Variability of the Great Plains Low-Level Jet: Large Scale Circulation Context and Hydroclimate Impacts

Title of Document: VARIABILITY OF THE GREAT PLAINS LOW-LEVEL JET: LARGE SCALE CIRCULATION CONTEXT AND HYDROCLIMATE IMPACTS Scott J. Weaver, Doctor of Philosophy, 2007 Directed By: Professor Sumant Nigam, Department of Atmospheric and Oceanic Science Variability of the Great Plains Low-Level Jet (GPLLJ) is analyzed from the perspective of larger-scale, lower-frequency influences and regional hydroclimate impacts; as opposed to the usual analysis of its frequency, diurnal variability and mesoscale structure. The circulation-centric core analysis is conducted with monthly and pentad data from the high spatio-temporal resolution, precipitation-assimilating North American Regional Reanalysis, and ERA-40 global reanalysis (as necessary) to identify the recurrent patterns of GPLLJ variability and their large-scale circulation and regional hydroclimate links. The analysis reveals that GPLLJ variability is, indeed, linked to coherent, largescale, upper-level height patterns over the Pacific, and NAO variability in the Atlantic. A Rossby Wave Source analysis shows the Pacific height pattern to be potentially linked to tropical diabatic heating anomalies in the west-central basin and in the eastern Pacific sector. EOF analysis of GPLLJ variability shows it to be comprised of three modes that exert profound influence on Great Plains precipitation variability, and together, account for ~75% of the variance. Ocean basin centered EOF analysis on summertime SLP anomalies shows similar GPLLJ and precipitation impacts as those found in the Great Plains centric perspective, supporting the claim for remotely generated influences on Great Plains low-level jet and hydroclimate variability. Pentad analysis of the atmospheric and terrestrial water balances during the 1988 drought and 1993 flood show that, jet variability, while influential over many of the subseasonal anomalous precipitation episodes was not a necessary condition for precipitation anomalies. Great Plains evaporation exhibited a 2-week delay with respect to precipitation suggesting a minor role for precipitation recycling during these events. ENSO and NAO variability are shown to contribute significantly to the large midsummer positive precipitation anomalies during 1993. EEOF analysis of pentad 900 hPa meridional winds during MJJ show three temporally stable modes of variability, each exhibiting similar spatial characteristics to the monthly EOF spatial patterns. Lead/lag regressions show a one pentad delay in moisture flux convergence generated precipitation anomalies, perhaps, suggesting the importance of moisture transports in generating Great Plains precipitation anomalies. Climate models are shown to be challenged in depicting the jet and precipitation variability over the Great Plains. VARIABILITY OF THE GREAT PLAINS LOW-LEVEL JET: LARGE SCALE CIRCULATION CONTEXT AND HYDROCLIMATE IMPACTS