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∘ at the Equator) to quantify this sinking. To this end we have calculated the Eulerian net vertical transport (W∑) from the modeled 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 W∑ 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 shelf: one within the first 90 km and onto the bathymetric slope at around the peak of the boundary current speed (regime I), the second between 90 and 250 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 at larger distances from the shelf where W∑ 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 W∑ in marginal seas located far from the overflow areas, although the regime boundaries can shift a few tens of kilometers 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, seasonal variability, and depth at which W∑ peaks vary for each region, thus revealing a complex picture of sinking in the subpolar North Atlantic.
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the Earth’s climate system (Lozier, 2012; Buckley and Marshall, 2016)
The remainder of this paper is structured as follows: in Sect. 2 we introduce the numerical simulation and assess the ability of the model to reproduce a realistic AMOC; in Sect. 3 we consider the main characteristics and the seasonal variability of sinking in the entire subpolar North Atlantic; in Sect. 4 we evaluate similarities and differences between sinking in the marginal seas, overflow regions, and midlatitude seas of the subpolar North Atlantic based on their different bathymetric profiles and driving local dynamical processes; and in Sect. 5 we show that in our simulation the connection between sinking variations and AMOC change fades when the dominant seasonal signal is removed
Based on a high-resolution ocean model simulation forced by a prescribed annual cycle of wind, precipitation, and heat fluxes, we have found that the amount of minimum timemean net vertical transport (W ) for the entire subpolar North Atlantic Ocean is consistent with the transport and vertical structure of the AMOC core at midlatitudes (45◦ N), with an average of about −14 Sv at a depth of 1139 m
Summary
The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the Earth’s climate system (Lozier, 2012; Buckley and Marshall, 2016). As pointed out by the recent work of Waldman et al (2018), significant sinking occurs in the first 50 km off the coast in the Mediterranean Sea, though it is much smaller than in the North Atlantic (∼ −1 Sv, where 1 Sv = 106 m3 s−1) At this location, sinking is catalyzed by the existence of a western boundary current that densifies along its way around the basin, a strong winter cooling in the interior due to northerly winds, and an active near-shelf eddy field. In order to better understand the contribution of geostrophic and ageostrophic processes to sinking, Brüggemann and Katsman (2019) used an idealized model with fine resolution (3 km in the horizontal), which is able to mimic the basic features of the Labrador Sea: a cyclonic boundary current circulating along a semicircular basin, with a small part dominated by a steeper topographic slope (change in depth of 3000 m in a few tens of kilometers), resulting in the generation of a vigorous eddy field.
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