Fig. 2: Pacific meridional overturning stream function, ideal mean age, and mean re-exposure time.

Stream function of the Pacific meridional overturning circulation (MOC) computed from the meridional flow below a given neutral density and remapped to a pseudo depth³² for the Pacific (a) and its half-range Δ (b) over the three states considered. Pacific zonal averages of the mean time since last surface contact (ideal mean age, Γ^↓) (c) and the mean time to next surface contact (mean re-exposure time, Γ^↑) (e), and their half-ranges ΔΓ^↓ (d) and ΔΓ^↑ (f). The heavy black lines delineate the Pacific Shadow Zone (PSZ) and the Pacific Abyssal Zone (PAZ) beneath. Because the North Pacific between 1500 and 3000 m depth is characterized by both weak overturning and the oldest ideal mean ages, we define both the PSZ and PAZ to lie in the North Pacific bounded to the south by the equator. The magenta lines indicate zonal mean neutral density surfaces (contour labels in units of kg m⁻³).