FY 2000

Interior pycnocline flow from the subtropical to the equatorial Pacific Ocean

Johnson, G.C., and M.J. McPhaden

J. Phys. Oceanogr., 29(12), 3073–3089, doi: 10.1175/1520-0485(1999)029<3073:IPFFTS>2.0.CO;2 (1999)

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 subtropics 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 and estimates an upper bound for its magnitude at 5 (±1) × 10⁹ kg s⁻¹. In contrast, the southern branch of the South Equatorial Current clearly brings pycnocline water estimated at 15 (±1) × 10⁹ kg s⁻¹ 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.