Submesoscale Vertical Velocities Enhance Tracer Subduction in an Idealized Antarctic Circumpolar Current

Abstract

Upper-ocean submesoscale fronts, with their associated strong vertical velocities, are often claimed to play a significant role in subducting tracers into the interior. The role of these submesoscale processes in restratifying the mixed layer is now well recognized, but whether they simultaneously flux tracers through the base of the boundary layer remains an open question. We vary the resolution in a semirealistic channel model to control turbulent processes at various scales and study their influence on tracers. It is found that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations: The 1-km simulation takes up 50% more tracer compared to the 20-km simulation, despite the increased restratifying influence of the resolved submesoscale processes. A full frequency-wave number cross-spectra of the vertical velocity and vertical tracer flux show that the high-frequency inertia-gravity waves that appear in the highest-resolution simulation play no role in irreversible downward tracer transport. Plain Language SummaryThe oceanic uptake and storage of anthropogenic carbon plays a central role in the global carbon budget, and a significant fraction of this uptake occurs in the Southern Ocean. It is well established that the eddies and fronts, turbulent fluctuations of the flow, are important for this uptake, but the relative influence of different scales is less understood. The eddies and fronts associated with the submesoscale (1-50 km) have been recognized to play a leading role in shallowing the surface boundary layer depths, but the role of the strong vertical velocities, which are generally associated with these features, in transporting fluid below the mixed layer remains unknown. Here we investigate these questions using an idealized Southern Ocean model with an imposed tracer source at the surface. The model resolution is varied as a means to include or omit turbulent processes at various scales. We find that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations, despite the reduction in the depths of the vigorously mixed boundary layers. We also found that inertia-gravity waves, which are ubiquitous in the ocean and are generally associated with very strong vertical velocities, had no impact on the net tracer flux.

Vertical eddy iron fluxes support primary production in the open Southern Ocean

“…In general, however, the mixed layer often used in observational studies can be deeper than the mixing layer as the former is defined purely by thermal dynamical properties 49 while as latter is defined by kinematic properties. We argue that the mixing layer is the relevant depth scale for tracer transport as it is the layer over which diapycnal mixing is active 15 . Figure 3c shows that diffusive fluxes are only active within the mixing layer in our simulation when eddies are explicitly resolved.…”

“…4). Although 2 km resolution is state-of-theart for a basin-scale simulation coupled to a full biogeochemical model, it is not sufficient to fully resolve submesoscale processes including MLI 15 . Based on the resolution dependence, we would expect the role of eddies in supplying iron to increase further with higher resolutions 42 , but this would only strengthen the central finding that eddy iron transport modulates primary productivity in the open Southern Ocean.…”

“…In addition to vertical diapycnal mixing and large-scale circulation, mesoscale eddies (on scales of roughly 20-200 km and to first-order geostrophically balanced) can make a major contribution to tracer transport 12,13 . In the Southern Ocean, upward vertical mesoscale eddy heat fluxes counteract the downward flux of heat due to Ekman pumping 14 , and mesoscale eddies help regulate the subduction of anthropogenic carbon from the surface into the interior 11,15 . At even smaller scales where the geostrophic approximation breaks down, submesoscale turbulence (roughly 1-20 km and associated with Rossby and Richardson numbers on the order of unity) generates vigorous vertical velocities near the surface 16,17 .…”

The Contribution of Submesoscale over Mesoscale Eddy Iron Transport in the Open Southern Ocean

“…The model setup is similar to Balwada et al (), but without a topographic ridge, using the hydrostatic configuration of the Massachusetts Institute of Technology general circulation model (MITgcm; Marshall et al, ). The channel domain ( L x =1,000km× L y =2,000km× H =2,985m) is flat bottom and zonally re‐entrant on a β ‐plane centered around 49S ( f 0 =−1.1×10 −4 s −1 , β =1.4×10 −11 m −1 s −1 ) in Cartesian coordinates.…”

Modulation of Lateral Transport by Submesoscale Flows and Inertia‐Gravity Waves

“…Lateral mixing in the upper ocean plays an important role in Earth's climate system. For instance, near-surface lateral mixing processes help determine the ocean's rate of uptake of tracers such as heat and anthropogenic CO 2 (Abernathey & Ferreira, 2015;Balwada et al, 2018;Gnanadesikan et al, 2017; The prevailing assumption is that mesoscale ocean flows, with horizontal scales of ∼100 km and evolutionary times of months, are the dominant contributor to lateral stirring on scales relevant for the large-scale circulation and transport. Phenomenologically, the mesoscale is characterized by distinctive coherent structures such as vortices (i.e., eddies), fronts, and filaments.…”