X as the arithmetic mean of the ensemble means, i.e., X 0 1 N P N j01 X j , where N is the total number of models in the IPCC archive and X j is the ensemble mean signal of the jth model. This weighting avoids undue emphasis on results from a single model with a large number of realizations. 33. One measure of ENSO variability is s(T NIÑ Oj3.4 ), the standard deviation of sea-surface temperatures in the Niño 3.4 region of the equatorial Pacific. Values of s(T S ) in the 49 IPCC realizations are

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... closely correlated with s(T NIÑ Oj3.4 ) (correlation coefficient r 0 0.92). 34. The theoretical expectation plotted in Fig. 3 was computed by taking the difference of two pseudoadiabats calculated from surface air parcels with temperatures of 28.0-and 28.2-C and 80% relative humidity. These are conditions typical of deep convective regions over the tropical oceans. The pseudo-adiabats correspond to equivalent potential temperatures of 353.2 and 354.1 K. The assumed temperature difference of 0.2-C corresponds approximately to the total change in tropical ocean temperature over the years 1979 to 1999. Theoretical scaling ratios are relatively insensitive to reasonable variations in the baseline values of surface air temperature and relative humidity, as well as to the magnitude of the surface air temperature increase.