1. Alfredo Ruiz-Barradas, and Sumant Nigam
The structure and nature of warm-season interannual variability of the Great Plains hydroclimate
Alfredo Ruiz-Barradas, and Sumant Nigam
Department of
Atmospheric and Oceanic Science
University of Maryland
Predicting Drought on Seasonal to Decadal Time Scales
Adelphi, MD.
May 17-19, 2005

2. Outline Precipitation variability over the Great Plains.
Structure of hydroclimate fields and their relative contributions associated with precipitation anomalies.
Pacific and Atlantic connections
Conclusions
References

3. Standard Deviation of monthly
precipitation during summer
(J,J,A) in NARR ( ).
Great Plains Precipitation index
in the warm-season (J,J,A) in
NARR and US-Mexico data sets.
1993
1998
1958
1998
1950s
1970s

4. Warm-season regressions of the GPP index show:
Precip anomalies are not linked
with precip anomalies over other continental regions
Significant moisture fluxes from the Gulf of Mexico and associated convergence
Weak connection with the Pacific
Low evaporation
Precipitation
Precipitation
Total Moisture Flux &
MF Convergence
St. Moisture Flux &
MF Convergence
This suggests the dominance of remote water sources over local recycling of precipitation for precipitation variability over the Great Plains.
Tr. Moisture Flux &
MF Convergence
Evaporation

5. Correlation between July’s precipitation and previous months’ precipitation
Low dependence of precipitation
anomalies on preceding precipitation anomalies suggests a weak local recycling of precipitation

6. SST-leading Correlations with July GPP index (1950-1998)
Links to both Pacific and Atlantic
basins
- From ENSO-like pattern
to PDO-like structure
- Atlantic forcing too?

7. Pacific Mode (7th)
Rotated EOFs of extratropical
(155°E-15°W,25°N-60°N)
Φ(700mb) & SST show
linkages to North American
summer hydroclimate variability
PNA-like pattern
Moisture transport at
mid-troposphere levels
Not moisture fluxes to/
from the Gulf of Mexico
Correlates with the GPP
index at
1970s
1988

8. Atlantic Mode (1st)
NAO-like pattern
Moisture transports at
low-troposphere levels
Moisture fluxes to/from
the Gulf of Mexico
Correlates with the GPP
index at
1976
1955
1993
1958
1998

10. Conclusions
Remote water sources dominate over local water sources (precipitation recycling) in the generation of precipitation variability.
Pacific’s link with the Great Plains hydroclimate is through northeastward moisture transports across the Gulf of California, at mid- troposphere levels.
The Atlantic connection, on the other hand, is through NAO’s influence on the Bermuda High, and the resulting interaction of circulation with North American orography; all of which serve to modulate the low-level moisture transports from the Gulf of Mexico

11. References
Nigam, S., and A. Ruiz-Barradas, 2005: Seasonal hydroclimate variability over North America in ERA-40, Regional Reanalysis and AMIP simulations. Submitted to J. Climate.
Ruiz-Barradas, A., and S. Nigam, 2005a: Warm-season Precipitation Variability over the US Great Plains in Observations, NCEP and ERA-40 Reanalyses, and NCAR and NASA Atmospheric Simulations. To appear in the 1 June issue of the J. Climate.
______, ______, 2005b: Great Plains Hydroclimate Variability: The View from the North American Regional Reanalysis. Submitted, J. Climate.