A description of interdecadal time-scale propagating North Atlantic sea surface temperature anomalies and their effect on winter European climate, 1948-2002

Eastward-propagating interdecadal time-scale sea surface temperature (SST) winter anomalies have been shown to exist at the North Atlantic subpolar/subtropical gyre boundary. Heat flux and surface air temperature signatures of these anomalies are investigated using satellite- and ship-based SST observations and atmospheric reanalysis. Using bandpass filter analysis, retaining periods between 9 and 25 yr, a succession of coherent propagating SST anomalies is identified. The size, speed, propagation path, and decay characteristics of propagating anomalies detected during the period 1948-2002 are documented. The behavior of the propagations changes between the periods 1948-70 and 1970-2002. In the former period, SST anomalies propagated from the east coast of North America to the British Isles in similar to 10 yr. The anomalies displayed a well-defined life cycle, growing in the western basin (west of 40 degrees W) and decaying in the eastern basin. During the period 1970-2002, SST anomalies did not propagate deep into the eastern basin, but grew in the western basin and then ceased propagating. Oceanic anomalies have a comparable marked signature in surface sensible and latent heat fluxes and in surface air temperature. Winter surface heat flux anomalies act to amplify SST anomalies during the middle of their lifetimes, normally in the west-central Atlantic. At other times, heat flux anomalies are associated with decay of anomalies. Surface heat fluxes do not always act to cause propagation, and it is likely that other processes such as advection play a role in the propagation mechanism. North European winter surface air temperatures are raised or lowered by up to +/- 0.5 degrees C over decadal time scales (similar to 1/3 of the total variation over the United Kingdom) when an SST anomaly reaches the eastern boundary. A variety of processes can cause SST variation on decadal time scales at the eastern boundary, but in the 1950s and 1960s the variability at these periods was the signature of features that had propagated across the Atlantic from the North American coast.