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Structure of the Atlantic Ocean Equatorial Deep Jets

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  • 1 NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 2 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington
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Abstract

The equatorial deep jets in the Atlantic Ocean are described using vertical strain, ξz, estimated from all available deep CTD stations in the region. Wavelet analysis reveals a distinct energy peak around 661-sdbar vertical wavelength, 1232-dbar pressure, and ±1.5° latitude from the equator. This high-vertical-wavenumber and off-equatorial maximum, coupled with previously published velocity data that show nodes in zonal velocity near ±1.5°, is grossly consistent with the structure of first-meridional-mode equatorial Rossby waves. However, the meridional scale obtained from the observations exceeds, by about 1.5, the theoretical meridional scale for these waves. The jets are strong, with zonal velocities similar in magnitude to the Kelvin wave phase speed for their vertical wavelength. Harmonics of ξz at vertical wavelengths of 1/2, 1/4, and perhaps 1/8 that of the primary peak provide evidence of a large-amplitude structure. Although sparse, available phase data at the 661-sdbar vertical wavelength suggest downward and westward phase propagation. Assuming sinusoidal character in time and longitude gives estimates of a 5- (±1) yr period and a 70° (±60°) zonal wavelength. These vertical, temporal, and zonal scales are roughly consistent with first-meridional-mode equatorial Rossby wave dynamics. However, although vertical and zonal phase propagation are discernible, there is no obvious signature of upward energy propagation in the variance vertical maxima, which is problematic for a simple linear Rossby wave interpretation.

Corresponding author address: Dr. Gregory C. Johnson, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way N.E., Bldg. 3, Seattle, WA 98115-6349. Email: gjohnson@pmel.noaa.gov

Abstract

The equatorial deep jets in the Atlantic Ocean are described using vertical strain, ξz, estimated from all available deep CTD stations in the region. Wavelet analysis reveals a distinct energy peak around 661-sdbar vertical wavelength, 1232-dbar pressure, and ±1.5° latitude from the equator. This high-vertical-wavenumber and off-equatorial maximum, coupled with previously published velocity data that show nodes in zonal velocity near ±1.5°, is grossly consistent with the structure of first-meridional-mode equatorial Rossby waves. However, the meridional scale obtained from the observations exceeds, by about 1.5, the theoretical meridional scale for these waves. The jets are strong, with zonal velocities similar in magnitude to the Kelvin wave phase speed for their vertical wavelength. Harmonics of ξz at vertical wavelengths of 1/2, 1/4, and perhaps 1/8 that of the primary peak provide evidence of a large-amplitude structure. Although sparse, available phase data at the 661-sdbar vertical wavelength suggest downward and westward phase propagation. Assuming sinusoidal character in time and longitude gives estimates of a 5- (±1) yr period and a 70° (±60°) zonal wavelength. These vertical, temporal, and zonal scales are roughly consistent with first-meridional-mode equatorial Rossby wave dynamics. However, although vertical and zonal phase propagation are discernible, there is no obvious signature of upward energy propagation in the variance vertical maxima, which is problematic for a simple linear Rossby wave interpretation.

Corresponding author address: Dr. Gregory C. Johnson, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way N.E., Bldg. 3, Seattle, WA 98115-6349. Email: gjohnson@pmel.noaa.gov

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