Abstract
Abstract. Wintertime convection in the North Atlantic Ocean is a key component of the global climate as it produces dense waters at high latitudes that flow equatorward as part of the Atlantic Meridional Overturning Circulation (AMOC). Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of North Atlantic Deep Water. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the AMOC lower limb. Air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation. We analyze these contributions to the transformation in order to better understand the connection between atmospheric forcing and the densification of surface water. More precisely, we study the impact of air–sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that the variance in SPMW transformation is mainly influenced by the variance in density at the ocean surface. This surface density is set by a combination of advection, wind-driven upwelling and surface fluxes. Our study shows that the latter explains ∼ 30 % of the variance in outcrop area as expressed by the surface area between the outcropped SPMW isopycnals. The key role of the surface density in SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–2015 over the Iceland Basin.
Highlights
Introduction and backgroundRecent 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 (Lozier et al, 2019)
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 (Bringedal et al, 2018; Chafik and Rossby, 2019; Petit et al, 2020)
Using 40 years of observations (1980–2019), we show that the production of subpolar mode water (SPMW) is correlated to the surface area of the source water and to buoyancy forcing over the Iceland Basin, at a similar level for each variable (R = 0.66 and R = 0.63, respectively)
Summary
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 (Lozier et al, 2019). The density anomalies associated with these property changes are expected to influence the overturning over the Irminger and Iceland Basin and the amount of dense water transported southward to the subtropical gyre (Jackson et al, 2016; Ortega et al, 2020; Zou et al, 2020) At odds with this expectation, is a recent study (Fu et al, 2020) that showed a relatively stable AMOC state since the 1990s, in spite of large hydrographic property changes over the subpolar gyre during the same time period.
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