Factors Contributing to Regional and Seasonal Trends in Temperature Extremes

Temperature extremes arise from multiple physical processes. Identifying the relative roles of these processes is essential for explaining past regional and seasonal trends in temperature extremes. Here, we examine regional trends in daily temperature extremes in winter and summer over Northern Hemisphere land areas since 1979. We first consider to what extent changes in external radiative forcing have constrained regional trends in seasonal-mean temperatures and highest and lowest daily extremes over this period. We then assess the sensitivity of the same trends to internal climate variations through climate model experiments subject to specified boundary forcings and unforced internal atmospheric variability. Several physical processes contribute to regional trends in temperature extremes over this period, with their relative importance varying by season and geographic location. The results highlight the need to consider such factors in estimating future regional trends in temperature extremes, as well as possible ranges in trends over specific time periods.