ENSO and PDO Effects on Hydroclimatic Variations of the Upper Colorado River Basin
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2003-02Author
Hidalgo León, Hugo G.
Dracup, John A.
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Linkages between tropical Pacific Ocean monthly climatic variables and the Upper Colorado River basin
(UCRB) hydroclimatic variations from 1909 to 1998 are analyzed at interseasonal timescales. A study of the
changes in these linkages through the years and their relationship to the Pacific Decadal Oscillation (PDO) is
also investigated. Tropical Pacific climate variations were represented by atmospheric/oceanic ENSO indicators.
For the UCRB, warm season (April–September) streamflow totals at Lee’s Ferry, Arizona, and precipitation
averages at different periods (cold season: October–March; warm season: April–September; and annual: October–
September) were used to study the UCRB’s response to tropical Pacific climatic forcing. A basinwide ENSO
signature was found in the significant correlations between warm season precipitation in the UCRB and warm
season SST averages from the Nin˜o-3 region in most of the stations around the UCRB. This link is more evident
during the warm phase of ENSO (El Nin˜ o), which is associated with an increase in warm season precipitation.
The analysis also showed a link between June to November ENSO conditions and cold season precipitation
variations contained in a principal component representing the high-elevation precipitation stations, which are
the main source of streamflow. However, the amplitude and coherence of the cold season ENSO signal is
significantly smaller compared to the general precipitation variations found in stations around the UCRB. Only
when very few stations in the high elevations are considered is the ENSO signal in cold season precipitation
in the basin revealed. Interdecadal hydroclimatic variations in the UCRB related to possible PDO influences
were also investigated. There are significant shifts in the mean of UCRB’s moisture-controlled variables (precipitation
and streamflow) coincident with the PDO shifts, suggesting a connection between the two processes.
It has been suggested in other studies that this connection could be expressed as a modulation on the predominance
of each ENSO phase; that is, strong and consistent winter El Nin˜o (La Nin˜a) patterns are associated with the
positive (negative) phase of the PDO. In the UCRB this apparent modulation seems to be accompanied by a
general change in the sign of the correlation between ENSO indicators and cold season precipitation in most
stations of the basin around 1932/33. From 1909 to 1932 the basin has a predominantly cold season ENSO
response characteristic of the northwestern United States (drier than normal associated with tropical SST warming
and vice versa); from 1933 to 1998 the response of the basin is predominantly typical of the southwestern United
States during winter (wetter than normal associated with tropical SST warming and vice versa). This apparent
correlation sign reversal is suggested to be related to interdecadal changes in the boundary of the north–south
bipolar response characteristic of the ENSO signal in the western United States during winter.
External link to the item
10.1175/1525-7541(2003)004<0005:EAPEOH>2.0.CO;2Collections
- Meteorología [509]