NAO-induced long-term changes in nutrient supply to the surface waters of the North Atlantic

Abstract

Since the late 1980s extensive observational campaigns like the Joint Global Ocean Flux Study (JGOFS) and the World Ocean Circulation Experiment (WOCE) have helped to considerably improve our understanding of marine biogeochemistry. By chance, this period corresponded to a phase of a positive swing of the North Atlantic Oscillation (NAO), whereas earlier studies in the North Atlantic generally took place during more negative phases of the NAO (Figure 1). This study demonstrates by means of a coupled ecosystem-circulation model that the long-term change in the NAO between the 1960s and 1990s may have induced significant regional changes in the upper ocean's nutrient supply. These include a decrease of nitrate supply by about 30% near Bermuda and in mid-latitudes, and a simultaneous 60% increase in the upwelling region off West Africa. The results suggest that a synthesis of biogeochemical observations taken during the past decades must take into account NAO-related climate variability.