Irminger Sea deep convection injects oxygen and anthropogenic carbon to the ocean interior

Abstract

Deep convection in the subpolar North Atlantic ventilates the ocean for atmospheric gases through the formation of deep water masses. Variability in the intensity of deep convection is believed to have caused large variations in North Atlantic anthropogenic carbon storage over the past decades, but observations of the properties during active convection are missing. Here we document the origin, extent and chemical properties of the deepest winter mixed layers directly observed in the Irminger Sea. As a result of the deep convection in winter 2014–2015, driven by large oceanic heat loss, mid-depth oxygen concentrations were replenished and anthropogenic carbon storage rates almost tripled compared with Irminger Sea hydrographic section data in 1997 and 2003. Our observations provide unequivocal evidence that ocean ventilation and anthropogenic carbon uptake take place in the Irminger Sea and that their efficiency can be directly linked to atmospheric forcing.

Inorganic carbon and water masses in the Irminger Sea since 1991

“…South of the Denmark Strait, they largely recirculate to the south. In the centre of the cyclonic circulation of the Irminger Gyre, preconditioning for convection is fulfilled (Marshall and Schott, 1999;Bacon et al, 2003;de Jong et al, 2012) and depending on heat loss, deep convection can occur (Pickart et al, 2003a;Våge et al, 2008Våge et al, , 2011Fröb et al, 2016;de Jong and de Steur, 2016). The extent and strength of convective processes are mainly driven by the state of the NAO, which is the leading mode of atmospheric variability over the mid-North Atlantic (Curry et al, 1998;Hurrell and Deser, 2009).…”

“…Within the subpolar North Atlantic, high-quality carbon data have been collected almost every second year since the early 1990s , enabling the determination of subdecadal variability. This shows relationships between the anthropogenic CO 2 storage rate and the extent and intensity of ventilation processes primarily driven by the North Atlantic Oscillation (NAO) Wanninkhof et al, 2010;Fröb et al, 2016;Woosley et al, 2016).…”

The <sup>226</sup>Ra-Ba relationship in the North Atlantic during GEOTRACES-GA01

“…(Fig.5). These distributions can be related to the LSW which is formed by winter convection in the Labrador Sea (Fröb et al, 2016;Pickart et al, 2003;Yashayaev and Loder, 2016). When formed, the LSW transports the low 226 Ra activities and Ba concentrations characteristic of surface waters to the deep ocean.…”

Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic

“…The last stage of the transformation of SPMW is the Labrador Sea Water (LSW), which is formed in the Labrador and Irminger seas (e.g. Pickart et al, 2003;de Jong and de Steur, 2016;Fröb et al, 2016;Piron et al, 2017). After its formation, LSW enters the Deep Western Boundary Current (DWBC) (Fig.…”