Abstract
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. Utilising numerical simulations with an ocean–sea-ice model using the NEMO (Nucleus for European Modelling of the Ocean) framework, we assess where the ocean subducts water and, thus, takes up properties from the atmosphere; how ocean currents transport and redistribute these properties over time; and how, where, and when these properties are ventilated. Here, the strength and patterns of the net uptake of water and associated properties are analysed by including simulated seawater 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 the 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 mixed-layer properties, with implications for the uptake and storage of anthropogenic heat and carbon in the ocean.
Highlights
The ocean absorbs more than 90 % of anthropogenic warming and is the largest mobile carbon reservoir in the climate system that is accessible on millennial timescales
Waters recently exposed to the atmosphere, which reside in the ocean’s surface mixed layer, pass into the ocean’s interior; this is balanced by entrainment of waters from the ocean interior into the ocean mixed layer (Stommel, 1979)
Numerical simulations are run using a global configuration of the Nucleus for European Modelling of the Ocean (NEMO; Madec, 2014) ocean model, which is coupled to the Los Alamos Sea-Ice Model (CICE; Rae et al, 2015)
Summary
The ocean absorbs more than 90 % of anthropogenic warming and is the largest mobile carbon reservoir in the climate system that is accessible on millennial timescales. Ocean ventilation is the process by which air–sea fluxes of properties such as heat and carbon penetrate into the enormous reservoir that is the ocean interior. The values of mixed-layer properties such as temperature and carbon content depend on the rate of this exchange (Abraham et al, 2013; Banks and Gregory, 2006; Iudicone et al, 2016) as well as the uptake from the atmosphere. The atmospheric uptake of heat and carbon itself depends on ocean ventilation through this impact of exchange on mixed-layer temperature and carbon. Ocean ventilation plays a key role in modulating climate variability on interannual to decadal (and even centennial) timescales
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