A Comparison of Decadal Climate Oscillations in the North Atlantic Detected in Observations and a Coupled GCM

Abstract

Decadal climate variations in the Atlantic Ocean found in observational fields and a coupled general circulation model (CGCM) are investigated. In particular, 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 features: sea surface temperature (SST) anomalies in the western subtropical Atlantic sandwiched by those with opposite sign to the north and south, and a dipole of sea level pressure anomalies, which resemble the North Atlantic Oscillation. 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 the subtropical western Atlantic to the European coast along the Gulf Stream. Northward propagation of SST anomalies are also found along the western boundaries including the Gulf of Mexico. A budget analysis for the temperature 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 the oscillation to another. Anomalous gyre intensity appears to have little contribution to the phase switching process of the near-surface variability, although the influence of the anomalous gyre is found in the lower subsurface up to 500 m. In contrast, SST anomalies in the CGCM are more strongly tied with subsurface temperature anomalies that propagate westward, consistent with a slow gyre adjustment by the baroclinic Rossby wave propagation. The wave-induced advection acts to change the phase of SST as well as the subsurface temperature anomalies in the model. Subduction of temperature anomalies 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?ocean interaction is important for the decadal variability. The anomalous heat flux originated from the wind?evaporation feedback appears to play a dominant role in the formation of the tripolar structure of oceanic thermal anomalies 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 Rossby waves, respectively, both of which happen to have similar timescales.