The path‐density distribution of oceanic surface‐to‐surface transport

Abstract

A novel diagnostic for advective‐diffusive surface‐to‐surface paths is developed and applied to a global ocean model. The diagnostic provides, for the first time, a rigorous quantitative assessment of the great ocean conveyor's deep branch. A new picture emerges of a diffusive conveyor in which the deep North Pacific is a holding pen of long‐residence‐time water. Our diagnostic is the joint density, η, per unit volume and interior residence time, τ, of paths connecting two specified surface patches. The spatially integrated η determines the residence‐time partitioned flux and volume of water in transit from entry to exit patch. We focus on interbasin paths from high‐latitude water mass formation regions to key regions of re‐exposure to the atmosphere. For non‐overlapping patches, a characteristic timescale is provided by the residence time, τϕ, for which the associated flux distribution, ϕ, has its maximum. Paths that are fast compared to τϕ are organized by the major current systems, while paths that are slow compared to τϕ are dominated by eddy diffusion. Because ϕ has substantial weight in its tail for τ > τϕ, the fast paths account for only a minority of the formation‐to‐re‐exposure flux. This conclusion is expected to apply to the real ocean based on recent tracer data analyses, which point to long eddy‐diffusive tails in the ocean's transit‐time distributions. The long‐τ asymptotic path density is governed by two time‐invariant patterns. One pattern, which we call the Deep North Pacific pattern, ultimately dominates a secondary redistribution pattern.