Geostrophic calculations are made relative to a reference surface of 900 dbar, roughly n = 27.5 kg m. Inspection of the depth of this isopycnal (not shown) suggests that geostrophic shear at this surface is weak except in the far southwest portion of the region under investigation, where the subtropical pycnocline is deep. This assumption is supported by direct measurements at this level (Davis 1998). The 900-dbar surface sits well beneath the tropical pycnocline and its use as a reference surface captures all the geostrophic shear above. Three previous quantitative works on zonal mass transports across the Tropics used similar surfaces: 600 dbar at 165°E (Gouriou and Toole 1993), 1000 dbar between 150°W and 158°W (Wyrtki and Kilonsky 1984), and 500 dbar at 110°W (Hayes et al. 1983).
Quasi-meridional interior geostrophic mass transport estimates presented here are made in a layer extending from the base of the mixed layer to the base of the tropical pycnocline. The results are similar to those inferred independently from wind fields using Sverdrup dynamics (McPhaden and Fine 1988), but here the magnitudes of the interior meridional transports are quantified as well as the density range over which they occur. The mixed layer is excluded because the mean wind field over nearly the entire region dictates a poleward Ekman transport (Lu and McCreary 1995), which overwhelms the equatorward geostrophic velocity in the mixed layer. Our functional definition of the tropical pycnocline base in this instance is the neutral density at which the net interior meridional mass transport changes sign from equatorward above to poleward below within a 9° latitude band containing the deep cyclonic gyres in each hemisphere. In the Northern Hemisphere, from 3° to 11°N, the base sits at n = 25.9 ± 0.2 kg m. In the southern hemisphere, from 4 to 12°S, the base sits at n = 26.2 ± 0.2 kg m (see Fig. 1 for isopycnal winter outcrop locations). Interior meridional transports at 8°N and 8°S show convergent equatorward flow, hence interior communication from the subtropics to the equator above these isopycnals and divergent poleward flow in the deep cyclonic gyres below them (Fig. 6). Only interior transports are presented here. While western boundary currents in both hemispheres transport significant amounts of subtropical pycnocline water toward the equator, they have been extensively discussed elsewhere (Tsuchiya et al. 1989; Lukas et al. 1991; Butt and Lindstrom 1994; Wijffels et al. 1995).
Figure 6. Net meridional mass transport (10 kg s) excluding the mixed layer binned in n every 0.2 kg m from the South American coast to (a) 165°E at 8°S and (b) 135°E at 8°N. In the south, equatorward flow is concentrated near the eastern STMW density and poleward flow below n = 26.1 kg m. In the north, equatorward flow is at lighter densities and poleward flow below n = 25.9 kg m. (c) Downward accumulated integrals of (a) and (b) show the net interior transport within the pycnocline (to n roughly 26 kg m) in the northern hemisphere (6 × 10 kg s) is much less than that in the southern hemisphere (14 × 10 kg s).
In the Southern Hemisphere, meridional mass transport estimates within the pycnocline, from n = 26.2 kg m to the base of the mixed layer, are made from 17°S equatorward. South of 17°S data starts too far off the South American coast for a meaningful interior meridional transport estimate. At all latitudes the interior meridional transport ends well east of the western boundary, and equatorward of 14°S this transport starts west of South America. The interior transport occurs between 135°W and South America at 17°S, but shifts westward toward the equator and is found between 170°W and 110°W by 6°S. Between 17°S and 6°S the estimates have a minimum of 13 (± 5) × 10 kg s at 9°S and a maximum of 19 (± 2) × 10 kg s at 16°S, both latitudes where data are sparse and error estimates are large. From 17°S to 6°S the weighted average and standard deviation of interior meridional mass transport is 15 (± 1) × 10 kg s (Fig. 7a). This value is an estimate of the interior subtropical mass transport from the Southern Hemisphere toward the equator. For purposes of comparison, this interior meridional mass transport estimate slightly exceeds the 912 (× 10 m s) of southern origin estimated to join the EUC core from western boundary currents in the Southern Hemisphere (Butt and Lindstrom 1994). These results are in reasonable accord with analysis of a data assimilation product which suggest western boundary and interior transports at 10°S of 15 and 11 (× 10 m s), respectively (Huang and Liu 1999).
Figure 7. Quasi-meridional interior pycnocline (mixed-layer base to pycnocline base) mass transport (10 kg s) zonally accumulated from the Americas westward. Symbols are individual integration points and lines are objectively mapped assuming a Gaussian covariance with a correlation length scale of 20° longitude and an error-to-signal energy of 0.25. (a) Southern hemisphere transports at three latitudes (17°S solid line, 12°S dashed, and 7°S dot-dashed) with a pycnocline base at n = 26.2 kg m. (b) Northern hemisphere transport between the NEC and NECC with a pycnocline base at n = 25.9 kg m. (c) Northern hemisphere transport between the NECC and SEC with a pycnocline base at n = 25.9 kg m.
Equatorward of 6°S, meridional interior transport estimates increase from 20 (± 2) × 10 kg s between 170°W and 110°W at 5°S to 30 (± 4) × 10 kg s between 170°W and 120°W at 3°S. While low-latitude geostrophic calculations are noisy, the trend is clear. This rapid increase probably reflects the presence of a shallow meridional overturning cell at the equator, with interior equatorward geostrophic transport working to offset large poleward mass transports in the Ekman layer. This cell has been observed in profiling and acoustic Doppler current meter data from the Hawaii-Tahiti Shuttle Experiment (McPhaden 1984; Johnson and Luther 1994) and reproduced in numerical models (Lu et al. 1998).
The Northern Hemisphere is somewhat more complicated than the Southern Hemisphere, with the quasi-zonal NEC, NECC, SEC, and EUC alternating in flow direction. Instead of presenting simple meridional mass transports, which would contain portions of these quasi-zonal currents, we isolate the cross-gyre transports by calculating transports across the geopotential anomaly trough under the ITCZ that separates the NEC from the NECC (Fig. 7b). Another estimate is made across the lower-latitude ridge that separates the NECC from the SEC and 2°N at a few western longitudes where no ridge is evident (Fig. 7c). In both cases, there are regions of little net transport east and west of where the interior meridional transport occurs. Within the pycnocline, from n = 25.9 kg m to the base of the mixed layer, 5 (± 1) × 10 kg s flows across the trough from the NEC to the NECC between 165°E and 135°W. This value is an estimate of the interior subtropical mass transport from the Northern Hemisphere toward the equator, and is one third the estimate of 15 (± 1) × 10 kg s equatorward interior flow estimated for the Southern Hemisphere. A simple comparison of interior pycnocline meridional mass transports along 8°N and 8°S gives a similar result (Fig. 6c). The Northern Hemisphere interior transport estimate is probably an upper bound because it may contain southward flow from a tropical recirculation around the ITCZ centered near 11°N, 125°W (Fig. 2b). However, since there is no evidence of any significant northward component of the recirculation east of 125°W (Fig. 7b), the net tropical recirculation is likely to be small. For purposes of comparison, this northern interior transport is less than half of the 1316 (× 10 m s) of northern origin water estimated to reside within the core of the EUC well to the west at 153°E (Butt and Lindstrom 1994). These results are again in reasonable accord with analysis of a data assimilation product, which suggests western boundary and interior transports at 10°N of 14 and 3 × 10 m s, respectively (Huang and Liu 1999).
Farther to the south, an estimated 35 (± 8) × 10 kg s flows across the low-latitude ridge from the NECC to the SEC between 165°E and 105°W (Fig. 7c). The 30 (± 8) × 10 kg s increase in equatorward transport with relation to the northern estimate seems large and may be an upper bound, since part of the transport calculation is made at 2°N, a fairly low latitude at which to apply geostrophy. However, this increase can fairly easily be accounted for by a combination of two similarly sized sources. First, the addition of northern subtropical water from the Mindanao Current (Lukas et al. 1991; Wijffels et al. 1995) at the western boundary transported eastward within the NECC could be contributing to equatorward pycnocline mass flux. The NECC carries 20 × 10 m s at 165°E (Gouriou and Toole 1993) and only 8 × 10 m s by 110°W (Hayes et al. 1983), allowing for an increase of 12 × 10 m s in equatorward transport from the NECC to the SEC. Second, the poleward Ekman transport across the low-latitude ridge is 45 × 10 kg s between 165°E and 105°W using the Hellerman and Rosenstein (1983) surface wind stress climatology after applying a recommended scaling factor of 0.8 to correct the drag coefficient (Harrison 1989). If 40 (± 25)% of the equatorward geostrophic transport balancing this Ekman transport occurs in a shallow tropical meridional cell extending below the mixed layer (Lu et al. 1998), it would account for the remaining mass budget within interior meridional transport errors.
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