Here properties on two neutral surfaces are discussed. The lighter surface, n = 25.0 kg m, is chosen for proximity to the equatorial pycnocline, the EUC, and the subtropical mode water (STMW) vertical potential vorticity minima (best seen in the meridional-vertical sections discussed below). The denser surface, n = 26.5 kg m, is chosen to lie below the pycnocline, but within the equatorial pycnostad and the SCCs. Winter outcropping locations of the lighter surface are in the subtropics and the denser surface in the subpolar regions (Fig. 1). Depth locates the neutral surface and its gradients give a sense of vertical geostrophic shear. Acceleration potential contours are streamlines of geostrophic velocity on that surface relative to 900 dbar. Salinity on a neutral surface is a passive tracer inversely related to temperature since saltier water must be cooler and fresher water must be warmer to remain on the neutral surface. Finally, the planetary component of the potential vorticity is a dynamically active tracer that (away from the equator and the western boundary) has seldom been described in the Tropics, but see Gouriou and Toole (1993) . In the absence of mixing and boundary forcing, both salinity and potential vorticity are conservative on neutral surfaces. However, in the ocean, these properties are not strictly conserved. Their distribution on neutral surfaces is governed by both advection and diffusion.
Figure 1. Winter (JanMar in the north and JulSep in the south) surface outcrops of n = 25.0 and 26.5 kg m and the pycnocline base (dashed lines) from seasonal climatologies of temperature (Levitus and Boyer 1994) and salinity (Levitus et al. 1994).
The n = 25.0 kg m surface has a specific volume anomaly of 301 (± 2) × 10 m kg and a potential density anomaly of 24.984 (± 0.004) kg m, very close to the second lightest surface of Tsuchiya (1968). The Northern and Southern Hemisphere patterns differ largely. Depth (Fig. 2a) shoals from over 225 m in the northwest part of the region to less than 25 m off South America. A nearly zonal ridge starts at 7°N off the western boundary, reaches 11°N by 125°W, and terminates at its shallowest in the Costa Rica Dome (somewhat obscured in this large-scale climatology) at 9°N, 90°W (Hoffman et al. 1981). This ridge lies below the upwelling-favorable wind stress associated with the ITCZ and separates the NEC and the NECC. A nearly zonal trough starts at 3°N off the western boundary and reaches 5°N by 105°W. East of 150°W, this trough separates the NECC to the north from the SEC to the south. A ridge along the equator marks the dome associated with the upper part of the EUC and separates the northern and southern branches of the SEC. At the southern edge a marked eastward shoaling occurs about 120°W, with isobaths fanning out equatorward and westward, a signature of the SEC. At least some of the shoaling just off South America is owing to upwelling-favorable alongshore winds there.
Figure 2a. Maps of properties on n = 25.0 kg m, within the tropical pycnocline. Color palettes are consistent for Figs. 25 except for acceleration potential, which has a varying offset. The locations of the mean hydrographic profiles (black dots) are given by the average positions of the CTD stations within bins mentioned in section 2. All panels are objectively mapped from values at these locations. (a) Depth, contour intervals of 25 m. (b) Acceleration potential relative to 900 dbar, contour intervals of 0.25 J kg.
Acceleration potential on this surface (Fig. 2b) has a similar pattern to that of depth, but reversed in sign. A trough extends from 6°N off the western boundary to 11°N, 125°W and terminates around 9°N, 90°W. The eastward fall of values along this trough from 165°E to 135°W reveals interior flow southward from the NEC into the NECC. Most of these isopleths can be traced northeast, originating off North America. While the eastern part of this southward flow may be part of a tropical recirculation centered at 11°N, 125°W, this pattern is weaker on lighter neutral surfaces (not shown). A ridge on the equator with values falling between the dateline and 105°W suggests equatorward pycnocline convergence feeding the EUC. In the north, between the dateline and 150°W the NECC supplies this convergence from the western boundary. From 150° to 100°W a ridge at 3°4°N separates the NECC from the SEC. Values falling eastward along this ridge again indicate some southward interior flow from the NECC into the SEC and subsequent convergence on the equator to feed the EUC. These southward flows from the NEC to NECC starting at 165°E extending at most to 135°W and from the NECC to the SEC from 150° to 100°W allow a somewhat convoluted interior route for subtropical water from the northeast Pacific to reach the EUC.
Figure 2b. (c) Salinity, contour intervals of 0.1 (PSS-78) . (d) Planetary potential vorticity, contour intervals of 200 × 10 m s.
The Southern Hemisphere is simpler, lacking an ITCZ except in the west. Values of acceleration potential (Fig. 2b) generally fall equatorward and eastward to a trough at 3°2°S from the date line to about 100°W. This trough separates the SEC from the EUC. The falling values all along this trough suggest a direct interior route for subtropical water to reach the EUC from the southeast Pacific. However, west of the date line a trough is evident at 6°S in the vicinity of the South Pacific convergence zone, blocking the interior route to the equator. Eastward flow north of this trough may be related to the South Equatorial Countercurrent (SECC). However, this surface is too dense to fully reveal the SECC. In the west the SEC feeds the New Guinea Coastal Undercurrent (Tsuchiya et al. 1989).
Salinity on this surface (Fig. 2c) is relatively high in the north-central Pacific, owing to a shallow salinity maximum of the North Pacific tropical water (NPTW; Tsuchiya et al. 1989) at n = 24.3 kg m. The NPTW is swept westward and slightly southward in the NEC. The southeast Pacific is very salty, owing to the strong South Pacific tropical water (SPTW) salinity maximum at n = 24.6 kg m. The SPTW is swept westward and equatorward in the SEC. In the northeast Pacific, fresher California Current water (CCW; Tsuchiya 1968), with a salinity minimum at n = 24.8 kg m, sweeps southward and westward in the NEC, forming the relative minimum on this surface that separates the NPTW and SPTW. In addition, the denser salinity minimum at n = 26.7 kg m, a signature of the North Pacific Intermediate Water (NPIW), may exert some influence, even at this shallow surface, through diapycnal fluxes into the strong shallow pycnocline under the ITCZ. Finally, freshness from excess rainfall under the ITCZ may also mix down to this surface. The equatorial convergence (Fig. 2b) of these three fresh northern sources and the salty SPTW creates a strong meridional salinity gradient at the equator. Slight northward penetration of SPTW across the equator is evident near the western boundary. In addition, the meridional salinity maximum at 1°S between 135° and 100°W is owing to SPTW advected eastward in the EUC.
The planetary component of potential vorticity on this surface (Fig. 2d) also exhibits contrasting patterns in the Northern and Southern Hemispheres. At the northern edge low potential vorticity, especially from 160°W to 125°W, is the remnant of eastern STMW (discussed below) formed to the north. Under the ITCZ values peak at 10°N, 125°W and again at 11°N, 95°W. This peak under the ITCZ is where the pycnocline is drawn closest to the surface by Ekman pumping associated with the large-scale wind stress curl and strengthened, perhaps by some combination of mixing and surface buoyancy flux. The pattern there is striking because acceleration potential contours (Fig. 2b) parallel isovorts (Fig. 2d), showing how the southward flow from the NEC to the NECC does not cross the high potential vorticity under the ITCZ, but moves around it. In the Southern Hemisphere, where the sign of potential vorticity is reversed, the only high magnitudes are off South America, where the pycnocline shoals and strengthens. Elsewhere, potential vorticity is low and relatively uniform. This pattern is associated with eastern STMW that has a vertical minimum around n = 25.3 kg m, visible in vertical sections. This minimum is strongest at the southern boundary around 120°W where the surface shoals rapidly from west to east (Fig. 2a). The uniformity of potential vorticity in the Southern Hemisphere shows there is no impediment to direct interior convergence of water from the subtropics to the equator in the SEC. Finally, isovorts around the equator are spread in the west and pinched in the east, a pattern consistent with acceleration potential and salinity distributions that suggest equatorward interior pycnocline convergence feeding the EUC.
The n = 26.5 kg m surface has a specific volume anomaly of 165 (±2) × 10 m kg and a potential density anomaly of 26.438 (±0.006) kg m, close to the densest surface described by Tsuchiya (1968). The fields for this surface are markedly different from those within the pycnocline. Poleward and westward the depth of this surface (Fig. 3a) exceeds 500 m in the NEC and 425 m in the SEC. A ridge under the ITCZ starts at 6°N off the western boundary, peaks at less than 150 m at 10°N, 135°W, and extends eastward to 7°N, 100°W. In the Southern Hemisphere, a similar ridge starts at 3°S off the western boundary and turns at 150°W to reach 15°S off South America. It is shallowest near 9°S, 95°W. An equatorial trough, from bowing at the base of the EUC, deepens toward both boundaries.
Figure 3a. Maps of properties on n = 26.5 kg m, below the tropical pycnocline. Details follow Fig. 2. (a) Depth, contour intervals of 25 m. (b) Acceleration potential relative to 900 dbar, contour intervals varying from 0.1 to 0.25 J kg.
Acceleration potential (Fig. 3b) again has some inverse correlation with depth. A nearly zonal trough starts at 5°N off the western boundary and reaches 6°N by 100°W. This trough delineates a cyclonic gyre with a narrow, rapid equatorward limb (the NSCC) and a wider, slower poleward limb starting at 20°N, 120°W and reaching 10°N by the western boundary (the deep NEC). These limbs are clearly connected by southward flow in the west and northward flow in the east. In the south, an analogous, but more tenuous gyre defined by the 11.5 J kg contour starts at 5°S in the west and reaches 15°S just off South America. The SSCC forms the equatorward limb and the deep SEC the poleward limb. The poleward flow connecting these currents in the east probably occurs so close to the continent that it is not resolved by the climatology, although diapycnal mixing may also play a role in the mass balance near the eastern boundary. Acceleration potential does not reveal flow at very low latitudes, but direct velocity measurements there suggest some westward return flow equatorward of the SCCs, implying deep anticyclonic gyres (Rowe et al. 2000). Notably, all these gyres effectively isolate the equator from deep interior subtropical ventilation.
Figure 3b. (c) Salinity, contour intervals of 0.1 (PSS-78). (d) Planetary potential vorticity, contour intervals of 25 × 10 m s.
This surface is fresh at the northern edge, owing to the combined influence of lighter CCW to the east and denser NPIW to the west (Fig. 3c). In the southeast the fresh influence of Antarctic Intermediate Water (AAIW), a vertical salinity minimum at n = 27.2 kg m, is evident. Higher Coral Sea and equatorial values encompassed by the 34.8 isohaline are owing to the mixing of salt from above. The meridional maximum starting at 4°S off the western boundary and reaching 7°S off South America is consistent with eastward advection in the SSCC. The salinity front at 2°N off the western boundary, reaching 4.5°N by 105°W (Fig. 3c), suggests that there is little meridional flow across the NSCC (Gouriou and Toole 1993). Similarly, another front starting at 10°N off the western boundary and reaching 20°N by 110°W suggests that meridional flow is also small at the northern limb of the deep gyre. The relatively homogenous region between these fronts, with slightly fresher subtropical waters advected eastward in the equatorward limb of the gyre and slightly saltier equatorial waters advected westward in the northern limb, is consistent with a tracer swirling around the gyre edge and homogenizing in the gyre interior.
Potential vorticity on this surface (Fig. 3d) increases nearly monotonically in magnitude poleward. However, lower values in the east are associated with the thickening equatorial pycnostad. In the Southern Hemisphere, a strong meridional front separating the equatorial pycnostad from the subtropical waters is evident, starting at 3°S off the western boundary and reaching 15°S off South America. This front runs along the SSCC, suggesting an absence of meridional exchange across this current. In the Northern Hemisphere, a similar front starts at 2°N off the western boundary and extends to about 5°N, 105°W. As in the salinity field, there is another front associated with the northern limb of this deep cyclonic gyre starting at 20°N, 110°W and reaching 10°N off the western boundary. Also paralleling the salinity field, between these fronts there is a relatively homogenous region where high subtropical values are advected eastward in the NSCC and low tropical values are swept northward in the eastern Pacific and then westward in the deep NEC. The low values under the ITCZ and off the South American coast suggest that upwelling-driven vortex stretching below the pycnocline is associated with the poleward transport in the deep cyclonic gyres.
In summary, property maps on two neutral surfaces highlight differences between the two hemispheres within the tropical pycnocline as well as changes at the pycnocline base. The pycnocline shoals from west to east, and under the ITCZ. In the north, equatorward flow of relatively fresh pycnocline water follows a serpentine route to skirt westward of the eastern potential vorticity maximum under the ITCZ. In the south, relatively salty pycnocline water follows a much more direct interior route to the equator through a region of homogenous and low potential vorticity characteristic of strong eastern STMW there. Below the pycnocline, deep cyclonic gyres are evident in each hemisphere, precluding any interior equatorward flow there. Acceleration potential, salinity, and potential vorticity distributions are consistent with advection around these deep gyres, and homogenization within them, especially in the northern hemisphere.
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