FLOOD10416

• County: Statewide
• Title: Historical Perspectives on Colorado Drought
• Date: 12/1/2003
• Prepared For: CWCB
• Prepared By: HDR
• Floodplain - Doc Type: Educational/Technical/Reference Information

Historical Perspectives on Colorado Drought

II.

Figure 21 was prepared by plotting the sum afthe basin decadal differences in the
number nf2-inch above and below annual precipitation events by dt.."'Cade for the
Platte, Colorado. Arkansas, and Rio Grande basins. This analysis portrays the
number of"extrcmc" events and their decadal changes.

The precipitation rich decades of the 191 Os and 19205 and the recent wet 19805 and
19905 arc easy to pick out. Conversely, the drought or dry periods orlhe "turn oflhe
century." 19305. and 1950's to 19705 can be looked at from a relative stance. NOle
that the two wet periods orlhe past millennium appear to provide less durational
impact than the entire extended dry period orlhe 19305 through the 19705.

The extended period oflhe dry 50s, 60s and 70s offers an amazing difference in
duration and intensity compared to the "spike" of the Dust Howl 1930s. Why'? Are
there differences from basin to basin or do the basins operate more in tandem then
separation'! Why'? Perhaps, the answer is in the meteorological causes of the dry and
wet periods.

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2005

2000

199~

o..tka eMI.,
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970 1975 1980 198~ 1990
Blue periods indicate La Nina periods

1950 1955 1960 1965
Red values Indicate EI Nino periods

1950-2003 (Wolter and Timn, 1998)

El Nino/Southern Oscillation (ENSO) is the most important coupled ocean.
atmosphere phenomenon to cause global chmate variability on interannual time
scales. Wolter and Timn (1998) monitor ENSO by basing the Multivariate ENSO
Index (MEI) on the six main observed variables over the tropical Pacific. These six
variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the
surface wind, sea surface temperature (5), surface air temperature (A), and total
cloudiness fraction of the sky (C).

Figure 22 shows the variation of this MEI index from 1950 to 2003. Note that the
red values above 0.5 indicate El Nino periods while the blue values of -0.5 or less
indicate La Nina periods. Note the regular cycles of the E1 Nino and La Nina.

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Annual variation of the MEI

Figure 22