On the interplay between downwelling, deep convection and mesoscale eddies in the Labrador Sea

Dynamics of Downwelling in an Eddying Marginal Sea: Contrasting the Eulerian and the Isopycnal Perspective

“…That is, the downward flow required to connect the upper and lower branches of the AMOC has to occur outside the deep convection areas (Marotzke and Scott 1999;Spall and Pickart 2001). Indeed, idealized model studies (e.g., Spall 2003Spall , 2004Spall , 2010Georgiou et al 2019) and realistic model studies (Katsman et al 2018;Waldman et al 2018;Sayol et al 2019) have shown that intense downwelling occurs within the buoyant boundary currents of the subpolar gyre rather than in the ocean interior. Regions with strong eddy activity are regions where this near-boundary sinking peaks (Spall 2003(Spall , 2004(Spall , 2010Georgiou et al 2019;Sayol et al 2019).…”

“…If this downwelling were to occur within the stratified boundary current, as suggested by among others the two-layer model of Straneo (2006), it needs to cross isopycnals. However, the time-mean near-boundary sinking typically occurs well below the local mixed layer depth (Spall 2004;Katsman et al 2018;Georgiou et al 2019) where diapycnal processes are small and water masses are expected to follow isopycnals.…”

“…Using an artificial subduction tracer and Lagrangian particles in an eddy-resolving model study of the North Atlantic, they found that nearly all the dense water that exits the Labrador Sea via the Labrador Current has been formed just upstream of this location, adjacent to the Labrador Current. In particular, this pathway does not pass through the area of intense eddy shedding and sinking along the west coast of Greenland (Spall 2010;Katsman et al 2018;Georgiou et al 2019). Rather, diapycnal fluxes along the west coast of Greenland diagnosed from an isopycnal model study in Xu et al (2018) are indicative of local upwelling, as they are directed from dense to light water masses.…”

Response to Short Comment 1

“…1b), the modeled AMOC is around 8 Sv weaker than at the RAPID array location but presents a more pronounced seasonal cycle (around 10 Sv), with the maximum in August and the minimum in February. Results from two dedicated campaigns in different years covering the OSNAP sections (∼ 50-60 • N) provided a similar range of variability for ψ o , ∼ 10-20 Sv (Holliday et al, 2018). The recent first assessment of the OSNAP observations by Lozier et al (2019) yields a mean estimate of ψ o that is smaller than our mean ψ o at 45 • N (around 6 Sv smaller) but that compares well with our mean modeled results at 55 • N (8.0 ± 0.7 Sv versus 8.1 ± 2.4 Sv; not shown).…”

“…Bi-harmonic viscosity and diffusion ∝ grid size 3 ; at the Equator k 0 = −30 m 4 s −1 Vertical mixing (K profile) 0.1 m 2 s −1 to solve gravitational instabilities Background vertical tracer diffusion From 10 −5 m 2 s −1 (surface) to 10 −4 m 2 s −1 (depth) 2018; Holliday et al, 2018), the scarcity of measurements below the surface still necessitates the use of numerical models to provide more insight into the spatiotemporal variability of sinking and to grasp the physical processes behind its dynamics. To this end, we use an ocean-only eddy-resolving numerical simulation with a nominal resolution of 0.1 • under a repeated climatological annual atmospheric forcing, not adding the additional complexity of historical variations (e.g., variations at interannual or inter-decadal scales) in surface forcing.…”