| A conceptual model of the mechanistic controls of North American climate variability on monthly-to-yearly time scales and regional-to-continental spatial scales is illustrated by cartographic visualization methods. Data from the NCEP/NCAR reanalysis project are analyzed to determine interactions among atmospheric circulation and surface energy- and water-balance components of the climate system. A general process-based description of intra-annual variations of the climate system of North America is presented to provide information on long-term average conditions, surface and atmospheric processes, and feedbacks.; Inter-annual climate variability related to ENSO variability is also examined. Composite-anomaly maps of the seasonal cycles of the two years that comprise strong El Niño and La Niña events are analyzed to address within-year progression of the events and among-year variations of anomalies. Temperature, mean sea-level pressure, 500mb geopotential heights, and 850mb specific humidity have anomaly patterns that exhibit the greatest degree of spatial and temporal coherence. In general, the composite-anomaly patterns for El Niño and La Niña events are of opposite sign. However El Niño events are of stronger sign and are more spatially coherent than La Niña events.; Examination of intra- and inter-annual variations of North American climate provides a context for understanding the processes that control extreme events like drought in the modern and paleo climate record. The Pacific-airmass model that has been used to describe mid-continental aridity (through enhanced westerlies) during the mid-Holocene is evaluated by examining modern circulation and surface-response features. The composite-anomaly patterns of 500mb geopotential heights show similar patterns for both anomalously wet and dry summer months, suggesting that primary circulation features alone (such as those that describe the strength of the westerlies) may not be appropriate indicators of surface-moisture anomalies. Instead, anomalously dry conditions in the mid-continent develop due to the dynamic interplay between surface conditions and atmospheric processes. Regional moisture-balance depends on the flux of atmospheric moisture into the region, recycling of soil moisture, and the atmospheric mechanisms that enhance or suppress precipitation. A consistent relationship exists today among moisture availability (determined by atmospheric moisture flux and soil moisture recycling), vertical motions in the atmosphere and surface conditions, which can be invoked to explain past variability. |
