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.
  • Technical Report
    Observations of internal Kelvin waves trapped round Bermuda
    (Woods Hole Oceanographic Institution, 1981-04) Hogg, Nelson G.
    Observations of the vertical and horizontal structure of motions near Bermuda have been made with two long-term moored arrays, one relatively far from and the other close to the island. Although not coincident in time, both arrays see horizontally coherent motions at II frequency bands ranging in period from 405 to 9.8 h. Only a peak at 26.1 h appears to be significant in the autospectra and, on several grounds, this is identified with the fundamental island-trapped mode (vertically and azimuthally). Additional resonant subinertial periods are at roughly 45, 54 and 90 h and these are vertical modes 2. 3 or 4 and azimuthal modes I or 2 propagating clockwise. The superinertial modes have less internal consistency but appear to have higher order vertical and azimuthal structures and both senses of azimuthal phase propagation. The subinertial vertical structure is modal and can be rationalized with baroclinic wave dynamics on a sloping bottom by defining an effective bottom depth as some reasonable average over the offshore decay scale. The subinertial motions are coherent with the surface wind stress and the phase between this forcing and the response changes by 180° across the trapped wave frequency bands consistent with a resonant system. The Q of the 26.1 h peak is at least 20 implying that dissipation has only a slight influence on the dynamics.
  • Technical Report
    A dynamical interpretation of low frequency motions near very rough topography : the Charlie Gibbs Fracture Zone
    (Woods Hole Oceanographic Institution, 1981-04) Hogg, Nelson G. ; Schmitz, William J. ;
    As a sequel to Schmitz and Hogg (1978), nine-month moored observations of current and temperature from the Charlie Gibbs Fracture Zone are further described, and then interpreted in terms of low frequency quasigeostrophic motions. Large vertical and horizontal changes are observed in the variance of both mean and fluctuating components. It is demonstrated that these changes could be associated with the (complex) nature of the topography. With regard to the mean flow, it is shown through an advective model that the topography is sufficiently steep to force this motion to closely follow isobaths. Time-dependent motions for periods from 2 to 96 days are described using the technique of empirical orthogonal functions. The most energetic mode is always bottom trapped, with flow oriented along isobaths at lower frequencies and approaching equipartition of along- and cross-isobath motions at higher frequencies. At the lowest frequencies a second mode which increases upward in energy is also judged significant, while for periods shorter than 3.6 days the second mode is again highly bottom trapped. We interpret these motions using linear wave theory. There is relatively close correspondence between theory and observation when the effects of both large- and small-scale topographic features are included in the model calculations. In addition to the usual topographic wave, the abrupt slope changes on the north wall allow for a baroclinic fringe mode with a ncar bottom node at low frequencies and small-scale bottom corrugations force highly bottom trapped waves above the smooth slope cut-off frequency.
  • 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.
  • Technical Report
    Space and time scales of mesoscale motion in the western North Atlantic
    (Woods Hole Oceanographic Institution, 2018-08) Richman, James G. ; Wunsch, Carl ; Hogg, Nelson G.
    From moored data, primarily temperature, of the Mid-Ocean Dynamics Experiment (ModeI) and its successor experiments we find a statistical description of the mesoscale variability. In the ModeI area itself the spectral characteristics of the thermocline and the deep water are different. The thermocline is conveniently described as being made up of three spectral bands: a ' low-frequency' band dominated by zonal velocity fluctuations, an 'eddy-containing' band in which the velocity field is nearly isotropic, and a 'high-frequency' band consistent with models of geostrophic turbulence. In the deep water the zonal dominance at low frequencies is not apparent, and there is enhanced energy at periods of 20-50 days. Vertical structure scales with WK BJ approximation in the high-frequency band but not in the lower frequencies, where low vertical modes dominate the motion. Linear models do not adequately describe the data in the ModeI region. Differences between rough and smooth topography regions are clearly seen only at 1500 m, where there is a loss of energy consistent with a reduced barotropic motion. Other differences, while apparently real, are small. It is found, consistent with the results of Schmitz (1976a), that the ModeI region is atypical of the midocean in that large changes of energy level are found elsewhere. A region due east of ModeI has slightly reduced kinetic energy levels in the main thermocline, but deep energy levels are much lower. Potential energy is less variable than kinetic; in the eastern region the frequency spectra change structure slightly. Linear models may be more adequate there. With more than 2 years of data, no statistically significant heat flux was found in the ModeI area, except for a weak zonal flux in the deep water. There is no direct evidence for baroclinic instability as a significant mechanism of eddy generation; the Gulf Stream is a possible, if unconfirmed, source.