Freshwater Origin of the Upper Shelf and Slope Waters of Southern Labrador and Newfoundland

Sea surface salinity (SSS) data, compiled for the period 1896-2013, indicate a large decrease of salinity during this period in the western Atlantic near 50-55°N. This decrease suggests an increase in time of the freshwater that is transported by the Labrador Current and on the shelves from high latitudes into the interior of the North Atlantic subpolar gyre. This time series illustrate also large decadal and multi-decadal variability with the last maxima in the late 1980s and early 2000s. Surface salinity has been investigated on an almost monthly frequency since 1993 from southern Newfoundland towards Iceland, indicating a large decrease of SSS in the shelf and slope region for the last few years. We investigate the origin of the water mass and in particular its fresh water component, combining salinity, nutrient and water istopolog data, collected on one hand seasonally from a merchant vessel in 2012-2015 and, on the other hand, from three cruises off southern Labrador in May-June 2014. The ship of opportunity data reveal a large seasonal cycle both in SSS and in the freshwater composition, which is different close to Newfoundland relative to the outer shelf and slope. A particularly large sea ice melt contribution is found in the summer 2014 on the outer shelf. We also find that the largest meteoric water contribution is found later than the maximum sea ice melt contribution and explains two thirds of the SSS seasonal cycle. The section data illustrated differences between 54-55°N where water mass distribution was rather homogeneous across the shelf (innermost part not sampled), and southern Labrador (52°N) where there were larger gradients between the near shore and the outer shelf. In both regions, water in the slope current was found devoid of Pacific water. Largest meteoric water contribution was found close to shore off southern Labrador, and there was some contribution of fresh water melt at the surface for these two section which took place later than the northern one. At subsurface, all sections indicate a large signal of ice formation, partially advected from higher latitudes.