Fig. 4: Origin and destination partitioned flow rates and interior zone volumes.

Surface origin and destination partition (a) of the surface-to-interior-zone (b) and interior-zone-to-surface (c) volume transport rates (arrows, with length to scale) in units of Sv. Parts (b) and (c) show an idealized exploded view of the Pacific Shadow Zone (PSZ) and the abutting Pacific Abyssal Zone (PAZ) beneath in latitude-depth space. The pie charts show the partition of the zone volume according to surface origin (b) and surface destination (c). The layout of the numerical values of the volume transport rates in units of Sv is given in (d). Arrows and pie charts are color coded as in (a) according to surface region: Antarctic marginal seas (AMS, dark blue), subpolar Southern Ocean (SP SO, light blue), low and mid latitudes (Low&Mid Lat, amber), the northern Pacific (NPac, dark green), and the northern Atlantic (NAtl, light green). The surface-to-interior transport rates are plotted at the face of the interior zones (PSZ or PAZ) with which surface-origin-labelled water makes first contact, and the interior-to-surface transport rates are plotted at the face where surface-destination-labelled water had last contact with the interior zones. Colored (non-red) numbers give the magnitude (Sv) of the correspondingly colored arrows that are ordered by region; there is no significance to their position on a given zone face. For a given zone face, the black transport rates are the sum of the (non-red) colored rates. The red arrows represent the volume flow rates through the faces of the interior zones; red numbers indicate the corresponding flow rate (Sv). To emphasize the deep coherent overturning traversing the PAZ (cf. Fig. 2), the flow rate across the vertical face of the PAZ is partitioned into the inflow below and the outflow above neutral density γ = 28.12 kg m⁻³. [Note that the transports from the surface to first zone contact, or to the surface since last zone contact, are physically distinct from the local stream flows across the zone faces unconditioned on transport pathway (see Methods); consequently, in the presence of eddy diffusion there is no simple correspondence between the transport rates from and to the surface (black numbers) and the local stream flows (red numbers)].