FIGURE 1. Mean circulation and potential vorticity averaged over 50 years (1950–99) in the upper pycnocline of the Pacific Ocean.

a, Geostrophic streamlines relative to 900 dbar (in m^-2 s^-2) and b, absolute value of potential vorticity (in 10^-10 m^-1 s^-1) on the 25.0 kg m^-3 potential density surface. Velocity vectors are overplotted on a. Distribution of hydrocasts down to 900 dbar is overplotted on b, with the size of the dots representing total number of casts in regions of 5° latitude by 15° longitude (smallest, 50–300; intermediate, 301–1,000; largest, 1,001 or more). Winter season outcrop lines are drawn in both panels. The outcrop lines define locations where wintertime surface mixing penetrates deepest into the pycnocline, creating new water masses that are subsequently sequestered from the atmosphere as seasonal heating restratifies the upper ocean in spring and summer. Potential vorticity is defined as f(/z)/₀, where f is the Coriolis parameter, /z is the vertical density gradient, and ₀ is a constant reference density (10³ kg m^-3). Water parcels conserve their potential vorticity in an ideal fluid.