HYDROCLIMATOLOGY OF CONTINENTAL WATERSHEDS .2. SPATIAL ANALYSES

We diagnose the spatial patterns and further examine temporal behavior of anomalous monthly-seasonal precipitation, temperature, and atmospheric circulation in relationship to hydrologic (soil water and potential evapotranspiration) fluctuations at two watersheds in the central United States. The bulk hydrologic balance at each of the two watersheds, Boone River, Iowa (BN), and Bird Creek, Oklahoma (BC), was determined from the rainfall-runoff-routing watershed model described in part 1. There are many similarities among the hydroclimatic linkages at the two basins. In both, relationships with precipitation and temperature indicate that the forcing occurs on regional scales, much larger than the individual watersheds. Precipitation exhibits anomaly variability over 500-km scales, and sometimes larger. Anomalous temperature, which is strongly correlated with potential evapotranspiration, often extends from the Great Plains to the Appalachian Mountains. Seasonally, the temperature and precipitation anomalies tend to have greatest spatial coherence in fall and least in summer. The temperature and precipitation tend to have out-of-phase anomalies (e.g., warm associated with dry). Thus low soil water conditions are reinforced by low precipitation and high potential evapotranspiration, and vice versa for high soil water. Soil water anomalies in each basin accumulate over a history of significant large-scale climate forcing that usually appears one or two seasons in advance. These forcing fields are produced by atmospheric circulation anomaly patterns that often take on hemispheric scales. BN and BC have strong similarities in their monthly circulation patterns producing heavy/light monthly precipitation episodes, the primary means of forcing of the watersheds. The patterns exhibit regional high or low geopotential anomalies just upstream over the western United States or near the center of the country. The regional circulation features are often part of a train, with teleconnections upstream over the North Pacific and downstream over the North Atlantic/Eurasia sector. Synoptic scale events exhibit very similar patterns to the monthly circulations, only more intense.