Secular Variations in Objectively Defined Climatological Seasons in the United States

Statistically significant increases in lower tropospheric and surface temperatures have been reliably documented over the period from the mid-1970s to the present. Many questions have been raised regarding the effects these increases may already have had on a variety of climatological factors. This thesis investigated the potential effect of global and continental scale temperature increases on the timing of the climatological seasons, based on records from 30 locations of varying characteristics in the contiguous United States. To accomplish this, Summer and Winter were defined by the warmest/coldest 91-day periods within each year (across calendar boundaries for winter). The resulting objective seasons were analyzed for secular trends of season start dates and season mean temperatures across the period 1974–2017. Additional relationships investigated included objective seasons compared to fixed season temperatures and analyses broken down by station characteristics. Mixed results were obtained from these analyses regarding statistically significant (α=0.05) relationships, except for the consistently close relationship between objective and fixed season temperatures. As would thus be expected, most stations showed significant secular temperature trends (upward) in the objective seasons. Despite the paucity of significant results regarding secular trends in the objectively defined seasons, the objective season development in this work opens the door for additional investigations regarding other relationships and effects with early or late onset of winter and summer, as well as the implications for shorter or longer transitional seasons.