Boundary Current and Mixing Processes in the High Latitude Oceans

<p>A substantial fraction of the deep ocean is ventilated in the high latitude North Atlantic. As a result, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a set of comprehensive Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined &#8216;North Atlantic Deep Water&#8217; (NADW) reveal the times and locations of subduction from the surface mixed layer at high temporal and spatial resolution. The major fraction (&#8764;60%) of NADW ventilation results from subduction directly into the Labrador Sea boundary current, with a smaller fraction (&#8764;25%) arising from open ocean deep convection in the Labrador Sea. There is a notable absence of ventilation arising from subduction in the Greenland&#8211;Iceland&#8211;Norwegian Seas, due to the re-entrainment of those waters as they move southward. Temporal variability in ventilation arises both from changes in subduction &#8212; driven by large-scale atmospheric forcing &#8212; and from year-to-year changes in the subsurface retention of newly subducted water, the result of an inter-annual equivalent of Stommel&#8217;s mixed layer demon. This interannual demon operates most effectively in the open ocean where newly subducted water is slow to escape its region of subduction. Thus, while subduction in the boundary current dominates NADW ventilation, processes in the open ocean set the variability, mediating the translation of inter-annual variations in atmospheric forcing to the ocean interior.</p>

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Locations and Mechanisms of Ocean Ventilation in the High-Latitude North Atlantic in an Eddy-Permitting Ocean Model

Journal of Climate ◽

10.1175/jcli-d-20-0191.1 ◽

2020 ◽

Vol 33 (23) ◽

pp. 10113-10131

Author(s):

Graeme A. MacGilchrist ◽

Helen L. Johnson ◽

David P. Marshall ◽

Camille Lique ◽

Matthew Thomas ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

High Latitude ◽

Circulation Model ◽

Deep Ocean ◽

Ocean Model ◽

Surface Mixed Layer ◽

Boundary Current ◽

Important Distinction ◽

Ocean Ventilation

AbstractA substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high-latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backward-in-time trajectories from a model-defined North Atlantic Deep Water (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major fraction of NADW ventilation results from subduction in the Labrador Sea, predominantly within the boundary current (~60% of ventilated NADW volume) and a smaller fraction arising from open ocean deep convection (~25%). Subsurface transformations—due in part to the model’s parameterization of bottom-intensified mixing—facilitate NADW ventilation, such that water subducted in the boundary current ventilates all of NADW, not just the lighter density classes. There is a notable absence of ventilation arising from subduction in the Greenland–Iceland–Norwegian Seas, due to the re-entrainment of those waters as they move southward. Taken together, our results emphasize an important distinction between ventilation and dense water formation in terms of the location where each takes place, and their concurrent sensitivities. These features of NADW ventilation are explored to understand how the representation of high-latitude processes impacts properties of the deep ocean in a state-of-the-science numerical simulation.

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