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

Abstract

Abstract. The ocean takes up 93\u2009% of the excess heat in the climate system and\napproximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean\nventilation and subduction are key processes that regulate the transport of\nwater (and associated properties) from the surface mixed layer, which is in\ncontact with the atmosphere, to the ocean s interior, which is isolated from\nthe atmosphere for a timescale set by the large-scale circulation. Utilising\nnumerical simulations with an ocean–sea-ice model using the NEMO (Nucleus for\nEuropean Modelling of the Ocean) framework, we\nassess where the ocean subducts water and, thus, takes up properties from the\natmosphere; how ocean currents transport and redistribute these properties over time;\nand how, where, and when these properties are ventilated. Here, the strength and patterns\nof the net uptake of water and associated properties are analysed by including\nsimulated seawater vintage dyes that are passive tracers released annually\ninto the ocean surface layers between 1958 and 2017. The dyes distribution is\nshown to capture years of strong and weak convection at deep and mode water\nformation sites in both hemispheres, especially when compared to observations\nin the North Atlantic subpolar gyre. Using this approach, relevant to any\npassive tracer in the ocean, we can evaluate the regional and depth\ndistribution of the tracers, and determine their variability on interannual to\nmultidecadal timescales. We highlight the key role of variations in the subduction\nrate driven by changes in surface atmospheric forcing in setting the different\nsizes of the long-term inventory of the dyes released in different years and\nthe evolution of their distribution. This suggests forecasting potential for\ndetermining how the distribution of passive tracers will evolve, from having\nprior knowledge of mixed-layer properties, with implications for the uptake\nand storage of anthropogenic heat and carbon in the ocean.\n