4.1. Mean circulation
The mean density structure and consequent geostrophic circulation are shown in plan view and as meridional sections in Figs. 2 and 6, respectively, and as a schematic in Fig. 5b (see also Fiedler and Talley, 2006).
4.1.1. Transition between the eastern and central Pacific
At the western edge of the region (125°W; Fig. 6, top), the well-known zonal currents of the central Pacific are seen. In the south, the thermocline slopes down into the bowl of the South Pacific subtropical gyre; south of about 10°S, sampling is insufficient to do more than sketch the broad westward and equatorward flow of the SEC that extends to about 25°S (Fig. 5b). The SEC is broken up into many branches and filaments whose structure and timescales remain poorly observed and understood (Morris et al., 1996; Kessler and Gourdeau, 2006). Near the equator, the two main lobes of the SEC at about 3°S and 3°N have mean surface speeds near 50 cm s; it is not known why they extend so deeply into the water column. The sharp SST front occurs along the axis of the northern branch; because the front is advected ±200 km north and south on 20–30-day timescales by tropical instability waves (Baturin and Niiler, 1997; Willett et al., 2006), it appears heavily smoothed in a time average like Fig. 6. Surface flow at the equator is slightly westward in the mean (but reverses in boreal spring, see Section 4.3) above the eastward EUC, centered near 80 m depth at 125°W. The thermocline is quite tight from 14 to 24 °C at ±5° latitude, but spreads at the equator, implying eastward geostrophic flow (the EUC) with its maximum in the center of the spreading, and westward shear (the SEC) above.
The trough at 5°N and ridge at 10°N bound the eastward NECC, and the downward slope north of the ridge gives the westward NEC, which is the southern limb of the North Pacific subtropical gyre. Below the thermocline, the paired eastward currents at about 125–400 m depth near ±4–5° latitude, associated with the deep bowl of isotherms from 10 to 12 °C (Fiedler and Talley, 2006), are the Subsurface Countercurrents (SSCCs or Tsuchiya Jets) that originate in the far western Pacific (Tsuchiya, 1975; Rowe et al., 2000; McCreary et al., 2002).
Ekman surface currents diverge from the equator in both hemispheres above about 30 m depth, balanced by equatorial upwelling (Johnson, 2001). Fig. 4 shows that the surface flow direction is dominated by Ekman transport; almost 90° to the left of the main SEC flow, especially under the strongest trade winds just south of the equator (Fig. 1). The general downward tilt of the thermocline towards the west (Fiedler and Talley, 2006) results in equatorward geostrophic flow (Fig. 2, top) that supplies the upwelling.
All these currents can be seen entering and leaving the region at the western edge of Fig. 2, and this pattern continues westward across the entire basin. Fig. 6 (top) is similar to sections that would be made as far west as about 170°E, except that all the surface currents would be somewhat stronger and the entire structure deepening to the west (Wyrtki and Kilonsky, 1984; Taft and Kessler, 1991; Johnson et al., 2002; Fiedler and Talley, 2006).
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