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Mesoscale Eddies Detached from Spatially Growing Meanders in an Eastward-Flowing Oceanic jet Using a Two-Layer Quasi-Geostrophic Model

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  • 1 Pacific Marine Environmental Laboratory, Environmental Research Laboratories, NOAA, Seattle, WA 98105
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Abstract

The process of eddy detachment from a strong, eastward-flowing oceanic jet, which is modeled after the Gulf Stream, is studied using a two-layer quasi-geostrophic model. Numerical calculations are performed, in which an initial perturbation having a small amplitude is superimposed on a basic flow consisting of the eastward jet in the upper layer. The initial perturbation is located in the western part of the jet and consists of one wavelength of meanders. The initial perturbation propagates eastward and grows rapidly, for the most part because of a baroclinic instability. The extremes of large-amplitude meanders are forced to go along the edges of recirculating gyres which are generated on both sides of the jet in the western part, and as a result the meanders are cut off and form eddies. In some cases, meander interactions—by which an upstream meander overtakes the nearest downstream meander—precedes the detachment.

The meander growth, which is the basic condition for the detachment, is highly dependent on the lower layer velocity of the basic jet, i.e., the fast eddy is detached later and farther downstream as the velocity increases from zero. The meander whose wavelength is ∼1.2 times as long as that of the fastest growing linear solution is the major source of the detached eddies, because nonlinearity has the stronger stabilizing effects on the shorter wavelength meanders and reduces group velocity of the longer wavelength meanders to a greater extent. A comparison of solutions on a β plane with ones on an f plane further reveals that the beta-effect assists detachment. The spatially growing solutions, which have larger amplitude in the eastern part, are more analogous to Gulf Stream meanders from which eddies are detached than are the spatially periodic, temporally growing solutions. The numerical results are applied to infrared images made from the NOAA-4 satellite of the Gulf Stream in which eddies are detached from meanders.

Abstract

The process of eddy detachment from a strong, eastward-flowing oceanic jet, which is modeled after the Gulf Stream, is studied using a two-layer quasi-geostrophic model. Numerical calculations are performed, in which an initial perturbation having a small amplitude is superimposed on a basic flow consisting of the eastward jet in the upper layer. The initial perturbation is located in the western part of the jet and consists of one wavelength of meanders. The initial perturbation propagates eastward and grows rapidly, for the most part because of a baroclinic instability. The extremes of large-amplitude meanders are forced to go along the edges of recirculating gyres which are generated on both sides of the jet in the western part, and as a result the meanders are cut off and form eddies. In some cases, meander interactions—by which an upstream meander overtakes the nearest downstream meander—precedes the detachment.

The meander growth, which is the basic condition for the detachment, is highly dependent on the lower layer velocity of the basic jet, i.e., the fast eddy is detached later and farther downstream as the velocity increases from zero. The meander whose wavelength is ∼1.2 times as long as that of the fastest growing linear solution is the major source of the detached eddies, because nonlinearity has the stronger stabilizing effects on the shorter wavelength meanders and reduces group velocity of the longer wavelength meanders to a greater extent. A comparison of solutions on a β plane with ones on an f plane further reveals that the beta-effect assists detachment. The spatially growing solutions, which have larger amplitude in the eastern part, are more analogous to Gulf Stream meanders from which eddies are detached than are the spatially periodic, temporally growing solutions. The numerical results are applied to infrared images made from the NOAA-4 satellite of the Gulf Stream in which eddies are detached from meanders.

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