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

Abstract

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC.

Role of air-sea fluxes and ocean surface density on the production of deep waters in the eastern subpolar gyre of the North Atlantic

“…Finally, we now understand that the preconditioning of the surface waters in the Iceland Basin is a major contributor of the total waters carried within the lower limb of the AMOC (Petit et al 2020), and thus a key determinant of AMOC variability within the subpolar North Atlantic. Hence, our study highlights the importance of understanding the factors that influence the surface density field in the Iceland Basin-whether by advection from the subtropics or the western subpolar gyre, or by the influence of local winds that bring cold water to the surface via mixing and/or upwelling, or through surface buoyancy loss-since that factor is of prime importance in determining the transformation of SPMW.…”

“…We begin our study of the influence of air-sea fluxes and ocean surface densities on the SPMW transformation in the Iceland Basin by evaluating the independence of these variables. We first examine the influence of the local buoyancy flux on the vertical density profile for each winter over the OSNAP period from 2014 to 2018, a period during which Petit et al (2020) vorticity < 4x10 -11 m -1 s -1 ) at the beginning of winter and its restratification at the onset of spring (Fig. 2a).…”

“…While these two regions produce approximately the same amount of dense waters, the interannual variability of the AMOC in the subpolar North Atlantic can be largely attributed to variability in the dense waters produced in the eastern subpolar region, rather than those imported from the Nordic Seas. A recent study based on observations shows that the production of dense waters in the Irminger and Iceland basins is mainly due to local buoyancy forcing (Petit et al, 2020). Specifically, between the Greenland-Scotland Ridge and the OSNAP East section (Fig.…”

“…Recent observational studies have identified two main source regions for the dense waters that constitute the lower limb of the Atlantic Meridional Overturning Circulation (AMOC): the Nordic Seas, and the Irminger and Iceland basins (Bringedal et al, 2018;Chafik & Rossby, 2019;Lozier et al, 2019;Petit et al, 2020). While these two regions produce approximately the same amount of dense waters, the interannual variability of the AMOC in the subpolar North Atlantic can be largely attributed to variability in the dense waters produced in the eastern subpolar region, rather than those imported from the Nordic Seas.…”

“…Because the transformation of dense waters mainly drives AMOC variability in this region, we seek to understand what factors drive variability in the transformation, which was twice as large in winter 2014-2015 as it was in winter 2015-2016 (Petit et al, 2020). Two variables are key to transformation estimates: the air-sea fluxes and the ocean surface densities.…”