Detecting warming trends in the U.S. Midwest using a synoptic climatological approach

With its highly variable climate, the Midwest is one of the most sensitive regions when it comes to human responses to excessive heat. The hottest and most oppressive air mass types (yielding increased heat deaths) are categorized as dry tropical and moist tropical plus (DT and MT+). The goal of this study was to evaluate how these air masses have changed in both frequency and meteorological characteristics in five mid-western cities: Chicago, Cincinnati, St. Louis, Minneapolis, and Detroit, over the period 1946-2010 for the summer season. We found that the number of hot, humid days has increased, on average, in these cities since the 1940s and 1950s, while hot, dry days have become warmer during the night. During summer, there are statistically significant increases in the frequency of both oppressive air masses, and decreases in the coolest summer air mass, dry polar. For example, in Chicago during the three month summer period, there are 8 more MT days and 2.4 more MT+ days today on average than there were 63 years ago in the summer. For St. Louis, over the same period there are 10 more MT+ days on average, thus creating considerably greater risk for heat-related mortality. The frequency of dry polar (DP), which often brings relief from the heat, has decreased across the region. For example, in Detroit, we see 12 less DP days today on average, than in the 1950's. Character analysis for the summer season reveals statistically significant increases in overnight (0300h) air temperatures within the three air masses (DT, MT+, and DP) in St. Louis, Detroit and Chicago. In St. Louis, the DT overnight temperatures average 3oC higher today than 60 years ago. Causes for these increases were tested against small cities paired to each large city to determine whether these increases are a phenomenon of the urban heat island or of widespread climate change. Trends in the frequencies of summer air masses generally moved in the same direction in both larger cities and their smaller counterparts, diminishing the role of the heat island impact and suggesting a generalized climate change. We also completed consecutive day analysis of three or more days of oppressively hot air masses to determine if the prevalence of such extreme episodes has also been increasing. Results of consecutive DT/MT+ days showed that the five cities are increasing in the prevalence of such extreme episodes by 3 to 9 episodes per year on average, with all cities but Chicago doing so in a statistically significant fashion. Further expansion of the analysis across the contiguous U.S. and assessing heat-related mortality can give us a clearer image of the spatial nature and magnitude of the changes.