Interactions between External and Intraseasonal Atmospheric Variability

An interaction between low-frequency intra-seasonal atmospheric variability and external (SST induced) forcing has been suggested by recent observational and modeling work. In particular the existence of regimes (in the Pacific North America sector), defined by non-Gaussian behavior of the multi-dimension probability distribution function in EOF coordinates, has been linked to the existence of cold tropical Pacific SSTs (La Nina events). This externally induced modification of intra-seasonal variability is documented in large ensembles using two atmospheric GCMs: (i) the COLA GCM (T63, L18), with 54 members in each ensemble using observed SSTs for the La Nina winters of 1984/85, 1988/89 and 1998/99, and (ii) the ECMWF GCM (T63L60), with 30 members in each of five ensembles using an idealized SST anomaly with varying magnitude and sign. The role of external forcing (SST) is verified by comparing the COLA GCM results with preliminary results using smaller (nine member ensembles) for each of the 18 winters 1981/82 - 1998/99.

The methodology used is based on EOF-expansion followed by cluster analysis using the dynamic cluster method (see Michelangeli, et. al., 1995 for details). Markov modeling provides the null hypothesis against which significance of multiple phase space maxima is assessed, and multiple analyses of sub-sampled data provides an estimate of robustness (reproduceability).

The barotropic (in)stability of the regimes will be discussed, as well as the roles of transient fluxes vis-a-vis mean flow advection and non-linearity in maintaining the regimes. The effects of model systematic error will be addressed.

Michangeli, P.-A., R. Vautard, and R. Legras, 1995: Weather regimes. recurrence, and quasi-stationarity. J. Atmos. Sci., 52, 1237-1256.