Hogg Nelson G.

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Hogg
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Nelson G.
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Technical Report

Eddies, islands, and mixing

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.

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Article

Eddy–mean flow interaction in the Kuroshio Extension region

2011-06 , Waterman, Stephanie N. , Hogg, Nelson G. , Jayne, Steven R.

The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

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Technical Report

Observations of internal Kelvin waves trapped round Bermuda

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.

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Article

Observations of the subtropical mode water evolution from the Kuroshio Extension System Study

2006-03 , Qiu, Bo , Hacker, Peter , Chen, Shuiming , Donohue, Kathleen A. , Watts, D. Randolph , Mitsudera, Humio , Hogg, Nelson G. , Jayne, Steven R.

Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of potential vorticity to be in the range of 2–5 × 10−4 m2 s−1. The latitudinal extent of the STMW formation was narrow, extending from 30°N to the Kuroshio Extension jet near 35°N. South of 30°N, STMW did not form locally but was transported from the recirculation gyre by lateral induction.

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Article

The Kuroshio Extension northern recirculation gyre : profiling float measurements and forcing mechanism

2008-08 , Qiu, Bo , Chen, Shuiming , Hacker, Peter , Hogg, Nelson G. , Jayne, Steven R. , Sasaki, Hideharu

Middepth, time-mean circulation in the western North Pacific Ocean (28°–45°N, 140°–165°E) is investigated using drift information from the profiling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (145°E) and the Shatsky Rise (156°E), and bordered to the north by the subarctic boundary along 40°N. This northern RG, which is simulated favorably in the eddy-resolving OGCM for the Earth Simulator (OFES) hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, the authors find the eddy dissipation effect to be important as well.

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Technical Report

Observations of energetic low frequency current fluctuations in the Charlie-Gibbs fracture zone

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.

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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

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.

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Article

Performance of a new generation of acoustic current meters

2007-02 , Hogg, Nelson G. , Frye, Daniel E.

As part of a program aimed at developing a long-duration, subsurface mooring, known as Ultramoor, several modern acoustic current meters were tested. The instruments with which the authors have the most experience are the Aanderaa RCM11 and the Nortek Aquadopp, which measure currents using the Doppler shift of backscattered acoustic signals, and the Falmouth Scientific ACM, which measures changes in travel time of acoustic signals between pairs of transducers. Some results from the Doppler-based Sontek Argonaut and the travel-time-based Nobska MAVS are also reported. This paper concentrates on the fidelity of the speed measurement but also presents some results related to the accuracy of the direction measurement. Two procedures were used to compare the instruments. In one, different instruments were placed close to one another on three different deep-ocean moorings. These tests showed that the RCM11 measures consistently lower speeds than either a vector averaging current meter or a vector measuring current meter, both more traditional instruments with mechanical velocity sensors. The Aquadopp in use at the time, but since updated to address accuracy problems in low scattering environments, was biased high. A second means of testing involved comparing the appropriate velocity component of each instrument with the rate of change of pressure when they were lowered from a ship. Results from this procedure revealed no depth dependence or measurable bias in the RCM11 data, but did show biases in both the Aquadopp and Argonaut Doppler-based instruments that resulted from low signal-to-noise ratios in the clear, low scattering conditions beneath the thermocline. Improvements in the design of the latest Aquadopp have reduced this bias to a level that is not significant.

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Technical Report

A compilation of moored current meter data from SYNOP arrays: one and two (September 1987 to July 1991), volume XLVI

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.

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Preprint

The Kuroshio Extension and its recirculation gyres

2009-07-01 , Jayne, Steven R. , Hogg, Nelson G. , Waterman, Stephanie N. , Rainville, Luc , Donohue, Kathleen A. , Watts, D. Randolph , Tracey, Karen L. , McClean, Julie L. , Maltrud, Mathew E. , Qiu, Bo , Chen, Shuiming , Hacker, Peter

This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of recirculation gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program program (June 2004 – June 2006) and include current meters, pressure and current recording inverted echo sounders, and sub-surface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114 Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio’s transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current’s transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.

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Thesis

The influence of topography on steady currents and internal waves

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.

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Technical Report

A dynamical interpretation of low frequency motions near very rough topography : the Charlie Gibbs Fracture Zone

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.

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Technical Report

Space and time scales of mesoscale motion in the western North Atlantic

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.