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Heat Waves in Mediterranean climate regimes: focus on California Alexander Gershunov Climate, Atmospheric Science and Physical Oceanography (CASPO) Scripps Institution of Oceanography La Jolla, California Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 Regional Heat Waves are Changing End of July 2006 INDIVIDUAL HEAT WAVES The heat wave of July 2006 was an unprecedented deadly event. 0 2 4 6 daytime magnitude nighttime magnitude 1950 1960 1970 1980 1990 2000 147 total deaths 99% of cases lived in zip codes where > 50% of residents live below Poverty Guide Line 8 TOTAL HEAT WAVE ACTIVITY 6 Day > Night magnitude Night > Day magnitude 4 California heat wave activity is increasing. 0 2 Specifically, nighttime-accentuated heat waves are on the rise… 1950 1960 1970 1980 1990 2000 Is this a feature of Mediterranean climates in general? Quantifying Regional Heat Waves • Heat Wave Index, related to impacts: MAGNITUDE = intensity + duration + spatial extent • Day and nighttime heat waves • Defined regionally from daily weather station data: local temperatures exceeding a threshold Working definition of extreme heat: Daytime Intensity Sacramento Tmax o o o o o o o o o o 2006 Tmax o threshold (42.2C/108F) o o oo o o o o oo o o o o oo o ooo o oo o oo o o o o o o o o ooooo o oo ooooooooooooooooooooooooo o o o o o o o o o o o o o oooo oooooooo oo o ooooooooo oooo o oo o oo o oooo o oo ooo ooo ooooo o o o o o o oo o oo o o o o oo o oo oo th percentile in C of JJA Tmax over oooo o o ooooooooo o 99 o o o the base period 1950 – 1999. o oo Temperatures have a climatological 20 25 30 35 40 45 • Locally extreme temperature exceeding a high percentile threshold t* (e.g. 99%-ile) 99th percentile thresholds o oo o o ooo o o o o o o o o o o o o oo o o o o oo oo o ooo o o ooo o oo ox o o o 43.3 o oo o o o o oo oo oo 42.3 o 41.7 o oo o oo o o 40.6 o oo o o o 39.4 o 38.7 o o oo o o ooo o 36.2 o oo o o o 35 o 32.4 o oo o 22.2 15 Mean Tmax 5 10 15 20 25 30 Jun 5 10 15 20 25 30 Jul 5 0.01 probability of exceeding these thresholds and should be considered locally extreme. 10 15 20 25 30 Aug Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 Working definition of extreme heat: Nighttime Intensity Sacramento Tmin 99th percentile thresholds • Locally extreme temperature exceeding a high percentile threshold t* (e.g. 99%-ile) o o ooo o o o o o o o o o o oooooo oo oo oo ooooooooooooooooooo ooo o o oooo oooo o o o o o o o ooo ooooooooooooo oooo oooooooooooooooooo o o o o o o o o o oo oo ooo ooo ooo ooooo o o o oo o o oo oooooooo o ooo o o o o 10 15 20 25 o oo o o ooo o o o o o o o o o o o o oo o o o o oo oo o oo o oo ooo x o oo o o o o o 29C/84F o o oo 26.5 o o o o o o oo o oo 24 2006 Tmin o o 22.8 o oo o oo o oo o o 21.1 o o o o oo o o o 18.9 o o o o o threshold (22.8C/73F) oo 18.3 oo o o ooo o 17.2 o o oo o o o o o o 16.1 o o oo o o oo 15 o o o o o 12.8 o o oo oo o o Mean Tmin 5 10 15 20 25 30 Jun 5 10 15 20 25 30 Jul 5 10 15 20 25 30 Aug 99th percentile in C of JJA Tmin over the base period 1950 – 1999. Temperatures have a climatological 0.01 probability of exceeding these thresholds and should be considered locally extreme. Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 An Index of California Regional Heat Wave Activity Nighttime Heat Wave Activity 2.0 1+nights 2+nights 3+nights 1.0 2006 2003 0.0 4 4 0.5 Daytime heat wave activity has increased somewhat 6 6 1+day s 2+day s 3+day s 1.5 8 8 Daytime Heat Wave Activity 1960 1970 1980 1990 2000 0 0 2 2 1950 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000 Nighttime heat wave activity is ramping up for all local durations Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 Tmin Tmax Seasonal Maxima of Regional Heat Wave Components Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 GREAT HEAT WAVES AND THEIR OVERALL REGIOINAL MAGNITUDES Local intensity + duration + spatial extent in degree days summed over the region for the six greatest daytime and six greatest nighttime events 6 daytime magnitude nighttime magnitude 4 2003 1972 1960 2 1961 jun 1961 aug 2002 1981 1983 1990 1992 2001 1990 2000 0 degree days per station 2006 1950 1960 1970 1980 Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 What is the meteorological difference between daytime and nighttime heat waves? Surface circulation (wind at sigma level 995, arrows in m/s) and mean sea level pressure in millibars (a,b), 500mb geopotential height in meters (c,d), and precipitable water kg/m2 (e,f) anomalies with respect to JJA mean. Anomalies are composited for the peak days of the largest five daytime events (a,c,e) and the largest five nighttime events (b,d,f). The data are from the NCEP/NCAR Reanalysis I. Red rectangles outline regions used for evolution plots presented below. Contours and colors represent the same anomalies, but only values statistically significant with 95% confidence (two-tailed test) determined via bootstrap re-sampling (performed with 1000 resampled 5-date composite anomaly maps) are plotted in color. Low level wind vectors are colored blue where significant according to similar re-sampling test performed for the u- and v-components separately. Significance is everywhere a function of magnitude and location. The reference period for computing anomalies is 1950 – 1999, as elsewhere. The anomalies are computed from 24hour averaged fields. Moisture is what makes the difference between day and nighttime heat waves in this arid region WHY THIS CHANGE IN HEAT WAVE ACTIVITY? 2 4 6 8 IS IT NATURAL? IS IT ANTHROPOGENIC? Day > Night magnitude IS IT REGIONAL? Night > Day magnitude IS IT GLOBAL? IS IT ALL OF THE ABOVE? 0 • • • • • 1950 1960 1970 1980 1990 2000 Is there a trend in humidity? Where does the moisture come from? a) Linear trend in Precipitable Water (Reanalysis 1) -0.5 0.0 b) PWTR Trend off Baja California oo 0.5 o 60.0 26 57.5 55.0 o o oo 52.5 ooo o 24 50.0 47.5 o 45.0 22 42.5 40.0 37.5 32.5 o o oo o oo o o o o o o o o o o o o o o o o o o o o o o o o o o ooo o o 20 35.0 o o o o o o o o 30.0 18 27.5 25.0 -160.0 -147.5 -135.0 -122.5 -110.0 -97.5 -87.5 -77.5 -67.5 o 1950 1960 1970 1980 1990 2000 Linear trend computed at each pixel of the PRWTR averaged for July. Significant trends (95% significance level in a two-tailed test) are colored. (b) July PRWTR in the box [132.5125W, 25-35N] and linear trend significant well above the 99% level for both the full and base (not shown) periods. Gershunov, Cayan and Iacobellis, Journal of Climate, 2009 Why this moisture trend? Regional SST warming Linear trend in sea surface temperature (C/decade) -0.1 0.0 0.1 0.2 o o o oo oo o o -0.5 20 0.0 0.5 40 o oo oo o o o -1.0 200 250 o o oo o o oo 0 150 o o 1.0 60 1.5 -0.2 SST Trend off Baja California o o o o oo o o o oo o o o o o o o 1950 o o o o o o o o o o o o oo 1960 1970 1980 1990 2000 As this region becomes warmer and more moist, the atmospheric circulation associated with heat waves, tends to bring humid air into California, giving heat waves a stronger nighttime expression. GLOBAL TEMPERATURE Variations of the Earth’s surface temperature Departures in temperature in oC (from the 1990 value) • As temperature around the globe continues to rise, we can expect changes in specific regional weather extremes: Projections • Heat waves • Cold spells • What changes do models show in summertime heat waves in California? 8 6 4 2 0 o F o C Proxy Instrument California Heat Waves and Global Climate Change: CNRM PAST PRESENT FUTURE 8 50 ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO 20 0 2 30 4 40 6 observations 1950 1960 1970 1980 1990 2000 Night > Day magnitude 0 10 Day > Night magnitude 1900 1950 2000 2050 2100 California Heat Waves and Global Climate Change: CNRM PAST PRESENT FUTURE 8 50 ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO SUGGESTS THAT THE OBSERVED CHANGE IS A TIP OF THE ICEBERG 20 0 2 30 4 40 6 observations 1950 1960 1970 1980 1990 2000 Night > Day magnitude 0 10 Day > Night magnitude 1900 1950 2000 2050 2100 California Heat Waves and Global Climate Change WHAT DO OTHER MODELS SHOW? California Heat Waves and Global Climate Change: GFDL 80 ANOTHER CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO SUGGESTS INCREASE, BUT IN THE OPPOSITE SENSE PRESENT FUTURE 8 PAST 0 40 2 4 6 60 observations 20 1950 1960 1970 1980 1990 2000 Day > Night magnitude 0 Night > Day magnitude 1900 1950 2000 2050 2100 What’s happening in other Mediterranean Climate Regimes? SPAIN Tmax Tmin At least one, two and three day/night local duration events Courtesy of Maria Jose Ortiz Bevia Conclusions In California: • Great heat waves are primarily day or nighttime events caused by specific pressure, wind and moisture patterns • Tendency towards more humid heat waves and unprecedented nighttime temperatures have implications for energy demand, agriculture, public and animal health… • Climate models suggest trends will continue and accelerate – BUT NOT ALWAYS IN THE OBSERVED SENSE • Is the observed trend over California and Nevada reflected over other world regions with similar “Mediterranean” climates? • Spanish heat waves are increasing somewhat consistently with their Californian sisters, but without a disproportionate nighttime intensification • Climate change is playing out with regional idiosyncrasies