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The Pacific Subsurface Countercurrents and an Inertial Model

Gregory C. Johnson and Dennis W. Moore

National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Physical Oceanography, 27, 2448-2459
Not subject to U.S. copyright. Published in 1997 by the American Meterological Society.

1. Introduction

The subsurface countercurrents (SSCCs) in the Pacific Ocean (Tsuchiya 1972 , 1975), or Tsuchiya jets, are persistent eastward subsurface currents found on either side of the equator with a pycnostad between them. The SSCCs are remarkably steady and extend across the entire Pacific Ocean. A recent study of mean hydrographic sections from CTD data averaged on potential density anomaly surfaces in the western Pacific quantifies the SSCC locations, velocities, and transports between 137 and 165E (Gouriou and Toole 1993). For the north SSCC they suggest that a velocity core, distinct from the Equatorial Undercurrent (EUC) and the North Equatorial Countercurrent (NECC), develops between 137 and 142E. At 165E Gouriou and Toole estimate a north SSCC volume transport of 11  106 m3 s-1 with a peak velocity of 0.27 m s-1 occurring at 2.5N near 280-m depth. For the south SSCC, they surmise an origin between 142 and 165E. At 165E they estimate a transport of 7  106 m3 s-1, with a peak velocity of 0.22 m s-1 at 2.5S near 280-m depth. They also note strong gradients in potential vorticity across the axes of both SSCCs and a strong gradient in salinity across the north SSCC, suggesting that the SSCCs are a barrier to meridional flow, but that between them water properties may be homogenized in gyres composed of the eastward flowing SSCCs and the westward flowing Equatorial Intermediate Current (EIC).

In the central Pacific, the Tsuchiya jets are distinctly visible in an annual mean hydrographic section nominally along 155W from CTD data averaged on pressure surfaces (Wyrtki and Kilonsky 1984). As in the western Pacific, the north SSCC is stronger with a transport of 9  106 m3 s-1 and a mean velocity of 0.07 m s-1, as compared to the south SSCC, with a 4  106 m3 s-1 transport and a mean velocity of 0.05 m s-1. Peak velocity values are not reported but a zonal velocity section shows peak speeds <0.15 m s-1 in the north SSCC and <0.10 m s-1 in the south SSCC near 240-m depth at 4 from the equator. These peaks are 40 m shallower and 1.5 poleward of the maxima at 165E. These peaks are also much weaker than those at 165E and 110W (see below), but without commensurate changes in transports. This seeming discrepancy arises because the cross-sectional areas of the SSCCs are larger at 155W than at 165E and 110W. These differences are at least partially artifacts of a shift in longitude of the mean station positions near the south SSCC which increases station spacing there. A salinity section suggests the gradient observed across the north SSCC in the western Pacific also exists in the central Pacific at 4N. In addition, dissolved oxygen and nutrient concentration sections show that both SSCCs are associated with oxygen-rich, nutrient-poor water near 4 latitude (Wyrtki and Kilonsky 1984), properties advected from the west (Tsuchiya 1975).

The Tsuchiya jets in the eastern Pacific are very well studied. East of 119W, they have been described extensively by Tsuchiya (1972, 1975) through analysis of many synoptic hydrographic sections. He notes that the SSCCs exist between 3 and 6N and 4 and 8S, and shift poleward to the east. The north SSCC mean transport is estimated at 8  106 m3 s-1 with an average peak velocity of 0.27 m s-1 at depths from 30 to 200 m; the south SSCC transport is estimated at 5  106 m3 s-1 with an average peak velocity of 0.15 m s-1 at depths from 80 to 250 m (Tsuchiya 1975). A similar, but more recent study of the north SSCC also using analysis of several synoptic CTD sections along 110W suggests a mean transport of 14  106 m3 s-1 with a mean peak velocity of 0.40 m s-1 at 4.6N near 120 m depth (Hayes et al. 1983). The discrepancy between these two transport estimates for the north SSCC near 110W may arise from differences in the definition of its upper boundary. The higher peak velocity estimate in the second study probably results from finer horizontal resolution of the synoptic sections. In any case, the SSCCs in the eastern Pacific are again shallower and poleward of their locations in the central Pacific with transports and velocities roughly consistent with those reported to the west. The poleward shift as the SSCCs flow eastward from 155 to 110W has also been noted in a diagnostic calculation of the upper ocean circulation (Bryden and Brady 1985).

Finally, the water in the equatorial pycnostad in the eastern Pacific has been traced westward along the south Tsuchiya jet and then along a counterclockwise route in the subtropical gyre to a surface origin near Tasmania by Tsuchiya (1981). This work demonstrates that the properties of water on a germane isanosteric surface are modified relatively little by mixing as it is advected over great distances around the South Pacific Ocean. It is argued that vertical mixing is dominant in influencing the water properties near their surface origin, but that lateral mixing plays the stronger role farther along the subsurface flow path.

We are aware of only one theory for the dynamics of the Tsuchiya jets (McPhaden 1984). A linear, vertically diffusive model simulates the SSCCs as lobes of the EUC, formed at the poleward edge of a broad diffusive equatorial boundary layer. Within the boundary layer, downward vertical diffusion of cyclonic relative vorticity is balanced by poleward advection of planetary vorticity. Outside the boundary layer, the planetary vorticity advection is balanced by vortex stretching, creating a pycnostad. In this model, the SSCCs are the result of geostrophic balance across the pycnostad. However, in the central and eastern Pacific Ocean, the SSCCs are separated from the EUC, suggesting nonlinear dynamics may be important there (McPhaden 1984). More recently, the EUC has been modeled as an inertial jet (Pedlosky 1987, 1988, 1991), but until now no inertial model has been proposed for the SSCCs.

In this paper we use historical CTD data to create mean hydrographic profiles averaged on neutral density anomaly surfaces (Jackett and McDougall 1997) for select areal bins in the tropical Pacific Ocean, reducing the data across the entire ocean in a uniform manner designed to preserve the small meridional and vertical scales of hydrographic features found in the Tropics. We present mean meridional sections of properties at 165E, 155W, and 110W together with maps of properties on and between appropriate isopycnals. These data are used in a trans-Pacific analysis to emphasize several points about the Tsuchiya jets. First, the SSCCs are separate from the EUC, at least in the central and eastern Pacific. Second, the SSCCs start near the equator in the west and shift poleward toward the east. Third, this poleward shift is correlated with a shoaling of the pycnocline and the building pycnostad between the SSCCs. We also present a simple inertial model of the SSCCs. This inertial model accounts for the roughly constant volume transports of the SSCCs, their advection of properties over long zonal distances, their persistent narrowness and rapidity, their poleward shift from west to east, the pycnostad between them that builds in size and strength from west to east, and the associated potential vorticity gradients across them.

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