Seasonal and regional variations of sinking in the subpolar North Atlantic from a high-resolution ocean model

Abstract

Abstract. Previous studies have indicated that most of the net sinking associated with the downward branch of the Atlantic Meridional Overturning Circulation (AMOC) must occur near the subpolar North Atlantic boundaries. In this work we have used monthly mean fields of a high-resolution ocean model (0.1 deg at the equator) to quantify this sinking. To this end we have calculated the Eulerian net vertical transport (WΣ) from the modelled vertical velocities, its seasonal variability and its spatial distribution under repeated climatological atmospheric forcing conditions. Based on this simulation, we find that for the whole subpolar North Atlantic WΣ peaks at about −14 Sv at a depth of 1139 m, matching both the mean depth and the magnitude of the meridional transport of the AMOC at 45° N. It displays a seasonal variability of around 10 Sv. Three sinking regimes are identified according to the characteristics of the accumulated W with respect to the distance to the coast: one within the first 110 km and onto the bathymetric slope at around the peak of the boundary current speed (regime I), the second between 110 km and 290 km covering the remainder of the shelf where mesoscale eddies exchange properties (momentum, heat, mass) between the interior and the boundary (regime II), and the third sinking regime at larger distances from the coast where WΣ is mostly driven by the ocean's interior eddies (regime III). Regimes I and II accumulate ∼ 90 % of the total sinking and display smaller seasonal changes and spatial variability than regime III. We find that such a distinction in regimes is also useful to describe the characteristics of WΣ in marginal seas located far from the overflow areas, although the regime boundaries can shift a few tens of km inshore or offshore depending on the bathymetric slope and shelf width of each marginal sea. The largest contributions to the sinking come from the Labrador Sea, the Newfoundland region and the overflow regions. The magnitude, the seasonal variability and the depth at which WΣ peaks vary for each region, thus revealing a complex picture of sinking in the subpolar North Atlantic.

Atlantic Deep Water Formation Occurs Primarily in the Iceland Basin and Irminger Sea by Local Buoyancy Forcing

“…The maximum volume of water exported out of the lower layer by the horizontal flow field lags the peak of densification by 5 months. The length of this delay—between the formation of dense water and its export to the boundary current in a marginal sea, such as the Irminger Sea—is consistent with modeling studies that attribute the exchange between the convective region and the boundary current to an eddy transport mechanism (Brüggemann & Katsman, 2019; Sayol et al, 2019). The delay depends upon the location of the convection and ranges from 3 months if convection takes place along the Irminger Current to more than 12 months if it is in the interior of the Irminger Sea (Le Bras et al, 2020).…”

Observation‐Based Estimates of Eulerian‐Mean Boundary Downwelling in the Western Subpolar North Atlantic

“…(2018) and 1.13 Sv from Sayol et al. (2019) within uncertainty estimates. Summer downwelling in the Labrador Sea is estimated to be 0.85 ± 0.15 Sv at depth 685 m by integrating the transport between the two ends of the AR7W line (Figure 2), which agrees quantitatively with Pickart and Spall (2007) observation‐based meridional transports of about 1 Sv at depth 800 m. The annual mean downwelling rate in the Labrador Sea is estimated here as 0.72 ± 0.07 Sv, similar to Holte and Straneo's (2017) and Lozier et al.…”

“…Furthermore, Sayol et al. (2019) used a high‐resolution model to find that seasonal variability in boundary sinking is rather insignificant in comparison to interior sinking, and that the variability is thought to be driven by ageostrophic dynamics that our large‐scale geostrophic balance cannot capture. Nevertheless, based on our observations, a potential upper bound on the seasonal cycle amplitude has been established.…”

Importance of Boundary Processes for Heat Uptake in the Subpolar North Atlantic

“…The strongest positive anomalies are found along the cyclonic path of the boundary current—from Rockall Trough down to Flemish Cap—which is therefore likely to represent an important gateway for surface decadal‐scale temperature signals to enter the intermediate layer. In fact, the confined boundary regions of both the Newfoundland area and the Irminger and Labrador seas are known to host the most significant vertical mass exchanges within the SPG domain, as demonstrated by theory and recently illustrated with a very high‐resolution simulation (Sayol et al, 2019). Moreover, basin‐scale observations of fine scale velocity and density variance show that the boundaries of the Labrador and Flemish Cap areas also host the most intense vertical mixing within the whole SPG region (Walter et al, 2005).…”