Line W measurements of the Deep Western Boundary Current reflect changes in Labrador Sea deep convection

The Line W moored array on the continental slope southeast of New England measured the North Atlantic's Deep Western Boundary Current (DWBC) properties and velocity from 2004 to 2014. The DWBC is the primary branch of the Atlantic Meridional Overturning Circulation's (AMOC) cold limb, bringing North Atlantic Deep Water (NADW) equatorward along the continental slope. Building on the work of Peña-Molino et al. 2011, we analyze intermediate water properties at five Line W moorings, focusing on the convectively-formed Classical Labrador Sea Water (CLSW) with its low planetary potential vorticity (PPV) signature. In all five moorings there is a trend of increasing PPV in the CLSW neutral density range, reflecting weakening convection in the Labrador Sea. The CLSW also warms (+0.016 °C/yr) and becomes saltier (+0.0014 /yr) over the course of the decade, consistent with decreasing dominance of cold, fresh CLSW. Shipboard hydrographic data extend our analysis back to the mid 1990s. Through this 20 year record the CLSW layer's T/S character changes. Ensembles of early T/S-profiles are broad in the CLSW range due to significant amounts of exceptionally cold, fresh water formed in the 1990s intermingling with warmer/saltier waters. By 2010, profile collections are narrow and lack the signature of especially cold, fresh CLSW. Our results are consistent with measurements in the Labrador Sea (Kieke et al. 2014), where there was intense convection in the 1990s but much weaker convection in the 2000s. The decay of the cold/fresh CLSW at Line W by 2010 indicates a travel time upper bound of approximately 10 years from the Labrador Sea to Line W. The observed warming is also consistent with measurements of the DWBC at 53 °N (Fischer et al. 2010) though at a decreased rate, most likely due to stirring with the interior. Future work will include a detailed comparison of these moored observations in order to quantify travel times of CLSW and bulk exchange with the interior.