Surface atmospheric forcing as the driver of long-term pathways and timescales of ocean ventilation

Abstract. The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air-sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water (and associated properties) from the surface mixed layer, which is in contact with the atmosphere, to the ocean's interior which is isolated from the atmosphere for a timescale set by the large-scale circulation. Using numerical simulations with an ocean-sea-ice model using the NEMO framework, we assess where the ocean subducts water and thus takes up properties from the atmosphere and how ocean currents transport and redistribute them over time and how, where and when they are ventilated. Here, the strength and patterns of the net uptake of water and associated properties are analysed by including simulated sea water vintage dyes that are passive tracers released annually into the ocean surface layers between 1958 and 2017. The dyes' distribution is shown to capture years of strong and weak convection at deep and mode water formation sites in both hemispheres, especially when compared to observations in the North Atlantic subpolar gyre. Using this approach, relevant to any passive tracer in the ocean, we can evaluate the regional and depth distribution of the tracers, and determine their variability on interannual to multidecadal timescales. We highlight the key role of variations in subduction rate driven by changes in surface atmospheric forcing in setting the different sizes of the long-term inventory of the dyes released in different years and the evolution of their distribution. This suggests forecasting potential for determining how the distribution of passive tracers will evolve, from having prior knowledge of the surface air-sea fluxes, with implications for the uptake and storage of anthropogenic heat and carbon in the ocean.