Gregg M. C.

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Gregg
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M. C.
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Flow and hydraulics near the sill of Hood Canal, a strongly sheared, continuously stratified fjord

2010-05 , Gregg, M. C. , Pratt, Lawrence J.

Hood Canal, a long fjord in Washington State, has strong tides but limited deep-water renewal landward of a complex constriction. Tide-resolving hydrographic and velocity observations at the constriction, with a depth-cycling towed body, varied markedly during three consecutive years, partly because of stratification variations. To determine whether hydraulic control is generally important and to interpret observations of lee waves, blocking, and other features, hydraulic criticality is estimated over full tidal cycles for channel wide internal wave modes 1, 2, and 3, at five cross-channel sections, using mode speeds from the extended Taylor–Goldstein equation. These modes were strongly supercritical during most of ebb and flood on the gentle seaward sill face and for part of flood at the base of the steep landward side. Examining local criticality along the thalweg found repeated changes between mode 1 being critical and supercritical approaching the sill crest during flood, unsurprising given local minima and maxima in the cross-sectional area, with the sill crest near a maximum. Density crossing the sill sometimes resembled an overflow with an internal hydraulic control at the sill, followed by a hydraulic jump or lee wave. Long-wave speeds, however, suggest cross waves, particularly along the shallower gentler side, where flow downstream of a large-amplitude wave was uniformly supercritical. Supercritical approaching the sill, peak ebb was critical to mode 1 and supercritical to modes 2 and 3 at the base while forming a sluggish dome of dense water over the sill. Full interpretation exceeds observations and existing theory.

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The CLIMODE field campaign : observing the cycle of convection and restratification over the Gulf Stream

2009-09 , Marshall, John C. , Ferrari, Raffaele , Forget, Gael , Andersson, A. , Bates, Nicholas R. , Dewar, William K. , Doney, Scott C. , Fratantoni, David M. , Joyce, Terrence M. , Straneo, Fiamma , Toole, John M. , Weller, Robert A. , Edson, James B. , Gregg, M. C. , Kelly, Kathryn A. , Lozier, M. Susan , Palter, Jaime B. , Lumpkin, Rick , Samelson, Roger M. , Skyllingstad, Eric D. , Silverthorne, Katherine E. , Talley, Lynne D. , Thomas, Leif N.

A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as “mode water” in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air–sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.