Changes in frost days in simulations of twentyfirst century climate

Global coupled climate model simulations of twentieth and twentyfirst century climate are analyzed for changes in frost days (defined as nighttime minima less than freezing). The model simulations agree with the observed pattern for late twentieth century of a greater decrease of frost days in the west and southwest USA compared to the rest of the country, and almost no change in frost days in fall compared to relatively larger decreases in spring. Associated with general increases of nighttime minimum temperatures, in the future climate with increased greenhouse gases (GHGs) the number of frost days is fewer almost everywhere, but there are greatest decreases over the western parts of the continents. The numbers of frost days are most consistently related to sea level pressure, with more frost days occurring when high pressure dominates on the monthly time scale in association with clearer skies and lower nighttime minimum temperatures. Spatial patterns of relative changes of frost days are indicative of regional scale atmospheric circulation changes that affect nighttime minimum temperatures. Increases of soil moisture and clouds also contribute, but play secondary roles. The linkages among soil moisture, clouds, sea level pressure, and diurnal temperature range are quantified by a statistical multiple regression model. Coefficients for present and future climate are similar among the predictors, indicating physical processes that affect frost days in present and future climates do not appreciably change. Only the intercept changes in association with the significant warming of the mean climate state. This study highlights the fact that, though there is a general decrease in the number of frost days with global warming, the processes that affect the pattern of those changes, and thus the regional changes of frost days, are influenced by several interrelated physical processes, with changes in regional atmospheric circulation generally being most important.