Interior circulation pathways from the subtropics to the equator are markedly
different in the Northern and Southern Hemispheres of the Pacific Ocean. In
the North Pacific the pycnocline shoals and strengthens dramatically under the
intertropical convergence zone, separating the North Equatorial Current from
the North Equatorial Countercurrent. While the high potential vorticity between
these currents would intuitively seem to inhibit meridional water-property exchange
between the subtopics and the equator, transient tracer analyses and some modeling
studies have suggested an interior pathway from the subtropics to the equator
in the pycnocline of the central North Pacific. This study delineates this pathway S
and estimates an upper bound for its magnitude at 5 (± 1) × 10
kg s1. In contrast, the southern branch of the South Equatorial
Current clearly brings pycnocline water estimated at 15 (± 1) × 10
kg s1 from the southern subtropics directly to the equator
in the South Pacific through an interior region of low and relatively uniform
potential vorticity. In both hemispheres, these interior pathways extend downward
as far as the lightest waters of the equatorial pycnostad. The subsurface countercurrents
flanking the pycnostad form the equatorward limbs of tropical subsurface cyclonic
gyres. These deep gyres are consistent with the absence of interior ventilation
of the equator from the subtropics below the pycnocline. Measured and derived
fields from a hydrographic climatology are presented on neutral surfaces and
in meridional-vertical sections to show that salinity, potential vorticity,
and acceleration potential are all consonant with these arguments. The vertical
extent of interior communication is also in agreement with the transient tracer
results.