Dilution of the Northern North Atlantic Ocean in Recent Decades

Abstract

Declining salinities signify that large amounts of fresh water have been added to the northern North Atlantic Ocean since the mid-1960s. We estimate that the Nordic Seas and Subpolar Basins were diluted by an extra 19,000 +/- 5000 cubic kilometers of freshwater input between 1965 and 1995. Fully half of that additional fresh water-about 10,000 cubic kilometers-infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year. Patterns of freshwater accumulation observed in the Nordic Seas suggest a century time scale to reach freshening thresholds critical to that portion of the Atlantic meridional overturning circulation.

Simulated Response of the Arctic Freshwater Budget to Extreme NAO Wind Forcing

“…In agreement with the work of, for example, Curry and Mauritzen (2005) and Peterson et al (2006), the increased export of freshwater to the North Atlantic when the NAO was positive in our model suggests that the observed freshening of the North Atlantic from the mid1960s to the mid-1990s may have had its origin in the Arctic Ocean. This period of North Atlantic freshening coincides with a change in the NAO from its strongly negative to strongly positive state in which a weakening of the Beaufort high would reduce Ekman convergence in the gyre, leading to an increase in the export of stored freshwater to the North Atlantic.…”

“…A change in the Arctic freshwater budget between 1965 and 1995 is believed to have increased the export of freshwater to the North Atlantic, as observed by a freshening of the North Atlantic by an amount equivalent to ;19 000 6 5000 km 3 [31 536 km 3 yr 21 5 1 Sv (Sv [ 10 6 m 3 s 21 )] of freshwater (Curry and Mauritzen 2005;Peterson et al 2006). About half of this freshwater appears to have been released from the Arctic during the Great Salinity Anomaly (GSA), which began in 1968, when increased ice export in Fram Strait discharged ;2000 km 3 yr 21 of freshwater into the northern North Atlantic over a 5-yr period (Dickson et al 1988).…”

Current estimates of freshwater flux through Arctic and subarctic seas

“…In hindcast experiments performed with different models of the NAOSIM hierarchy there is a long-term decrease of liquid freshwater amounting to about 10,000 km 3 between the late 1960s and late 1990s with a parallel gain of about 3000 km 3 for the Nordic Seas; the remainder drains to the North Atlantic (Karcher et al, 2005;Koeberle and Gerdes, 2007). These numbers are close to the results of the HYDROBASE analysis by Curry and Mauritzen (2005), mentioned above, which estimated an increase of 4000 km 3 in the freshwater content of the Nordic Seas and…”

“…Though our direct measurements are too brief as yet to comment, the analysis of the historic hydrographic record and modelling appear to agree that the efflux of freshwater from the Arctic has increased steadily over recent decades (Curry and Mauritzen, 2005), and model-based projections suggest that this will continue, reflecting increased circumarctic river inputs (Wu et al, 2005), a progressive thinning and retraction of the Arctic perennial sea-ice (eg Comiso, 2002), an increase in the freshwater production of Greenland (Fichefet et al, 2003) and increases in the combined ice and freshwater outflows through the CAA and Fram Strait (eg Haak et al, 2005).…”

“…The Labrador Sea is the main receiving volume in which ocean fluxes from northern seas are stored, recirculated, transformed and discharged, and it is of interest in the present context to determine the total change in freshwater 'received' in relation to the freshwater 'lost' from northern seas. From their analysis of HYDROBASE (see http://www.whoi.edu/science/PO/hydrobase), Curry and Mauritzen (2005) provide us with a modern answer. They calculate that from the mid 1950s to the mid 1990s, the freshwater storage in the watercolumn of Nordic Seas increased by 4000 km 3 and the fresh-…”