The Kuroshio Extension Northern Recirculation Gyre: Profiling Float Measurements and Forcing Mechanism

Bo Qiu Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

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Shuiming Chen Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

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Peter Hacker Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

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Nelson G. Hogg Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Steven R. Jayne Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Hideharu Sasaki Earth Simulator Center, JAMSTEC, Yokohama, Japan

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Abstract

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.

Corresponding author address: Dr. Bo Qiu, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd., Honolulu, HI 96822. Email: bo@soest.hawaii.edu

Abstract

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.

Corresponding author address: Dr. Bo Qiu, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd., Honolulu, HI 96822. Email: bo@soest.hawaii.edu

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