WMOD00274 ‎(2)‎

• Title: Hydro-Climate Index Study - Final Report
• Prepared By: HDR Engineering
• Date: 2/14/2008
• County: Statewide
• State: CO
• Weather Modification - Doc Type: Report

It may seem at first that many of these regions are far away from Colorado
however, while keeping the large-scale system in mind we think of the words of Robert
A. Heinlein who once said “Climate is what you would expect, weather is what you get”.
These regions of atmospheric and oceanic variability may essentially act as large-scale
‘drivers’ that change the long-term position of the jet stream which in turn, effects the
frequency and potency of storm activity that drives some of the variability in seasonal
precipitation and temperature values. It should be noted that the variables used in the
tabulation of these HCI are driven by atmospheric and/or oceanic measurements that
were consistently and/or accurately measured starting in the late 1940’s. However,it
should be noted that some research groups have extrapolated certain HCI values back
and extra 50-120 years back based on courser datasets. Despite this the determinate
values such as precipitation and streamflow are still limited by observations that date
back to the late 1940s’.
A brief summary and explanation of the HCI values can be found below:
Southern Oscillation Index (SOI)
: This index is one of the first and most basic indexes
that have been used in climatic-scale analysis. The variability that is inherent in this
index is embedded in the overall phenomenon known as the El Nino-Southern Oscillation
(ENSO) that is one of the most well-known factors in climatic/seasonable variability.
This calculation of the index is a derivative of the measurement of differences in
atmospheric pressure between Tahiti and Darwin, Australia. Given that simple
atmospheric pressure instruments are one of the first and most basic measurements made
by man, this index extends back a long period of time into the late 1800’s.
Nino 3.4
: This index is another variant of the ENSO phenomenon. Nino 3.4 is simply
defined as the Sea Surface Temperatures (SST) of the region defined by an area that is
from 5 degrees north to 5 degrees south latitude and from 170 degrees west longitude to
120 degrees west longitude. As of January 2005, the National Atmospheric and Oceanic
Administration officially declares conditions to be in an ‘El Nino’ condition when SST in
this region are 0.5 degrees Celsius warmer than the 1971-2000 average for three-
consecutive months. If the SST in this ‘box’ is 0.5 degrees Celcius or colder than the 30-
year average then conditions are considered to be in a ‘La Nina’ state.
Multi-Variate ENSO Index (MEI):
This index can best be summarized as an ‘all
encompassing’ index to measure the strength and duration of ENSO events. This index is
comprised of indexed measurements of surface winds, pressure and temperatures along
with cloud cover and SST measurements.
Pacific Decadal Oscillation (PDO)
: This index is essentially measure a ‘north-south’
and to some extent and ‘east-west’ difference in SST across the Pacific Ocean from the
equatorial regions to the North Pacific. This index does indicate its’ highest degree of
variability on multi-decadal scales (mainly on the range of 30 years or so), however
embedded within these periods are year-to-year variations which is the reasoning for
inclusion in this study.
North Atlantic Oscillation (NAO):
The NAO encompasses the most dominant mode of
variability in the atmosphere in the Atlantic Ocean and examines departures in the large-
scale departures in upper-level atmospheric patterns over the North Atlantic mid-and-
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