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Observations on the Variability of the Gulf Stream Path between 74°W and 70°W

Harilaos KontoyiannisGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island

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D. Randolph WattsGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island

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Abstract

The Gulf Stream path varies in a wavelike fashion in the deep water downstream of Cape Hatteras, energetic within a range of wavelengths λ from 150 km to over 1000 km, and periods τ from 4 d to over 500 d. The authors determined the path from the 12°C isotherm at 400-m depth, which was obtained daily for 740 days (April 1983–May 1985) from objective analysis maps of the thermocline topography measured by inverted echo sounders (IESs). The IESs were deployed in a region 400 km long from 74° to 70°W. To find propagating and standing mode characteristics of this large array of data, frequency-domain empirical orthogonal functions and wavenumber-frequency spectra were calculated, using FFT in time combined with autoregressive (AR) modeling in space. This study reveals smooth transitions from (i) downstream propagating and growing short-period meander modes, to (ii) standing modes at intermediate period, to (iii) upstream propagating long-period modes. For periods shorter than semiannual, meanders propagate downstream with speeds monotonically increasing from 6 km d−1 for (λ, τ)∼(1000 km, 160 d), to 55 km d−1 for (λ, τ) ∼ (180 km, 3 d). Their maximum growth rates occur at (λ, τ) ∼ (260 km, 8 d), with e-folding spatial scales ∼250 km, and e-folding temporal scales ∼3 d. For periods between semiannual and annual, the entire path shifts like a standing mode with no detectable phase lag within our study region, and its envelope accounts for the local minimum ("node") that has been observed near 70°W. For periods from annual to at least 500 d, fluctuations propagate upstream at about 9 km d−1, and their amplitude decreases to the west in the direction of propagation.

Abstract

The Gulf Stream path varies in a wavelike fashion in the deep water downstream of Cape Hatteras, energetic within a range of wavelengths λ from 150 km to over 1000 km, and periods τ from 4 d to over 500 d. The authors determined the path from the 12°C isotherm at 400-m depth, which was obtained daily for 740 days (April 1983–May 1985) from objective analysis maps of the thermocline topography measured by inverted echo sounders (IESs). The IESs were deployed in a region 400 km long from 74° to 70°W. To find propagating and standing mode characteristics of this large array of data, frequency-domain empirical orthogonal functions and wavenumber-frequency spectra were calculated, using FFT in time combined with autoregressive (AR) modeling in space. This study reveals smooth transitions from (i) downstream propagating and growing short-period meander modes, to (ii) standing modes at intermediate period, to (iii) upstream propagating long-period modes. For periods shorter than semiannual, meanders propagate downstream with speeds monotonically increasing from 6 km d−1 for (λ, τ)∼(1000 km, 160 d), to 55 km d−1 for (λ, τ) ∼ (180 km, 3 d). Their maximum growth rates occur at (λ, τ) ∼ (260 km, 8 d), with e-folding spatial scales ∼250 km, and e-folding temporal scales ∼3 d. For periods between semiannual and annual, the entire path shifts like a standing mode with no detectable phase lag within our study region, and its envelope accounts for the local minimum ("node") that has been observed near 70°W. For periods from annual to at least 500 d, fluctuations propagate upstream at about 9 km d−1, and their amplitude decreases to the west in the direction of propagation.

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