Wind-driven transport of fresh shelf water into the upper 30 m of the Labrador Sea

Abstract

Abstract. The Labrador Sea is one of a small number of deep convection sites in the North Atlantic that contribute to the meridional overturning circulation. Buoyancy is lost from surface waters during winter, allowing the formation of dense deep water. During the last few decades, mass loss from the Greenland ice sheet has accelerated, releasing freshwater into the high-latitude North Atlantic. This and the enhanced Arctic freshwater export in recent years have the potential to add buoyancy to surface waters, slowing or suppressing convection in the Labrador Sea. However, the impact of freshwater on convection is dependent on whether or not it can escape the shallow, topographically trapped boundary currents encircling the Labrador Sea. Previous studies have estimated the transport of freshwater into the central Labrador Sea by focusing on the role of eddies. Here, we use a Lagrangian approach by tracking particles in a global, eddy-permitting (1/12∘) ocean model to examine where and when freshwater in the surface 30 m enters the Labrador Sea basin. We find that 60 % of the total freshwater in the top 100 m enters the basin in the top 30 m along the eastern side. The year-to-year variability in freshwater transport from the shelves to the central Labrador Sea, as found by the model trajectories in the top 30 m, is dominated by wind-driven Ekman transport rather than eddies transporting freshwater into the basin along the northeast.

Controls on the Transport of Meltwater From the Southern Greenland Ice Sheet in the Labrador Sea

“…EGMR, on the other hand, can be transported to offshore of the 3,000‐m isobath where it can potentially influence vertical stratification and winter convection when winds are upwelling favorable during the melting season, especially if meltwater input is large. This contrast in behavior is consistent with recent modeling results that showed enhanced wind‐driven cross‐isobath displacement of surface drifters off West Greenland to the south of 61 °N, decreasing to the north (Schulze Chretien & Frajka‐Williams, ). While enhanced cross‐isobath transport to the south of 61 °N can result in offshore export of EGMR toward the interior of the basin (Figure f), WGMR would not be significantly impacted since most of the input occurs farther north in regions of weaker cross‐isobath transport (Figure a).…”

“…That was the case in 2012 between June and early November, but winds in 2008 were anomalously upwelling favorable from July to late October, which could help explain the differences in offshore transport between these years (Luo et al, ). Indeed, Schulze Chretien and Frajka‐Williams () used results from a 20‐year long model simulation to show that the anomaly in the probability of cross‐isobath displacements of modeled drifters in the top 30 m in the Labrador Sea is correlated with wind‐driven Ekman transport but not with eddy kinetic energy.…”

“…Here we use a high‐resolution ocean model to identify the contributions of different transport mechanisms on the offshore export of meltwater off West Greenland. Building on Luo et al () and Schulze Chretien and Frajka‐Williams (), we investigate which conditions favor the transport of meltwater from West and East Greenland toward the interior of the Labrador Sea. We also identify which mechanisms are necessary for effective offshore transport of meltwater, and how their impact is different on meltwater originating along the West and East Greenland coasts.…”

Seasonality of Freshwater in the East Greenland Current System From 2014 to 2016

“…This is because oceanic flows conserve Potential Vorticity and the bathymetric gradient of the continental slope gives rise to a large Potential Vorticity gradient. Hence, the water most likely to be diverted offshore at Cape Farewell and in the West Greenland Current is the water that is closest to the interior to begin with, as Schulze Chretien and Frajka‐Williams () found in a recent modeling study. In order to spread into the interior, then, water must also be transported off the shelf.…”

“…Furthermore, several recent modeling studies have found that freshwater originating east of Greenland is more likely to enter the Labrador Sea than freshwater originating west of Greenland (Gillard et al, ; Luo et al, ; Marson et al, ; Tesdal et al, ; Wang et al, ). In particular, water over the slope east of Greenland is more likely to enter the Labrador Sea than water originating on the shelf (Schulze Chretien & Frajka‐Williams, ).…”