Hogg Nelson G.

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Hogg
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Nelson G.
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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.
  • Article
    Performance of a new generation of acoustic current meters
    (American Meteorological Society, 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.
  • Article
    Observations of the subtropical mode water evolution from the Kuroshio Extension System Study
    (American Meteorological Society, 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.
  • Article
    The Kuroshio Extension northern recirculation gyre : profiling float measurements and forcing mechanism
    (American Meteorological Society, 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.