Unravelling the effects of natural variation and climate change in the North Atlantic
A new, British-American study reveals that although the North Atlantic has warmed over the past 50 years, this warming has not been uniform, as the subpolar regions have got cooler while the subtropical and tropical waters have warmed up. The research is published online by the journal Science. The researchers attribute this pattern of temperature changes to the North Atlantic Oscillation (NAO). The NAO is a natural and cyclical wind circulation pattern driven by differences in atmospheric pressure over the Azores and Iceland. 'The winds have a tremendous impact on the underlying ocean,' commented Susan Lozier of Duke University in the US. The scientists drew on five decades of ocean temperature data and complex computer models to investigate the effects of winds, evaporation and precipitation on the ocean. Their studies showed that the water in the sub-polar ocean, between 45° North and the Arctic Circle, cools as the water directly loses heat to the air above it. Meanwhile further south, NOA-driven winds 'pile up' sun-warmed waters in the southern part of the North Atlantic south of 45 degrees. This serves to retain heat at the surface and push underlying cooler water further down. The study highlights the difficulties faced by scientists seeking to differentiate between natural variation in the climate and human-induced climate change. The scientists note that in this case, although natural cycles appear to be behind the temperature changes, this natural variability is so strong that it may well be masking more subtle changes due to human induced climate change. 'The take-home message is that the NAO produces strong natural variability,' explained Professor Lozier. 'The simplistic view of global warming is that everything forward in time will warm uniformly. But this very strong natural variability is superimposed on human-caused warming. So researchers will need to unravel that natural variability to get at the part humans are responsible for.' A further twist to the tale comes from studies which suggest that the NAO itself could be affected by human-caused warming, although other studies have cast doubt on this connection. 'Continued long-term monitoring of North Atlantic temperatures is needed to answer the question of whether the basin-average warming is reflecting anthropogenic forcing and/or natural variability,' the researchers recommend.
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United Kingdom, United States