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It may seem at first that many of these regions are far away from Colorado <br />however, while keeping the large -scale system in mind we think of the words of Robert <br />A. Heinlein who once said "Climate is what you would expect, weather is what you get" <br />These regions of atmospheric and oceanic variability may essentially act as large -scale <br />`drivers' that change the long -term position of the jet stream which in turn, effects the <br />frequency and potency of storm activity that drives some of the variability in seasonal <br />precipitation and temperature values. <br />A brief summary of the HCI values can be found below: <br />Southern Oscillation Index (SOI): This index is one of the first and most basic indexes <br />that have been used in climatic -scale analysis. The variability that is inherent in this <br />index is embedded in the overall phenomenon known as the El Nino - Southern Oscillation <br />(ENSO) that is one of the most well -known factors in climatic /seasonable variability. <br />This index is simply a measure of difference in atmospheric pressure between Tahiti and <br />Darwin, Australia. Given that simple atmospheric pressure instruments are one of the <br />first and most basic measurements made by man, this index extends back a long period of <br />time into the late 1800's. <br />Nino 3.4: This index (not shown on Figure 1) is another variant of the ENSO <br />phenomenon. Nino 3.4 is simply defined as the Sea Surface Temperatures (SST) of the <br />region bounded by an area that is from 5 degrees north to 5 degrees south latitude and <br />from 170 degrees west to 120 degrees west. As of January 2005, the National <br />Atmospheric and Oceanic Administration officially declares conditions to be in an `El <br />Nino' condition when SST in this region are 0.5 degrees Celsius warmer than normal for <br />a three - consecutive month period and vice versa for La Nina. <br />• Multi - Variate ENSO Index (MEI): This index can best be described as an `all <br />encompassing' index to measure the strength and duration of ENSO events. This index is <br />comprised of indexed measurements of surface winds, pressure and temperatures along <br />with cloud cover and SST measurements. <br />Pacific Decadal Oscillation (PDO): This index is essentially measure a `north - south' <br />and to some extent and `east -west' difference in SST across the Pacific Ocean from the <br />equatorial regions to the North Pacific. This index does indicate its' highest degree of <br />variability on multi - decadal scales (mainly on the range of 30 years or so), however <br />embedded within these periods are year -to -year variations which is the reasoning for <br />inclusion in this study. <br />North Atlantic Oscillation (NAO): The NAO encompasses the most dominant mode of <br />variability in the atmosphere in the Atlantic Ocean and examines departures in the large - <br />scale departures in upper -level atmospheric patterns over the North Atlantic mid -and- <br />high latitudes (from 35 degrees to 70 degrees North). Although Pacific -based indexes are <br />more `intuitive' for Colorado in that most of the `weather' that effects arrives from areas <br />of Pacific origin. It is important to examine large -scale variability in the Atlantic as <br />weather in this region of the Northern Hemisphere can act to influence the storms that <br />arrive from Pacific regions, especially if the departures are strong. <br />5 <br />