On the Mechanism of North Atlantic Decadal Variability

North Atlantic decadal climate variability is studied with a coupled atmosphere–ocean–sea ice model (ECBILT). After having reached an approximate statistical equilibrium in coupled mode without applying flux corrections, a subsequent 1000-yr integration is performed and analyzed. Compared to the current climate, the surface temperatures are 2°C warmer in the Tropics to almost 8°C warmer in the polar regions.

The covariability between the atmosphere and ocean is explored by performing a singular value decomposition (SVD) of boreal winter SST anomalies and 800-hPa geopotential height anomalies. The first SVD pair shows a red variance spectrum in SST and a white spectrum in 800-hPa height. The second mode shows a peak in both spectra at a timescale of about 16–18 yr. The geopotential height pattern is the model’s equivalent of the North Atlantic oscillation (NAO) pattern; the SST anomaly pattern is a north–south oriented dipole.

Additional experiments have revealed that the decadal oscillation in ECBILT is basically an oscillation in the subsurface of the ocean. The oscillation is excited by anomalies in the atmospheric NAO pattern, both through anomalous surface heat fluxes and anomalous Ekman transports. The atmospheric response to the SST anomaly enhances the oscillation and slightly modifies it, but is not essential. The atmospheric response consists primarily of a local surface air temperature adjustment to the SST anomaly. An important element in the physical mechanism of the oscillation is the geostrophic response of the ocean circulation to the forced temperature anomalies creating surface salinity anomalies through anomalous horizontal advection. These salinity anomalies influence the convective activity in the area of the temperature anomaly such as to break down the subsurface temperature anomaly. Both temperature and salinity anomalies slowly propagate eastward at a rate consistent with the mean current.