Heat wave probability in the changing climate of the Southwest US
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Analyses of observed non-Gaussian daily minimum and maximum temperature probability distribution functions (PDFs) in the Southwest US highlight the importance of variance and warm tail length in determining future heat wave probability. Even if no PDF shape change occurs with climate change, locations with shorter warm tails and/or smaller variance will see a greater increase in heat wave probability, defined as exceedances above the historical 95th percentile threshold, than will long tailed/larger variance distributions. Projections from ten downscaled CMIP5 models show important geospatial differences in the amount of warming expected for a location. However, changes in heat wave probability do not directly follow changes in background warming. Projected changes in heat wave probability are largely explained by a rigid shift of the daily temperature distribution. In some locations where there is more warming, future heat wave probability is buffered somewhat by longer warm tails. In other parts of the Southwest where there is less warming, heat wave probability is relatively enhanced because of shorter tailed PDFs. Effects of PDF shape changes are generally small by comparison to those from a rigid shift, and fall within the range of uncertainty among models in the amount of warming expected by the end of the century.
This work was supported by DOI via the Southwest Climate Science Center on a grant titled: “Natural variability in the changing climate: Interaction of interannual, decadal, and century timescales with daily weather”, by the National Science Foundation via Grant #F12078-2013-005, and by NOAA via the RISA program through the California and Nevada Applications Program. The observational temperature dataset is freely and publicly available from the University of Washington (ftp://ftp.hydro.washington.edu/pub/blivneh/CONUS/). The LOCA downscaled GCM data are publically available from the USGS Center for Integrated Data Analytics. We thank Mary Tyree for data retrieval and handling. We additionally thank three anonymous reviewers for providing helpful comments during the evaluation of this paper.
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