Ocean Evidence for a Shifting Atlantic Fresh Water Balance Over the Past Four Decades

Two long meridional transects through the western and eastern deep basins respectively are used to investigate salinity differences throughout the water columns of the North and South Atlantic Ocean over the past forty years. Toward both poleward limits of these sections we observe freshening over much of the water column, while within the intervening tropics and subtropics, the upper water column in particular has become markedly more saline. Where the dataset permits, it supports the idea that this growing contrast between high and low latitudes is the result of a progressive, if non-steady, evolution throughout the time period examined. For the low latitudes, an evaluation of the amount of annual fresh water loss that could be attributed to evaporation-minus-precipitation (E-P) changes suggests a 5-10% increase in net evaporation rates in recent decades. The radical freshening of the Northern subpolar seas is linked, at least in part, to enhanced efflux of ice and freshwater from the Arctic. In the South Atlantic, freshening is observed in the watermasses that are ventilated in regions where precipitation dominates evaporation. Combined with recent studies in the Mediterranean, Indian and Pacific Oceans, the present investigation extends the evidence that shifts in ocean salinity and fresh water distributions are occurring globally in ways that could reflect an amplification of the hydrologic cycle in recent decades.