Dynamics of Downwelling in an Eddying Marginal Sea: Contrasting the Eulerian and the Isopycnal Perspective

Abstract

In this study, we explore the downward branch of the Atlantic meridional overturning circulation (AMOC) from a perspective in depth space (Eulerian downwelling) as well as from a perspective in density space (diapycnal downwelling). Using an idealized model, we focus on the role of eddying marginal seas, where dense water is formed by deep convection due to an intense surface heat loss. We assess where diapycnal mass fluxes take place, investigate the pathways of dense water masses, and elucidate the role of eddies. We find that there are fundamental differences between the Eulerian and diapycnal downwelling: the strong Eulerian near-boundary downwelling is not associated with substantial diapycnal downwelling; the latter takes place in the interior and elsewhere in the boundary current. We show that the diapycnal downwelling appears to be more appropriate to describe the pathways of water masses. In our model, dense water masses are exported along two routes: those formed in the upper part of the boundary current are exported directly; those formed in the interior move toward the boundary along isopycnals due to eddy stirring and are then exported. This study thus reveals a complex three-dimensional view of the overturning in a marginal sea, with possible implications for our understanding of the AMOC.

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).…”

Direct and Indirect Pathways of Convected Water Masses and Their impacts on the Overturning Dynamics of the Labrador Sea

“…Our investigation of the residence time of the major pathways (i.e. withinBC and interiorShort particles) shows that lighter water masses formed close to or within the boundary current are exported faster than the denser water masses formed in the central‐LS (Figure 6), in line with the more qualitative results obtained from the idealized studies of Brüggemann and Katsman (2019) and Georgiou et al. (2020): 90% of the volume transport carried by the withinBC and interiorShort particles reaches 53°N within 1.5 and 3 years, respectively (Figure 6c).…”

Rapid Export of Waters Formed by Convection Near the Irminger Sea's Western Boundary

“…Eddy dynamics feature prominently in the Southern Ocean overturning literature (e.g., Marshall & Radko, ; Thompson et al, ). However, the role of eddies in Atlantic overturning is rarely discussed outside of modeling studies (e.g., Brüggemann & Katsman, ; Spall, ; Straneo, ). We presented observational evidence in support of the idea that eddies mediate the export of waters formed by convection in the subpolar North Atlantic, and that eddy dynamics facilitate the export of waters formed near boundary currents in particular.…”

“…In order to compensate, eddy fluxes from the interior must cool the boundary current. The net result is that the boundary current becomes denser, which leads to overturning in density space (Brüggemann & Katsman, ; Spall, ; Straneo, ). Observations confirm that high‐latitude boundary currents become denser (colder) as they circulate around marginal seas (Holte & Straneo, ; Mauritzen, ; Pickart & Spall, ; Våge et al, ), but the processes which contribute to this density change have yet to be observed in detail.…”