Daily NCEP/NCAR reanalysis data are used that cover the boreal winters of 1958-97. A set of non-orthogonal basis functions that span the continuum are derived. The leading basis functions correspond to well known patterns such as as the Pacific North/American and North Atlantic Oscillation. Evidence for the continuum perspective is based on the finding that (1) most members of the continuum tend to have similar variance and autocorrelation time scales, and (2) that members of the continuum show dynamical characteristics that are intermediate between those of the surrounding basis functions. The latter finding is obtained by examining the streamfunction tendency equation both forthe basis functions and some members of the continuum.
The streamfunction tendency equation analysis suggests that North Pacific patterns (basis functions and continuum) are primarily driven by their interaction with the climatological stationary eddies, and that North Atlantic patterns are primarily driven by transient eddy vorticity fluxes. The decay mechanism for all patterns is similar, being due to the impact of low-frequency (period greater than 10 days) transient eddies and horizontal divergence. Analysis with outgoing longwave radiation shows that tropical convection is found to play a much greater role in exciting North Pacific patterns. A plausible explanation for these differences between the North Atlantic and North Pacific patterns is presented.