Alongshelf variability of a coastal buoyancy current during the relaxation of downwelling favorable winds

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Abstract

High-Frequency Doppler radars synoptically mapped surface currents over the inner-shelf off the North Carolina Outer Banks during the Duck94 experiment. The radar system sampled for the month of October, 1994 over a region 45 to 66 km southeast of the Chesapeake Bay mouth up to 34 km offshore. Concurrently, salinity and temperature were measured at locations 0.5, 1.6, 5.3 and 16.6 km offshore along a cross-shelf transect. Salinities over the shelf ranged from 26.6 to 32.7, with the lower salinities resulting from the intermittent presence of the coastal buoyancy current from the Chesapeake Bay. Three events were observed in which downwelling favorable winds persisted for at least three days, relaxed and then rotated to upwelling favorable. Downwelling favorable winds produced coastal sea level set-up. When the buoyancy current was present, cross-shelf salinity gradients were established between fresher water near the coast and the more saline water offshore. The response of the buoyancy current was sensitive to the temporal evolution of the wind field from downwelling to upwelling favorable. Following downwelling favorable winds the wind typically shifted to an onshore direction and decreased in magnitude. When the alongshelf velocities decreased, the onshore Coriolis force also decreased and the buoyancy current exhibited significant alongshelf variability over scales less than 10 km. Offshore flowing jets and undulations in the flow developed that formed vortices with diameters of 5-10 km following two of the downwelling events. Offshore surface layer velocities were observed between 4 and 16 hours before the wind switched to upwelling favorable. Formation of the vortices was likely triggered by an imbalance in the cross-shelf momentum due to nearshore water level set-up and the decreased onshore Coriolis component.

Original languageEnglish (US)
Pages (from-to)409-420
Number of pages12
JournalJournal of Coastal Research
Volume19
Issue number2
StatePublished - Mar 1 2003

Keywords

  • Chesapeake Bay outflow
  • Doppler velocities
  • High-Frequency radar
  • Mid-Atlantic Bight
  • Outer Banks
  • Remote sensing
  • Stratified flow
  • Surface currents

ASJC Scopus subject areas

  • Ecology
  • Water Science and Technology
  • Earth-Surface Processes

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