Climate‐Driven Changes in the Ocean's Ventilation Pathways and Time Scales Diagnosed From Transport Matrices

Abstract

AbstractWe diagnose changes in the decadal‐mean ocean transport over the 21st century as simulated by the ACCESS‐1.3 model under RCP8.5 forcing. A matrix formulation of advection‐diffusion is used to quantify transport into the ocean interior and from the interior back to the surface. We compute the ocean volumes last ventilated at the surface, as well as the volumes next reexposed to the atmosphere, per unit area, and the distributions of water in transit between key ventilation regions. We find that for the 2090s circulation, Antarctic Bottom Water formation and Antarctic ventilation have collapsed but are compensated by increased mode water formation and ventilation at the poleward edge of the southern subtropical gyres. Reexposure of deep water back to the atmosphere in the Antarctic Southern Ocean is also reduced but compensated by enhanced reexposure in the subpolar Southern Ocean. Labrador Sea ventilation is suppressed, while there is increased ventilation in the eastern subpolar Atlantic. The 2090s surface‐to‐surface interbasin transport is much more sluggish than that of the 1990s, with 30–40% reduced volume flow rates. For transport from last ventilation in the subpolar North Atlantic to next ventilation in the tropical Eastern Pacific, the 2090s residence‐time distribution has a ∼40% longer mode with much more slowly decaying tails and correspondingly lagged path densities. The 2090s mean times since last ventilation, as well as the mean times to reexposure are older in the Antarctic Southern Ocean and elsewhere progressively with depth, by up to ∼1,000 years in the abyss.