A comparison of decadal climate oscillations in the north Atlantic detected in observations and a coupled GCM

Decadal climate variations in the Atlantic Ocean found in observational fie lds and a coupled general circulation model (CGCM) are investigated. In par ticular, physical processes responsible for the phase reversal are compared . Observed and modeled decadal variations have dominant periodicities around 12.3 and 9.9 yr, respectively. Both variations show similar spatial feature s: sea surface temperature (SST) anomalies in the western subtropical Atlan tic sandwiched by those with opposite sign to the north and south, and a di pole of sea level pressure anomalies, which resemble the North Atlantic Osc illation. Their temporal evolutions are, however, different from each other suggestive of different dynamics of the oscillation. In the observations, SST and surface-layer (0-100 m) temperature anomalies move eastward from th e subtropical western Atlantic to the European coast along the Gulf Stream. Northward propagation of SST anomalies are also found along the western bo undaries including the Gulf of Mexico. A budget analysis for the temperatur e equation shows that these features are the manifestation of the advection of SST anomalies by the mean current, which acts to switch one phase of th e oscillation to another. Anomalous gyre intensity appears to have little c ontribution to the phase switching process of the nearsurface variability, although the influence of the anomalous gyre is found in the lower subsurfa ce up to 500 m. In contrast, SST anomalies in the CGCM are more strongly ti ed with subsurface temperature anomalies that propagate westward, consisten t with a slow gyre adjustment by the baroclinic Rossby wave propagation. Th e wave-induced advection acts to change the phase of SST as well as the sub surface temperature anomalies in the model. Subduction of temperature anoma lies is found to occur on decadal timescales both in the observation and in the model over the eastern basin where the winter mixed layer is deepened, although the consequence of such a process is not very clear. In agreement with previous studies, it is suggested that the atmosphere-oce an interaction is important for the decadal variability. The anomalous heat Bur originated from the wind-evaporation feedback appears to play a domina nt role in the formation of the tripolar structure of oceanic thermal anoma lies both in observations and in the CGCM. On the other hand, the dominant timescales of observed and simulated decadal modes are largely dominated by the mean subtropical gyre velocity, and by the propagation speed of long R ossby waves, respectively, both of which happen to have similar timescales.