Surface freshwater flux variability and recent freshening of the North Atlantic in the eastern subpolar gyre

Abstract

Variability in the air‐sea flux of freshwater (precipitation‐evaporation (P‐E)) and its connection to the observed freshening of the eastern half of the North Atlantic subpolar gyre over the period 1960 to present is investigated using atmospheric model reanalyses and observational data. Similar results are obtained from both the National Centers for Environmental Prediction/National Center for Atmospheric Research and ERA‐40 reanalyses, each of which shows a major increase in P‐E (primarily due to increased precipitation) in the gyre region in the mid‐1970s, from about 0.10 m yr−1 to 0.27 m yr−1. This increase is supported by independent rain gauge observations recorded in Iceland, the Faeroes, and Ireland. When integrated over a box centered on the eastern gyre, the atmosphere to ocean freshwater flux in the period 1975–1989 is 4 × 1012 m3 greater than that in the earlier period 1960–1974. This increase is about twice as large as the freshwater excess associated with major advective events such as the Great Salinity Anomaly. The link between the increase in P‐E and the two major modes of atmospheric variability in the region, the North Atlantic Oscillation (NAO) and the East Atlantic Pattern (EAP), is investigated. The EAP is found to be the dominant cause of the increase, with the NAO only playing a minor role. The impact of the surface freshwater flux trend on sea surface salinity is also examined through comparison with hydrographic measurements at 60°N, which show freshening since the mid‐1970s. The observed freshening can be largely explained as a direct response to changes in the air‐sea freshwater exchange.