Forced and Internal 20th Century SST Trends in the North Atlantic

LDEO Publication: 
Publication Type  Journal Article
Year of Publication  2009
Authors  Ting, M.; Kushnir, Y.; Seager, R.; Li, C.
Journal Title  Journal of Climate
Volume  22
Issue  6
Pages  13
Start Page  1469
Journal Date  March/2009
LDEO Publication Number  7204
Key Words  Atlantic Multi-decadal Variability; Anthropogenically forced SST trends; Signal to Noise maximizing EOF analysis

In recent years, two alarming trends in North Atlantic climate have been noted: an increase in the 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 both; and if both, what their relative roles are. Here models and observations are used to detect and attribute long-term (multidecadal) twentieth-century North Atlantic (NA) SST changes to their anthropogenic and natural causes. A suite of Intergovernmental Panel on Climate Change (IPCC) twentieth-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 NA 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 multidecadal ‘‘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 yr 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 twentieth-century NA 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.

DOI  10.1175/2008JCLI2561.1