Hogg Nelson G.

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Nelson G.

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  • Technical Report
    Eddies, islands, and mixing
    (Woods Hole Oceanographic Institution, 1978-12) Hogg, Nelson G. ; Katz, Eli J. ; Sanford, Thomas B.
    As part of a field study of the relation between fine scale and large‐scale variations of water properties in the western North Atlantic, the waters in the vicinity of Bermuda were investigated in detail. Previous work in the area had revealed regions of intense temperature fine structure confined to the sides of the island. Generally quieter levels of activity elsewhere in the midocean have suggested that significant mixing might only occur at the solid and fluid boundaries of the ocean. During the course of our investigation, two Gulf Stream rings were found in the vicinity of the island. The exchange of water between them caused three regions of strong alongshore flow. In these three areas we find elevated levels of temperature fine structure in the upper 800 m as measured by the variance in the temperature gradient normalized by the square of the mean temperature gradient over the interval. The normalized temperature variances on small scales (0.2–1 m) are most energetic in patches tightly bound to the island sides, whereas the fine structure on larger scales (5–25 m) is also energetic away from the island in a region of outflow. Velocity profiles show that vertical scales shorten as one approaches the island, and the energy increases in the counterclockwise component. There is no correlation evident between the shear measurements of the internal wave field and the intensity of the fine structure. Possible mechanisms for the production of fine structure are explored within the context of these observations.
  • Technical Report
    Observations of energetic low frequency current fluctuations in the Charlie-Gibbs fracture zone
    (Woods Hole Oceanographic Institution, 1979-02) Schmitz, William J. ; Hogg, Nelson G.
    Relatively energetic low frequency fluctuations in horizontal currents are found to exist below the thermocline in the northern trough of the Charlie-Gibbs Fracture Zone. For example, deep eddy kinetic energy levels there are about twice as large as those observed at similar relative depths in the MODE-I region. Eddy kinetic energies are about 2-10 times larger than mean kinetic energies. The vertical distribution of eddy kinetic energy is frequency dependent, increasing toward the thermocline for the longer time scales and intensifying toward the bottom at higher frequencies. In addition to the expected mean westward motion of Norwegian Sea Overflow Water through the northern trough of the fracture, rather consistent mean southward flow is observed at a depth immediately above the overflow.
  • Technical Report
    A moored array along the southern boundary of the Brazil Basin for the Deep Basin Experiment : report on a joint experiment 1991-1992
    (Woods Hole Oceanographic Institution, 1994-02) Tarbell, Susan A. ; Meyer, Ralf ; Hogg, Nelson G. ; Zenk, Walter
    The Deep Basin Experiment (DBE) is an international effort and a part of the World Ocean Circulation Experiment with the principal objective of improving our knowledge of the subthermocline circulation. The DBE fieldwork is focussed on the Brazil Basin and this report is concerned with a moored array situated along its southern boundary which was installed in early 1991 to measure the inflow and outflow to the Basin and to investigate the Brazil Current near 30S. This moored array was a joint undertaking by the Institut für Meereskunde of the University of Kiel and the Woods Hole Oceanographic Institution. Moorings were deployed on Meteor Cruise 15, leg 1 and retrieved on Meteor cruise 22, legs 3 and 4. A total of 57 conventional current meters and two Acoustic Doppler Current Profilers were set on 13 moorings with some concentration within the Brazil Current and the Vema Channel. CTDs were taken at each mooring site as well as in between. Some of the recovered instruments were reset in the Hunter Channel, a suspected additional connection between the Argentine Basin and the Brazil Basin. A later report will summarize this data after it is recovered in May 1994.
  • Thesis
    The influence of topography on steady currents and internal waves
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1971-01) Hogg, Nelson G.
    Observations of the ocean in the vicinity of Bermuda on two different occasions show systematic distortions of the isotherms close to the island and an area of intensive mixing on the northern coast. Two mechanisms are investigated and each produces some agreement with data from different flow regimes. Firstly, the island is modeled as a circularly symmetric obstacle with steep sides and a small aspect ratio. A steady, rotating, and stratified flow which, far from the island, is uniform in the horizontal and a linear function of the vertical coordinate is taken to be flowing past the island. Neglecting circulation effects, the problem is solved to first order in a small parameter, α, which measures the steepness of the island and a small Rossby number, ε. This allows a calculation of the depth contours of isotherms to 0(ε2,εα). For one set of data the flow is such that the slope effect of 0(εα) predominates while for another period of observation both slope and Rossby number influences are of the same magnitude. In both cases qualitative agreement between fact and theory is remarkably good. In addition, it is shown that the north slope (for a west-east current) is the most favored area for mixing as there the Richardson number is a minimum and the flow is most likely to separate from the boundary. A second means of producing isotherm distortion and mixing areas close to the island concerns the nonlinear effects of shoaling internal gravity waves. For normal incidence on a two-dimensional beach the Reynolds stresses produced by the fundamental wave motion are shown to force a mean Eulerian current which is equal hut opposite in sense to the Stokes drift. This causes the mean Lagrangian current to vanish so that the physical constraint that there be no net motion of fluid particles along isopycnals into the beach is satisfied. In addition, isotherms are distorted in a fashion analogous to the surface set-down produced by shoaling surface waves. The mean isopycnal shift can be as much as 10m where the theory has some validity. Distortions of the predicted form are observed in the data from a period when the mean currents were small. Consideration of the oblique incidence problem shows that this generalization has little effect on the expected magnitude of the shifts but that a significant longshore current can be forced by the breaking of the waves.
  • Technical Report
    A compilation of moored current meter data from SYNOP arrays: one and two (September 1987 to July 1991), volume XLVI
    (Woods Hole Oceanographic Institution, 1992-11) Tarbell, Susan A. ; Worrilow, Scott E. ; Hogg, Nelson G.
    The Synoptic Ocean Prediction Experiment (SYNOP) was an ambitious, multi-faceted program focused on the dynamics and predictailty of the Gulf Stream and its recirculations. The moored array component contained the arrays; one just downstream of Cape Hatteras (the "Inlet Array"), one near 68°W (the SYNOP "Central Array") and one near 55°W ("SYNOP East") to which this report is addessed. There were two settings of the SYNOP East array, the first, from fall 1987 to summer 1989, contained 42 current meters on 13 moorings straddling the mean axis of the Stream and extending north and south into the two recirculations. The second extended the southernmost six moorings for an additional two years until summer 1991. Performance was excellent and all instruments but one were recovered.