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Temporal and Spatial Structure of the Equatorial Deep Jets in the Pacific Ocean

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  • 1 NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 2 Applied Physics Laboratory, University of Washington, Seattle, Washington
  • | 3 NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

The spatial and temporal structure of the equatorial deep jets (EDJs) in the Pacific Ocean is investigated using CTD station data taken on the equator from 1979 through 2001. The EDJs are revealed in profiles of vertical strain, ξz, estimated from the CTD data in a stretched vertical coordinate system. The majority of synoptic meridional sections were occupied over an 8-yr span west of the date line. Two-decade equatorial time series are available at both 110°W and 140°W. Analysis shows the expected equatorial trapping of ξz but yields little new detailed information about the EDJ meridional structure. Analysis of the equatorial data yields novel results. The EDJs are most easily seen in the eastern Pacific (at and east of 140°W). There, they may be isolated from the influence of higher-frequency Rossby waves generated by surface forcing. Spectral analysis of equatorial ξz profiles shows a significant and coherent peak at 400-sdbar vertical wavelength (with No = 1.56 × 10−3 s−1) from 95°W to 142°W. This peak has very long zonal scales. It exhibits a very slow mean downward migration of 4.2(±0.5) × 10−7 sdbar s−1 [13(±2) sdbar yr−1]. Whether this migration is steady or intermittent is difficult to ascertain. However, over the two-decade record length, the EDJs shift downward by only about two-thirds of a vertical wavelength. Hence it is no surprise that previous observational analyses of the EDJs in the Pacific, limited to 16 months or less, had difficulty finding any significant vertical migration.

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 spatial and temporal structure of the equatorial deep jets (EDJs) in the Pacific Ocean is investigated using CTD station data taken on the equator from 1979 through 2001. The EDJs are revealed in profiles of vertical strain, ξz, estimated from the CTD data in a stretched vertical coordinate system. The majority of synoptic meridional sections were occupied over an 8-yr span west of the date line. Two-decade equatorial time series are available at both 110°W and 140°W. Analysis shows the expected equatorial trapping of ξz but yields little new detailed information about the EDJ meridional structure. Analysis of the equatorial data yields novel results. The EDJs are most easily seen in the eastern Pacific (at and east of 140°W). There, they may be isolated from the influence of higher-frequency Rossby waves generated by surface forcing. Spectral analysis of equatorial ξz profiles shows a significant and coherent peak at 400-sdbar vertical wavelength (with No = 1.56 × 10−3 s−1) from 95°W to 142°W. This peak has very long zonal scales. It exhibits a very slow mean downward migration of 4.2(±0.5) × 10−7 sdbar s−1 [13(±2) sdbar yr−1]. Whether this migration is steady or intermittent is difficult to ascertain. However, over the two-decade record length, the EDJs shift downward by only about two-thirds of a vertical wavelength. Hence it is no surprise that previous observational analyses of the EDJs in the Pacific, limited to 16 months or less, had difficulty finding any significant vertical migration.

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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