Abstract
Labrador Slope Water (LSLW) is a relatively fresh and cool water mass that originates from the Labrador Current in the subarctic and is known to occur in the Eastern Slope Sea on the US-Canadian shelf-slope north of the Gulf Stream. It has potential densities of 27.4–27.65 kg m−3. Using ocean observations, we show here that the LSLW penetrates as a boundary current deeply into the Western Slope Sea (west of 66°W) as a salinity minimum between 400 and 600 m, bringing it into close proximity with the Gulf Stream. The LSLW at Line W (near 69°W) also spreads across, and brings fresher and thicker waters to, the Slope Sea north of the Gulf Stream. A high-resolution ocean model simulation shows that the spreading of the LSLW occurs throughout the entire Slope Sea through the extrusion of fine-scale filaments from the boundary current following interaction with Gulf Stream meanders and eddies. At Line W, the LSLW is also found to be fresher and thicker between 2003 and 2008, when the Atlantic Meridional Overturning Circulation (AMOC) at 26°N is higher (by 3 Sv), and the Shelf Slope Front is further south (by 0.7°), compared to AMOC low conditions in 2009–2014. The thicker LSLW causes lighter isopycnals to rise over the shelf slope, and through increasing the lateral density gradient contributes an additional 1.3 Sv to the Gulf Stream transport. These changes to the LSLW and the Shelf Slope Front are likely to result from an enhanced flow of the Labrador Current into the Slope Sea, caused by changes in the wind stress in the subpolar gyre. The transport of the LSLW (as opposed to the deeper Labrador Sea Water) thereby offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting changes in the subarctic with subsequent variability in the Gulf Stream and AMOC.
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) carries warm, salty near-surface waters northwards, a major component of this being the Gulf Stream, and cooler, fresher deeper waters southwards, in the Deep Western Boundary Current (DWBC), the whole being a key component of the climate system (e.g. Smeed et al 2018)
An increasing proportion of LSLW in the Gulf of Maine may result from decreases in either the Nova Scotia Current (Townsend et al 2015) or a shift from a positive to a negative phase of the North Atlantic Oscillation (NAO, Mountain 2012). This latter relationship is supported by Marsh (2000), who forced a 1◦ ocean model with extreme negative NAO conditions, resulting in the subpolar gyre expanding to the south, which in turn allowed an enhanced flow of Labrador Current waters westwards from the Grand Banks, and cooling in the Eastern Slope Sea
We have shown that the LSLW penetrates deeply into the Western Slope Sea as a boundary current where it is characterised by a salinity minimum between 400 and 600 m on the slopes, and where it interacts strongly with the Gulf Stream
Summary
A L New1,∗ , D A Smeed , A Czaja , A T Blaker , J V Mecking , J P Mathews and A Sanchez-Franks. Labrador Current in the subarctic and is known to occur in the Eastern Slope Sea on the US-Canadian shelf-slope north of the Gulf Stream. It has potential densities of 27.4–27.65 kg m−3. The LSLW at Line W (near 69◦W) spreads across, and brings fresher and thicker waters to, the Slope Sea north of the Gulf Stream. The transport of the LSLW (as opposed to the deeper Labrador Sea Water) thereby offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting changes in the subarctic with subsequent variability in the Gulf Stream and AMOC
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