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Climatic means can be changed in two distinct ways: by a small change acting over the entire averaging period or by a changed number of extreme events within the averaging period. Thus a summer can be especially hot if the daily temperature is hotter every day during the summer or if there are, say, three heat waves instead of the usual two. Extreme events therefore contribute in an important way to climatic means but the events themselves are weather, not climate.

Other climatic statistics include the variances of quantities averaged over the climatic period. For example, two winters with the same mean temperature may differ in that one has a wider range of maximum and minimum temperatures. Orange growers in Florida would certainly be more concerned by a winter in which the lowest daily temperatures often went below freezing than a winter in which they did not, even if the mean winter temperature were the same for both.

The climatic statistics for a given month (say December) are not the same each year. When there is significant variability of December averaged temperature from year to year (compare December 1982 with December 1983, say), the climate is said to vary interannually. Although a certain amount of interannual variability is intrinsic to any monthly averaged process (the time average over any varying short-term weather process will vary depending on the statistics of the weather process), there are global patterns of interannual variation that have characteristic properties in space and time.

The strongest known pattern of interannual variability in the earth's climate system is El Niño/Southern Oscillation (ENSO): it consists of both a warming and a cooling of the waters of the equatorial Pacific Ocean occurring irregularly every few years and a concomitant set of worldwide climatic changes that statistically depend on these changes of sea surface temperature in the equatorial Pacific. A detailed description of the ENSO phenomenon appears in National Research Council (1996a) and a simple description may be found on the web at <http://www.pmel.noaa.gov/toga-tao/el-nino/home.html>. The regions affected by ENSO are shown in Figure 2-1. Although ENSO is the strongest known interannual signal, it is not the only one. A region may have interannual variability for reasons that may or may not include ENSO.

How Seasonal-to-Interannual Climate Forecasts Are Made

The Weather Forecasting Paradigm

Society has come to take for granted the benefits of weather of forecasting and is accepting of the considerable costs incurred to make the fore-



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