The frequency of heatwaves (defined as daily temperature exceeding the local 90th percentile for at least three consecutive days) during summer in the U.S. is examined for daily maximum and minimum temperature and maximum apparent temperature, in recent observations and in 10 CMIP5 models for recent past and future. The annual average percentage of days participating in a heatwave varied between approximately 2 and 10% in observations and in the model’s historical simulations during 1979-2005. Applying today’s temperature thresholds to future projections, heatwave frequencies rise to more than 20% by 2035-2040. However, based on the models’ slight overestimation of frequencies and positive trend rates during 1979-2005, these projected heatwave frequencies should be regarded cautiously. The models’ overestimations may be associated with their higher daily autocorrelation than found in observations. Heatwave frequencies defined using apparent temperature, reflecting both temperature and atmospheric moisture, are projected to increase at a slightly (and statistically significantly) faster rate than for temperature alone.

Analyses show little or no changes in the day-to-day variability or persistence (autocorrelation) of extreme temperature between recent past and future, indicating that the future heatwave frequency will be due predominantly to increases in standardized (using historical period statistics) mean temperature and moisture content, adjusted by the local climatological daily autocorrelation. Using nonparametric methods, the average level and spatial pattern of future heatwave frequency is shown to be approximately predictable based only on projected mean temperature increases and local autocorrelation.

These model-projected changes, even if only approximate, would impact infrastructure, ecology, and human well-being.

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