Synoptic and dynamic climate controls of North American mid-continental aridity

The mid-continent of North America was likely drier than present during the mid-Holocene, based on inferences from fossil-pollen data and estimates of past lake levels. Mid-Holocene dry conditions have often been explained by increases in the dominance (frequency and/or duration) of Pacific airmasses, zonal flow patterns, or enhanced westerlies. We analyzed modern episodes between 1959 and 1997 when westerly flow was anomalously strong (an indication of zonal flow) to determine if zonal flow is associated with drier-than-normal conditions in the mid-continent. In contrast to that expectation, composite-anomaly patterns of 500 mb geopotential heights show similar meridional, as opposed to zonal, patterns for both anomalously dry and anomalously wet years, suggesting that large-scale circulation patterns alone may not provide a full explanation of surface-moisture anomalies. Anomalous moisture conditions in the mid-continent at present develop due to the dynamic interplay between surface conditions and atmospheric processes. Consideration of both moisture availability (determined by atmospheric moisture flux and soil-moisture recycling) and mechanisms for enhancing or suppressing precipitation (vertical motions in the atmosphere) is therefore required to describe the establishment of anomalously dry or wet conditions in the region today, as well as during the mid-Holocene.