Variability of the Arctic freshwater export west of Greenland: a proxy from 65 years of hydrographic observations on the Labrador Shelf

Abstract

The Arctic Ocean has increased significantly its storage of freshwater over the past two decades. It is a major source of freshwater to the subpolar North Atlantic, where it has the potential to freshen deep-water formation regions, disrupt the meridional overturning circulation and severely affect the European (and even global) climate. This manuscript describes, for the first time, the multidecadal (1950 to 2014) variability of the Arctic freshwater export west of Greenland from observations, with the purpose of investigating how the freshwater budgets of the Arctic and the North Atlantic connect through these fluxes. First, this thesis processes repeated hydrographic observations at the Seal Island section (on the Labrador shelf) and produces a multidecadal dataset of gridded summer sections of temperature, salinity and density. Then, a high-resolution coupled ice-ocean global general circulation model is used to describe the circulation on the Labrador shelf region and to put the observations in context. The regional circulation is dominated by the Labrador current, which presents a dual core with different dynamics. Its upper component carries the Arctic freshwater export west of Greenland and, at the Seal Island section, it fills the mid-shelf with this water. The newly named Labrador Coastal current is a continuation of the Hudson Strait outflow and contains the Hudson water inshore (in the first 50 km of the shelf). This makes it possible to identify the signal of Arctic freshwater at the Seal Island observations. The observed freshwater transport (referenced to salinity 35.0) of this Arctic signal is used as a proxy for the variability of the Arctic freshwater export west of Greenland over 65 years of data. Two periods of enhanced export (1955-1960 and 1968-1994) and two periods of decreased transport (1960-1968 and 1994-present) are identified. The variability shown by the proxy is compared to observed and modelled changes in the Arctic and the North Atlantic freshwater budgets. This is to test the hypothesis that an increased storage of freshwater in the Arctic results in a decreased export and a reduction in the freshwater content of the subpolar North Atlantic, and otherwise. The transport shown by the proxy totals to an accumulated increase of ⇠22000 km3 from 1965 to 1995 and an accumulated decrease of ⇠16000 km3 from 1995 to 2014. This fully accounts for the magnitude of the changes observed in the Arctic and the North Atlantic budgets during the same periods, and comparable changes are also found throughout the full time series. This indicates that the advection of freshwater west of Greenland has a key role in the freshwater balance of these budgets. This also underlines the relevance of the freshwater export west of Greenland to the total magnitude of the export and the impacts on the North Atlantic.

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