Transport Variability of the Irminger Sea Deep Western Boundary Current from a Mooring Array

Abstract

The Deep Western Boundary Current (DWBC) in the subpolar North Atlantic is the lower limb of the Atlantic Meridional Overturning Circulation and a key component of the global climate system. Here, a mooring array deployed at 60${}^{\circ }$N in the Irminger Sea, between 2014 and 2016 provides the longest continuous record of total DWBC volume transport at this latitude. The 1.8 year averaged transport of water denser than $\sigma þeta$ = 27.8 kg m-3 was -10.8 textpm 4.9 Sv (mean textpm 1 std; 1 Sv = 106 m3 s-1). Of this total, we find -4.1 textpm 1.4 Sv within the densest layer ($\sigma þeta$ $>$ 27.88 kg m-3) that originated from the Denmark Strait Overflow. The lighter North East Atlantic Deep Water layer ($\sigma þeta$ = 27.8-27.88 kg m-3) carries -6.5 textpm 7.7 Sv. The variability in transport ranges between 2 and 65 days. There is a distinct shift from high to low frequency with distance from the East Greenland slope. High frequency fluctuations (2-8 days) close to the continental slope are likely associated with topographic Rossby waves and/or cyclonic eddies. Here, perturbations in layer thickness make a significant (20-60%) contribution to transport variability. In deeper water, toward the centre of the Irminger Basin, transport variance at 55 days dominates. Our results suggest that there has been a 1.8 Sv increase in total transport since 2005-2006, but this difference can be accounted for by a range of methodological and data limitation biases.