U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Interior pycnocline flow from the Subtropical to the Equatorial Pacific Ocean

Gregory C. Johnson and Michael J. McPhaden

NOAA, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, Washington  98115-6349

Journal of Physical Oceanography, 29, 3073–3089, 1999.
This paper is not subject to U.S. copyright. Published in 1999 by the American Meteorological Society

6. Discussion

The focus of this manuscript is on describing interior pycnocline pathways from the subtropics to the equator in the Pacific Ocean and quantifying mass fluxes of these pathways. The serpentine pathway in the Northern Hemisphere has been described using the potential vorticity and salinity fields, as well as quantified with geostrophic calculations. Relatively fresh water flows westward in the NEC, skirting the high potential vorticity under the ITCZ, then flows south from the NEC to the NECC between 165°E and 135°W, then eastward in the NECC, then south between 150°W and 100°W to join the SEC and flow westward and converge on the EUC. The more direct interior pathway in the Southern Hemisphere has also been studied, with salty low potential vorticity water flowing northward and westward. At 17°S this flow occurs between 135°W and the South American coast and by 6°S between 170°W and 110°W, continuing northward and westward to the equator, augmented by a shallow meridional overturning cell.

The Southern Hemisphere interior route carries about three times more subtropical water towards the equator than does the northern interior route. At least two, probably related, factors contribute to this difference. First, a large body of modeling work reviewed in the introduction as well as the data analysis presented here has demonstrated that high potential vorticity under the ITCZ is obviously linked to the relatively small equatorward interior flow found in the north. Most equatorward flow in the Northern Hemisphere pycnocline flows around this region of high potential vorticity to the western boundary where it can more easily turn southward. Second, there is evidence for a hemispheric asymmetry in eastern STMW production rates and locations. The Northern Hemisphere region of high subduction rate associated with eastern STMW production is centered near 25°N, 130°W (Huang and Qiu 1994; Hautala and Roemmich 1998). The analogous southern region is centered near 20°S, 120°W (Huang and Qiu 1998). The area of large subduction rate associated with eastern STMW production in the south is nearly twice the magnitude and twice the area of that in the north. In addition the southern region is located eastward and equatorward of the northern region. All of these differences contribute to a stronger equatorward interior flux of eastern STMW in the Southern Hemisphere. The low potential vorticity evident in the pycnocline of the Southeast Pacific (Figs. 2d and 5b) as well as the large equatorward transport of water around the density of eastern STMW in the south compared to that in the north (Fig. 6) reflect this asymmetry in production rates and locations.

The other significant result of this manuscript is that the salinity, potential vorticity, and geostrophic velocity fields are all consistent with deep cyclonic gyres below the pycnocline, hence an absence of interior equatorward flow of subtropical water there. The poleward interior flows of these gyres are in opposition to the equatorward flows within the pycnocline, the base of which we defined to be near the 26 kg m neutral surface. The eastward flowing SCCs serve as the equatorward limbs of these deep cyclonic gyres and the deeper portions of the westward flowing NEC and SEC serve as their poleward limbs. Salinity and potential vorticity signals are advected around these gyres, with reduced meridional gradients in the gyre interiors.

The dynamics of these deep gyres are not clear. In addition to the cyclonic gyres poleward of the SCCs there is evidence for anticyclonic gyres equatorward of the SCCs (Rowe et al. 2000). The eastward zonal flows within the SCCs have been modeled as inertial jets (Johnson and Moore 1997). The westward quasi-zonal return flows poleward of the SCCs are part of the deep NEC and SEC, but are not well studied. More return flow equatorward of the SCCs may be in the Equatorial Intermediate Current (Rowe et al. 2000). Flows completing the gyres at the western boundaries presumably follow western boundary current dynamics. Examination of acceleration potential and potential vorticity (Figs. 3b and 3d) suggests that in both deep cyclonic gyres contours of these quantities cross in the western boundary, where dissipation is expected. Upwelling under the ITCZ and off the South American coast may force the poleward flow within these deep cyclonic gyres through vortex stretching and diapycnal processes. In the southern deep gyre the effects of upwelling are likely confined to the coast, and are not well resolved with this data set. However, in the northern deep gyre contours cross again in the region of poleward flow under the ITCZ, where vortex stretching and diapycnal processes are posited. The eastward lightening of the equatorial influences in these deep gyres discussed here, as well as the change in density of the SCCs (Johnson and Moore 1997) is consistent with the hypothesized influence diapycnal processes.

Acknowledgments. This work was stimulated by conversations with E. Firing, W. Kessler, J. McCreary, D. Rowe, and, as always, L. Thompson. E. Firing, W. Kessler, J. McCreary, and an anonymous review all helped to improve the manuscript. K. McTaggart helped with calibration and processing of CTD data. This work was funded by the NOAA Office of Oceanic and Atmospheric Research, the NOAA Office of Global Programs, and the NASA Physical Oceanography Program.

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