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  • Author or Editor: Kristene E. McTaggart x
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Gregory C. Johnson and Kristene E. McTaggart

Abstract

Argo float profile data are used to analyze warm, salty, weakly stratified, subthermocline eddies of tropical origin in the eastern subtropical South Pacific Ocean. These eddies contain anomalous signatures of the equatorial Pacific “13°C Water” that is carried poleward within the Peru–Chile Undercurrent (PCU) as it flows along the west coast of South America. From their source along the Chilean coast between ∼29° and 39°S, the eddies spread westward and slightly northward, likely at least partly advected by the subtropical gyre. The eddy water properties contrast strongly with the colder, fresher, more strongly stratified waters of subantarctic origin being carried northward then westward by the gyre. Near the eddy source, about 6% of Argo profiles sample eddies that are above selected thresholds for both salinity and potential vorticity anomalies relative to maps of the mean distributions of these properties on and around the core isopycnal for the eddies. The proportion of such profiles diminishes to about 1% near the northwestern limit of the eddy range, near 15°S and 115°W. These eddies are anticyclonic, with a subsurface radial velocity maximum near the core isopycnal for water property anomalies, hence a reduced surface expression. Their geostrophic signature sometimes extends below 1000 dbar, suggesting the eddies may influence float subsurface trajectories. Radial transports around the eddy centers are estimated to be on the order of 2 × 106 m3 s−1 for the potential density layer 26.0 < σθ < 27.0 kg m−3, about the same magnitude as the mean poleward transport of the PCU.

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Gregory C. Johnson, Eric Kunze, Kristene E. McTaggart, and Dennis W. Moore

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, ξ z, 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 ξ z 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 ξ z profiles shows a significant and coherent peak at 400-sdbar vertical wavelength (with N o = 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.

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