport of mass and heat in the North Atlantic (Halkin and Rossby, 1985, hereafter HR; Rago and Rossby, 1987). These repeat sections also revealed the following

  • a remarkable stiffness or rigidity to the Gulf Stream itself, i.e. the width, shape and magnitude of the velocity field remained comparatively invariant regardless of position of the current or direction of flow (HR).
  • two thirds of the eddy kinetic energy (EKE) obtained from the ensemble of sections disappears after reordering them into stream coordinates. This means that much of the Eulerian EKE observed at a point in the current results from the meandering of this fixed structure, and not from some general mesoscale eddy activity.
In more recent years another long-term program to examine the variability of the Gulf Stream on decadal time scales has been in operation. The key element in this program consists of regular and continuous measurements of upper ocean currents from the container vessel MV Oleander with a shipboard ADCP during its weekly roundtrips between Bermuda and Port Elizabeth, New Jersey. In addition to the velocity data, an expendable bathythermograph (XBT) section is taken once a month from the same vessel. These sections provide valuable information on the thermal structure and thereby a link to the corresponding density field.

A remarkable result to emerge from the first four and a half years of operation indicated that the lateral structure of the current was even 'stiffer' than anticipated from the Pegasus program (Rossby and Gottlieb, 1998). Thanks to the high-resolution sampling afforded by the ADCP, we found that the near-surface Gulf Stream can be characterized quite effectively as having a single velocity maximum with an exponentially decaying velocity profile to either side. In this paper we use all data from Fall 1992 to late 1999 to take a detailed look at the velocity and vorticity fields of the Gulf Stream.

  • We first examine the Gulf Stream and its variability in both Eulerian and natural coordinates.
  • We then test the hypothesis that the observed double exponential velocity structure in stream coordinates results from a uniformity of potential vorticity to either side of but not across the velocity maximum. Stommel (1957) had shown how the shoaling pycnocline of the Gulf Stream could be explained as a consequence of uniform potential vorticity on the Sargasso Sea side of the current. Here we extend Stommel's idea to include the Slope Sea side as well.
  • We also consider whether the hypothesized uniformity of potential vorticity results from a mean inflow or an eddy-driven homogenization process that maintains a uniform PV pool to each side of the current.

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