The influence of midlatitude ocean-atmosphere coupling on the low-frequency variability of a GCM. Part II: Interannual variability induced by tropical SST forcing

This study extends the investigation of the impact of midlatitude ocean-atm osphere interactions on the atmospheric circulation to the interannual time scale by incorporating SST variability in the tropical Pacific representati ve of observed conditions. Two perpetual January GCM simulations are perfor med to examine the changes in the low-frequency atmospheric variability bro ught about by the inclusion of an interactive slab mixed layer in midlatitu des, in particular the changes in the extratropical response to ENSO-like t ropical 90-day mean SST anomalies. It is found that midlatitude coupling alters the spatial organization of th e low-frequency variability in qualitatively the same manner (but not to th e same extent) as tropical SST variability-namely, by selectively enhancing (in terms of amplitude, persistence, and/or frequency of occurrence) certa in of the preexisting (natural) dominant modes without significantly modify ing them or generating new ones. While tropical SST forcing results in a no table amplification of the Pacific-North American (PNA) mode of the model, midlatitude SST anomalies appear to favor the regional zonal index circulat ions in the eastern and western Pacific (through decreased thermal damping at the surface). As a result, the PNA response to ENSO-like tropical SST fo rcing is not reinforced but slightly weakened by the presence of interactio ns with the underlying mixed layer. On the other hand, coupling increases t he persistence of the overall extratropical signal and causes it to acquire distinct Western Pacific-like features, thus improving its resemblance to the observed ENSO teleconnection pattern. sThe leading mode of covariability between the hemispheric atmospheric circ ulation and North Pacific SST qualitatively reproduces its observational co unterpart, with the atmosphere leading by about one month and surface atmos pheric variations consistent with the notion that the atmosphere is driving the ocean. This agreement suggests that, even on interannual timescales, t wo-way air-sea interactions and ocean dynamics do not play an essential rol e in establishing the large-scale spatial structure of this observed domina nt mode of ocean-atmosphere interaction. In addition, the simulated pattern s of covariability in this Sector possess the same kind of interannual-intr aseasonal duality exhibited by the observations. In the North Atlantic the model essentially recovers the results from Part I of this study.