The Labrador boundary current system in summer and fall: mean, variability and influence on cross-shelf exchange

Abstract

The Labrador boundary current system significantly impacts freshwater pathways and upper-ocean stratification in the subpolar North Atlantic, which in turn influences the state of the Atlantic Meridional Overturning Circulation. In this study we analyze an extensive collection of shipboard hydrographic and velocity sections across the Labrador shelf and slope, occupied in the summer and fall seasons since 1940 along the Seal Island transect. The mean sections provide a quantitative view of the boundary current system and its velocity structure, consisting of the Labrador Coastal Current (LCC), the Labrador Current (LC), and a permanent recirculation associated with Hamilton Bank situated between the two currents. The LCC, which flows adjacent to the coast, is strongly influenced by local wind forcing. It has exhibited increasing volume and freshwater transports since around 2010, while its baroclinic component has progressively strengthened since the early 1970s, largely influenced by increased river runoff from northern Canada. By contrast, the LC, situated at the shelfbreak, is more closely linked to larger scale atmospheric patterns – captured by the North Atlantic Oscillation (NAO) – rather than local winds. Over the past decade the LC has been in a strengthened regime, associated with a positive phase of the NAO, with enhanced equatorward freshwater transport. On the mid-shelf, the recirculation and its associated mixing facilitate a net offshore flux of freshwater.