Changes in the spread of the variability of the seasonal mean atmospheric states associated with ENSO

For a fixed sea surface temperature (SST) forcing, the variability of the o bserved seasonal mean atmospheric states in the extratropical latitudes can be characterized in terms of probability distribution functions (PDFs). Pr edictability of the seasonal mean anomalies related to interannual variatio ns in the SSTs, therefore, entails understanding the influence of SST forci ng on various moments of the probability distribution that characterize the variability of the seasonal means. Such an understanding for changes in th e first moment of the PDF for the seasonal means with SSTs is well document ed. In this paper the analysis is extended to include also the impact of SS T forcing on the second moment of the PDFs. The analysis is primarily based on ensemble atmospheric general circulation model (AGCM) simulations forced with observed SSTs for the period 1950-94. To establish the robustness of the results and to ensure that they are not unduly affected by biases in a particular AGCM, the analysis is based on s imulations from four different AGCMs. The analysis of AGCM simulations indicates that over the Pacific-North Amer ican region, the impact of interannual variations in SSTs on the spread of the seasonal mean atmospheric states (i.e., the second moment of the PDFs) may be small. This is in contrast to their well-defined impact on the first moment of the PDF for the seasonal mean atmospheric state that is manifest ed as an anomalous wave train over this region. For seasonal predictions, t he results imply that the dominant contribution to seasonal predictability comes from the impact of SSTs on the first moment of the PDF, with the impa ct of SSTs on the second moment of the PDFs playing a secondary role.