Demons in the North Atlantic: Variability of Deep Ocean Ventilation

Translation of atmospheric forcing variability into the ocean interior via ocean ventilation is an important aspect of transient climate change. On a seasonal timescale in the subtropics, this translation is mediated by a so-called Demon that prevents access to all except late-winter mixed-layer water. Here, we use an eddy-permitting numerical circulation model to investigate a similar process operating on longer (interannual) timescales in the subpolar North Atlantic. We find that variations in atmospheric forcing are mediated in their translation to the ocean interior, with year-to-year changes in the late-winter mixed layer depth being the critical factor. The signature of persistent strong atmospheric forcing driving deep mixed layers is preferentially ventilated to the interior when the forcing is ceased. Susceptibility to this effect depends on the location and density of subduction-with the rate at which newly ventilated water escapes its region of subduction being the crucial factor. Plain Language Summary Water that leaves the ocean's surface boundary layer-where water is in direct contact with the overlying atmosphere-to be transported into the subsurface, is said to be ventilated (the name arising from the abundance of oxygen in newly ventilated water). The ventilation process, which carries implications for the ocean storage of climate-relevant substances such as carbon dioxide, occurs only at certain times and under certain conditions. In describing a mechanism for the selective nature of ventilation over the seasonal cycle, Henry Stommel imagined a Demon sitting at the base of the surface boundary layer, granting access only to parcels of water that meet certain characteristics (namely their speed of escape). Thus, Stommel's Demon was born. Here, we investigate this same process as it operates in more northerly regions and on longer timescales. In so doing we give birth to a new interannual Demon, and describe its characteristics. Key Points Temporal variability of high-latitude ocean ventilation is investigated using Lagrangian trajectories in an eddy-permitting ocean model High-latitude ocean ventilation adheres to Stommel's Demon, such that only water subducted in late-winter is retained in the subsurface An interannual Demon also operates to mediate exchange between the atmosphere and subsurface ocean on longer timescales

Automated summary

Variability in atmospheric forcing is mediated by the oceanic ventilation, with changes in late-winter mixed layer depth being a critical factor. Strong atmospheric forcing can lead to deep mixed layers being preferentially ventilated into the ocean interior when the forcing is ceased. The location and density of subduction play a crucial role in determining the susceptibility to this effect.