Multidecadal Variability of North Atlantic Temperature and Salinity during the Twentieth Century

Substantial changes occurred in the North Atlantic during the twentieth century. Here the authors demonstrate, through the analysis of a vast collection of observational data, that multidecadal fluctuations on time scales of 50–80 yr are prevalent in the upper 3000 m of the North Atlantic Ocean. Spatially averaged temperature and salinity from the 0–300and 1000–3000-m layers vary in opposition: prolonged periods of cooling and freshening (warming and salinification) in one layer are generally associated with opposite tendencies in the other layer, consistent with the notion of thermohaline overturning circulation. In the 1990s, widespread cooling and freshening was a dominant feature in the 1000–3000-m layer, whereas warming and salinification generally dominated in the upper 300 m, except for the subpolar North Atlantic where complex exchanges with the Arctic Ocean occur. The single-signed basin-scale pattern of multidecadal variability is evident from decadal 1000–3000-m temperature and salinity fields, whereas upper-ocean temperature and salinity distributions have a more complicated spatial pattern. Results suggest a general warming trend of 0.012° 0.009°C decade 1 in the upper-3000-m North Atlantic over the last 55 yr of the twentieth century, although during this time there are periods in which short-term trends are strongly amplified by multidecadal variability. Since warming (cooling) is generally associated with salinification (freshening) for these large-scale fluctuations, qualitatively tracking the mean temperature–salinity relationship, vertical displacement of isotherms appears to play an important role in this warming and in other observed fluctuations. Finally, since the North Atlantic Ocean plays a crucial role in establishing and regulating global thermohaline circulation, the multidecadal fluctuations of the heat and freshwater balance discussed here should be considered when assessing long-term climate change and variability, both in the North Atlantic and at global scales.