Atmospheric low frequency variability and itsrelationship to midlatitude SST variability : studiesusing the NCAR Climate System

The characteristics of atmospheric low-frequency variability and midlatitude SST variability as simulated by the NCAR Climate System Model are analyzed in the vicinity of the North Paciic and North Atlantic basins. The simulated spatial patterns of variability correspond quite well to those seen in observational datasets, although there are some diierences in the amplitudes of variability. Companion uncoupled integrations using the atmospheric component of the coupled model are also analyzed to identify the mechanisms of midlatitude SST variability on interannual timescales. These integrations are subject to a hierarchy of SST boundary conditions, ranging from the climatological annual-cycle to global monthly-mean observed SST. Even uncoupled atmospheric model integrations forced by climatological SST boundary conditions are capable of simulating the spatial patterns of atmospheric variability fairly well, although coupling to an interactive ocean does produce some improvements in the spatial patterns. However, the presence of realistic SST variability, especially in the tropics, is necessary to obtain the right variance amplitudes for the diierent modes of variability. It appears that coupling to an interactive ocean essentially reorders , rather than reshapes, the dominant modes of atmospheric low-frequency variability. The results indicate that the dominant modes of SST variability in each ocean basin are forced by the respective dominant modes of atmospheric low-frequency variability in the vicinity of the ocean basin. The relationship between atmospheric variability and the surface heat ux is also analyzed. Evidence is found for a local thermal feedback in the coupled integration, associated with the nite heat capacity of the ocean, that acts to damp surface heat ux variability. It is also shown that the relationship between midlatitude SST anomalies and the surface heat ux in AMIP-type of atmospheric model integrations is quite different from that in the coupled model integration.