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

1. Introduction

Recently, it has been suggested (Gu and Philander 1997), supported by observational analysis (Zhang et al. 1998), that interior thermal anomalies, apparently originating where water is subducted in the subtropics of the North Pacific (Deser et al. 1996), may propagate to the equatorial region in the Pacific. This propagation may occur through advection by the ocean general circulation (Gu and Philander 1997), or more rapidly through Rossby and Kelvin waves (Lysne et al. 1997). Thermal anomalies may be owing to subduction poleward of 18°N and variations of wind-stress equatorward of this latitude (Schneider et al. 1999). It has been further hypothesized that these anomalies may contribute to the decadal changes observed in the structure of the equatorial pycnocline and that this modulation of the equatorial pycnocline may in turn be linked to a shift in the dynamics, frequency, and intensity of El Niño (Gu and Philander 1997). Taken together, these hypotheses are one reason that the pathways by which and rates at which subtropical water reaches the equatorial region is of interest in understanding and predicting seasonal-to-interannual climate variability. Of course, describing intergyre oceanic mass and water property exchanges, understanding their dynamics, and quantifying their magnitudes are also important in themselves to the study of the ocean general circulation and its role in climate.

Tritium data have been interpreted to suggest an interior pathway along which pycnocline water ventilated in the subtropical North Pacific reaches the equator between 145°W and 125°W (Fine et al. 1987) in the central Pacific, crossing the intertropical convergence zone (ITCZ) that separates westward North Equatorial Current (NEC) from the eastward North Equatorial Countercurrent (NECC). These data suggest this northern ventilation ranges in potential density from = 23 to 26.2 kg m–3 and that at = 25.0 kg m the timescale for water to reach the equator is less than 14 years.

Depth-integrated geostrophic mass transport in the North Pacific Ocean as inferred from Sverdrup dynamics clearly indicates this interior pathway (McPhaden and Fine 1988). In addition, mean surface dynamic height relative to a pressure of 1000 dbar (Wyrtki 1975) shows a similar pattern. However, water in the ocean interior moves along neutral surfaces. The near-surface geostrophic circulation likely has a pattern similar to the surface dynamic height field or the depth-integrated mass transport predicted using the wind stress curl, but these studies do not reveal how the circulation changes with increasing density (and depth).

Modeling provides a third approach to the question of pathways for subtropical-equatorial exchange. Ventilated thermocline dynamics (Luyten et al. 1983), extended to the tropics in a reduced-gravity model formulation, shows that the presence of an interior pathway for northern hemisphere ventilation depends critically on the location of the poleward edge of the shadow zone (McCreary and Lu 1994; Liu 1994). A complex configuration of the model with a Pacific-like geometry under a smoothed climatological wind field suggests that the region of high potential vorticity beneath the ITCZ may constitute a barrier to interior subtropical-equatorial exchange in the Northern Hemisphere (Lu and McCreary 1995). However, reducing the model thermocline thickness to a smaller value results in an interior pathway as one of three possibilities (Lu et al. 1998; Liu and Huang 1998).

Lagrangian analysis of more complex ocean general circulation models also shows that water subducted in the Northern Hemisphere subtropics takes one of three pathways (Liu et al. 1994; Gu and Philander 1997; Liu and Huang 1998; Rothstein et al. 1998). Water subducted in the western North Pacific reaches the western boundary and heads north in the Kuroshio, avoiding immediate subtropical-equatorial exchange. Water subducted in the central North Pacific reaches the western boundary and heads south in the Mindanao Current to join the eastward flowing Equatorial Undercurrent (EUC), after reaching the dateline in the NECC. Water subducted in the eastern North Pacific describes a zigzag toward the equator, initially flowing southwestward in the NEC, then abruptly turning south to flow southeastward in the NECC, then turning south again to flow southwestward in the northern branch of the South Equatorial Current (SEC) before joining the EUC in the central Pacific. In the Southern Hemisphere subtropics the three pathways for subducted water are similar, with the significant exception that the waters subducted farthest east flow directly northwestward toward the equator in the southern branch of the SEC. In general, water parcels take under 16 years to reach the equatorial region after subduction (Gu and Philander 1997), roughly consonant with the time estimated from the tritium measurements (Fine et al. 1987).

A comprehensive description of the tropical Pacific Ocean circulation and its variation with density using conventional physical oceanographic measurements exists (Tsuchiya 1968). The work here adds to this description with discussions of potential vorticity, vertical sections, and an emphasis on interior pathways for subtropical-equatorial exchange, only briefly discussed in Tsuchiya (1968). In addition, 25 times more stations are used than were available to Tsuchiya (1968), and the present data are all high vertical resolution CTD data rather than the bottle data used before. Maps of depth, salinity, potential vorticity, and acceleration potential on selected neutral surfaces are discussed along with meridional-vertical sections of neutral density, salinity, and potential vorticity. The density above which interior subtropical-to-equatorial exchange ceases in both hemispheres is quantified and shown to coincide with the base of the tropical pycnocline. This density surface outcrops in winter at the poleward limits of the subtropical gyres. It is also the density above which the equatorial pycnostad and the eastward flowing Tsuchiya jets, that is the North and South Subsurface Countercurrents (NSCC and SSCC, or SCCs), are found (Tsuchiya 1975; Johnson and Moore 1997; Rowe et al. 2000). These jets form the equatorward limbs of basin-wide subsurface cyclonic tropical gyres, where equatorward flow is absent except at the western boundary. Finally, interior meridional transport estimates within the tropical pycnocline are presented.

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