Determining the Spring to Summer Transition in the Missouri Ozarks Using Synoptic Scale Atmospheric Data

Abstract

There is abundant anecdotal evidence available to suggest that the transition from a spring to summer season flow regime is often quite abrupt. This same transition renders longrange forecasting problematic as the forecast time period crosses through the spring and summer seasons. Despite these problems, the transition from spring-to-summer flow regimes is a problem that has not been examined in detail in the published literature. In this study, the transition is examined from a regional perspective over a 20-year period (1981 - 2000) and includes the development of a criterion for identifying the transition based on using routinely available synoptic observations. Within the East-central Ozarks region of Missouri, the transition from spring-to-summer season flow regimes is often abrupt, and is identified as occurring in mid-June. The transition could also be identified for the entire Northern Hemisphere using the 500 hPa wave amplitude index for some years during the 1980's. The results found here are consistent with the results of the one other study found in the literature that also addresses the spring-to-summer transition issue for the entire Northern Hemisphere. Additionally, this study found that the average date of summer onset in the region is June, and the 20-year set of summer onset dates was normally distributed with respect to this mean. It was also shown that there is an abrupt change in the average period between heavy precipitation events. Finally, it is demonstrated that late arriving summers are generally associated with a transition in the phase of the El Nino and Southern Oscillation (ENSO), especially the La Nina phase, while early arriving summers are generally associated with steady-state ENSO conditions.