Transcript ppt
The structure and nature of warmseason 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 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 Standard Deviation of monthly precipitation during summer (J,J,A) in NARR (1979-1998). Great Plains Precipitation index in the warm-season (J,J,A) in NARR and US-Mexico data sets. 1993 1998 1958 1950s 1970s 1983-84 1998 Precipitation Precipitation Tr. Moisture Flux & Total St. Moisture Flux&& MoistureFlux Evaporation MF Convergence MF Convergence MF Convergence 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 This suggests the dominance of remote water sources over local recycling of precipitation for precipitation variability over the Great Plains. 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 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? 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 -0.43. 1970s 1988 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 -0.33. 1955 1958 1976 1983-84 1993 1998 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 midtroposphere 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 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.