Graeme A. MacGilchrist (University of Oxford)
Lagrangian perspectives of ocean ventilation in the high-latitude North Athlantic.
The ventilation of intermediate and deep water masses in the high-latitude North Atlantic plays a crucial role in the oceanic uptake of anthropogenic heat and carbon dioxide. Despite many years of observations, establishing the connection between variability in local dense water formation and a downstream ventilation signature remains challenging. This is due, in part, to the varied and diffusive pathways followed by newly-ventilated water in the deep ocean. Here, we elucidate the mechanisms and variability of ventilation in the high-latitude, North Atlantic Ocean of an eddy-permitting ocean circulation model. We adopt an approach that directly links water in the deep North Atlantic Ocean to the time and location that it left the mixed layer, via backwards-in-time, non-diffused Lagrangian trajectories. We show that, in the model, ventilation occurs almost exclusively in the Labrador Sea, with the majority occurring directly within the boundary current. We find that subduction in the boundary current results from the shallowing of the late-winter mixed layer depth along the path of the barotropic streamfunction. On multi-annual (3-5 years) timescales, temporal variability in surface forcing is reflected in the age distribution of the subsurface. Shorter timescales in the subsurface age distribution are established by inter-annual variability in re-entrainment. Finally, there is evidence that trajectories leaving the mixed layer during years of anomalously high ventilation are exported southward at a greater rate, on average, than those from low ventilation years.