Principal Modes of Atmospheric Variability in Model Atmospheres with and without Anomalous Sea Surface Temperature Forcing in the Tropical Pacific

Principal modes of low-frequency atmospheric variability and the influence of sea surface temperature anomalies on such modes are investigated by examining the output from two general circulation model experiments. In the first experiment (the “control run”), all boundary forcings were constrained to evolve through 15 identical annual cycles. In the second experiment (the “SST runs”), the sea surface temperature conditions in the tropical Pacific Basin were prescribed to follow the actual month-to-month changes observed during the period 1962–76, which encompasses several El Niño events. The analysis tools employed here include empirical orthogonal functions, teleconnections and composite charts. Two prominent modes of variability have been identified in the wintertime Northern Hemisphere eddy streamfunction of the SST experiment. The first mode bears a strong spatial resemblance to the observed characteristic circulation pattern over the North Pacific–North American sector. It is demonstrated that this mode is highly correlated with the changing SST forcing imposed over the tropical Pacific, and with various meteorological phenomena accompanying El Niño-Southern Oscillation (ENSO). The second mode exhibits no significant correlation with the SST forcing, but is linked instead to a characteristic structure of the zonally averaged zonal wind. The circulation features related to this mode are reminiscent of observed anomalies accompanying the North Atlantic Oscillation, as well as variations of the “zonal index.” The fluctuations associated with the first and second modes in the SST experiment are of comparable amplitudes. It is further demonstrated that the essential characteristics of the first and second modes are highly reproducible in another 15-year simulation initiated from a completely independent set of atmospheric conditions. A parallel diagnosis of the model behavior in the control run reveals no signal of the first mode (i.e., that related to FNSO) as previously described. However, circulation features accompanying the second mode are still discernible in the control experiment. It is noted that the predominant anomalous phenomena in the control run are manifestations of the zonal/eddy relationship associated with the second mode. The mode appearing in both the control and SST runs is likely related to internal dynamical processes in the model atmosphere subjected to a fixed boundary forcing.