n recent years, two alarming trends in North Atlantic climate have been noted: an increase in intensity and frequency of Atlantic hurricanes and a rapid decrease in Greenland ice sheet volume. Both of these phenomena occurred while a significant warming took place in North Atlantic sea surface temperatures (SSTs), thus sparking a debate on whether the warming is a consequence of natural climate variations, anthropogenic forcing or, if due to both, what are their relative roles. Here models and observations are used to detect and attribute long-term (multi-decadal) 20th Century North Atlantic (NAtl) SST changes to their anthropogenic and natural causes. A suite of IPCC 20th century (C20C) coupled model simulations with multiple ensemble members and a signal to noise maximizing empirical orthogonal function analysis are used to identify a model-based estimate of the forced, anthropogenic component in NAtl SST variability. Comparing the results to observations, it is argued that the long-term, observed, North Atlantic basin-averaged SSTs combine a forced global warming trend with a distinct, local multi-decadal “oscillation” that is outside of the range of the model-simulated, forced component and most likely arose from internal variability. This internal variability produced a cold interval between 1900 and 1930, followed by 30 years of relative warmth and another cold phase from 1960 to 1990, and a warming since then. This natural variation, referred to previously as the Atlantic Multidecadal Oscillation (AMO), thus played a significant role in the 20th Century NAtl SST variability and should be considered in future, near-term climate projections as a mechanism that, depending on its behavior, can act either constructively or destructively with the region's response to anthropogenic influence, temporarily amplifying or mitigating regional climate change.

Full Article: http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2008JCLI2561.1

Last Updated: 2009-03-13