Atmospheric Processes Associated with Summer Floods and Droughts in the Central United States

Abstract

Persistent wet and dry events over the central United States are examined during summer. Composites based on selected persistent wet and dry events reveal common atmospheric processes and circulation features. During summer, heavy precipitation in the central United States is accompanied by less precipitation to the south, in a band that extends from the tropical eastern Pacific through the Gulf of Mexico into the western Atlantic. Dry conditions also occur along the western coasts of Canada and Mexico during persistent wet episodes in the central United States. This rainfall pattern is supported by an inverse temperature?rainfall relationship over North America. During dry events, high pressure extends throughout a vertical column in a pattern that covers North America from 30° to 60°N. In contrast, during wet events, the high pressure is confined to the eastern half of North America, with low pressure prevailing in the western half. Increased northward meridional winds are found between this cyclonic?anticyclonic dipole, leading to increased moisture flux from the Gulf of Mexico at low levels. A significant precursor to wet events is the enhancement of westerlies over the eastern Pacific and western North America from 30° to 40°N. Synoptic-scale eddies intensify prior to onset and accelerate this westerly flow as revealed by Eliassen?Palm flux diagnostics. One pentad before onset, rainfall begins in Texas, and the low level jet (LLJ) in the Great Plains strengthens. The intensified LLJ transports moisture into the central United States and the moisture convergence downwind from the LLJ maintains rainfall. For dry events, heating occurs in the tropical eastern Pacific associated with the northward shift of the ITCZ roughly one pentad prior to onset. The prevailing easterly flow over subtropical portions of North America is not conducive to moisture transport into the United States, and without the support of moisture influx from the Gulf of Mexico, dry conditions prevail.