Diagnosing a meander of the shelf break current in the Middle Atlantic Bight

Authors:
  • RPRobert S. Pickart
    Robert S. Pickart
  • DTDaniel J. Torres
    Daniel J. Torres
  • TMTheresa K. McKee
    Theresa K. McKee
  • MCMichael J. Caruso
    Michael J. Caruso
  • JPJennifer E. Przystup
    Jennifer E. Przystup
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Abstract
Two wintertime hydrographic crossings of the shelf break current in the Middle Atlantic Bight are analyzed. Separated by 3 days, the two sections sampled opposite phases of a meander of the current. The shipboard acoustic Doppler current profiler (ADCP) data reveal a strong, convergent jet during the first crossing, transporting 0.32 Sv of water equatorward. During the second crossing the transport is the same but the jet is weak and divergent. The measured Rossby numbers imply that the jet is significantly nonlinear in the convergent state. The associated potential voracity Q distributions differ substantially; the core of the weak jet is characterized by a region of uniform Q, whereas the strong jet contains no such feature. Surface thermal imagery indicates that the leading edge of a steep meander trough was sampled during the first crossing, followed by a broad crest during the second crossing. This is consistent with the convergent versus divergent nature of the flow in the two sections. A nearby Gulf Stream ring likely caused the steepening of the trough. After integrating the spatially low‐passed thermal wind shear, the resulting geostrophic sections are compared to the similarly low‐passed ADCP fields. This, together with the scales of the meander deduced from the surface imagery, indicates that the trough was in gradient wind balance, whereas the crest was predominantly geostrophic. These observations are consistent with the structure and dynamics of modeled baroclinic jets.

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