Future Projections of the Large-Scale Meteorology Associated with California Heat Waves in CMIP5 Models

Previous work showed that climate models capture historical large-scale meteorological patterns (LSMPs) associated with California Central Valley heat waves including both ways these heat waves form. This work examines what models predict under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 scenarios. Model performance varies, so a multimodel average weights each model based on its historical performance in four parameters. An LSMP index (LSMPi) defined using upper atmosphere variables captures dates of past extreme surface temperature maxima. LSMPi correlates well with all values of California Central Valley-average surface maximum temperature. LSMPi distributions in future simulations shift similar to 0.6 standard deviations higher between 1961-2000 and 2061-2100 for RCP8.5 data. Based on the historical climatology, future scenarios show a large increase in the frequency and duration of heat waves in every model. Four times as many heat waves occur and their median duration doubles, using historical thresholds. Of the two ways heat waves form, Type 1 has similar frequency in the future. But, Type 2 becomes much more common because Type 2 has a preexisting hot anomaly in Southwestern Canada, much like the historical to future climatological change in that region (a "global warming" signal). The 20-year return value anomaly increases by 30-40%. The average of the 50 hottest temperatures increases 3.5-6 K depending on the scenario. When extreme values are defined using the future climatology, the models and their average have no consistent increase or decrease of distribution properties such as shape, scale, and return values of the extremes compared to historical values. Plain Language Summary The hottest heat waves in California occur during specific weather patterns. Computer models that simulate global climate can include such patterns because the patterns have large horizontal size. We average results from 13 climate models. More weight is given those models that are better than other models at generating these weather patterns and other properties of California heat waves during 1961-2000. A heat wave is when the daily maximum temperature at each Central Valley location is among the warmest 5% of June-September days during the years 1971-2010. We examine two scenarios for the future, one with continued increase in "greenhouse" gases and another that has less increase. Our averages find several things for 2061-2100. In the future, depending on which scenario, daily maximum temperatures will exceed those highest 5% values 6 to 8 times as often as now, about one fifth to one quarter of summer. The hottest days will be 3.5-6 degrees C (about 7-12 degrees F) hotter than the hotter days in recent memory. Those heat wave weather patterns are neither stronger nor occur more often in the future; instead, many more heat waves occur in future simulations from a general warming of western North America.