Atlantic Hurricanes and Climate Change: Modeling Studies Hurricane Katrina, Aug GFDL model simulation of Atlantic hurricane activity Tom Knutson Geophysical Fluid Dynamics Lab/NOAA Princeton, New Jersey 1 Collaborators: Joe Sirutis Isaac Held Gabe Vecchi Bob Tuleya Morris Bender Steve Garner Ming Zhao S.-J. Lin

Source: Vecchi et al. Science (2008) Projection 1: Absolute SST ~300% projected increase in Power Dissipation Indirect attribution: CO2  SST  Hurricanes Projection 2: Relative SST Projected change: sign uncertain, +/- 80% No Attribution Statistical projections of 21 st century Atlantic hurricane activity have a large dependence on the predictor used. 4

Zetac Regional Model reproduces the interannual variability and trend of Atlantic hurricane counts ( ) 18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses 3 Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.

Projected Atlantic region climate changes: 18-Model CMIP3 ensemble Higher shear Higher potential intensity

1) Decreased frequency of tropical storms (-27%) and hurricanes (-18%). 3) Caveat: this model does not simulate hurricanes as strong as those observed. The model provides projections of Atlantic hurricane and tropical storm frequency changes for late 21 st century, downscaled from a multi-model ensemble climate change (IPCC A1B scenario): Storm Intensities (Normalized by frequency) 4) A more consistent intensity increase is apparent after adjusting for decreased frequency 2) Increased frequency and intensity of the strongest hurricanes (5  12) Source: Knutson et al., 2008, Nature Geoscience. 5

6 Zetac Model Downscaling: 9 individual CMIP3 models now completed (plus the 18-model ensemble mean) A consistent decrease in tropical storms becomes a mixture of decreases and increases for major (Cat 3) hurricanes (in terms of central pressure) Provisional Results: Do not quote or cite.

Tropical Cyclones Frequency Projections (Late 21 st century) - Summary Blue = decrease Red = increase Source: Knutson et al. 2010

The new model simulates increased hurricane rainfall rates in the warmer climate (late 21 st century, A1B scenario) …consistent with previous studies… Present Climate Warm Climate Warm Climate – Present Climate Rainfall Rates (mm/day) Avg. Rainfall Rate Increases: 50 km radius: +37% 100 km radius: +23% 150 km radius: +17% 400 km radius: +10% Average Warming: 1.72 o C Source: Knutson et al., 2008, Nature Geoscience. 8

A further downscaling step to 9-km triply nested GFDL hurricane model Morris Bender, et al., Science, Feb

18km grid Zetac regional climate model 9 km GFDL hurricane model observed Simulated distributions of maximum wind speeds (Atlantic tropical storms and hurricanes, ) Down-scaled GFDL hurricane prediction model produced much more realistic distribution of maximum wind speeds compared to Zetac Maximum Wind Speed (m/s) Normalized occurrences Source: Bender, et al., Science, 2010.

Distributions (CDF’s) of Atlantic tropical cyclone intensities ( ). Red: (inactive) Blue: (active) Fraction of storms above indicated intensity Source: Bender et al., Science, GFDL Hurricane Model intensity distribution is also shifted to higher intensities in active years, but the difference is smaller than observed. 11 Active era Inactive era Active era Inactive era Maximum Wind Speed (m/s) Observed intensities Simulated intensities

12 In a warmer climate (late 21 st century A1B scenario) the hurricane model simulates an expanded distribution of Atlantic hurricane intensities. The strongest hurricanes increase in number for the downscaled ensemble-mean climate warming… …and increase for 3 of 4 individual climate models tested. Control Source: Bender et al., Science, 2010.

Late 21 st Century Climate Warming Projection-- Average of 18 CMIP3 Models (27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010

Control Climate (Odd Years Only) GFDL-CM2.1 MRI-CGCM MPI-ECHAM5 UKMO-HADCM3 Degrees Longitude East Degrees Latitude NWS VERSION (GFDL) Tracks of Storms that Reached Category 4 or 5 Intensity Degrees Longitude East Degrees Latitude Late 21st Century Warmed Climate Projection based on 4 Individual CMIP3 Climate Models

SUMMARY OF PROJECTED CHANGE Colored bars show changes for the18 model CMIP3 ensemble (27 seasons); dots show range of changes across 4 individual CMIP models (13 seasons). Cat 4+5 frequency: 81% increase, or 10% per decade Source: Bender et al., Science, Estimated net impact of these changes on damage potential: +28%

Emergence Time Scale: If the observed Cat 4+5 data since 1944 represents the noise (e.g. through bootstrap resampling), how long would it take for a trend of ~10% per decade in Cat 4+5 frequency to emerge from noise? Answer: ~60 yr (by then 95% of cases are positive) 16 Source: Bender et al., Science, Instead, assume residuals from a 4 th order polynomial: 55 yr Instead, resample chunks of length 3-7 yr: yr

GFDL HIRAM 50km grid global model: Simulated vs Observed Tropical Storm Tracks ( ) Source: Zhao et al. J. Climate (2009) 17

HIRAM 50 km grid model TC correlations for several basins North Atlantic East Pacific West Pacific red: observations blue: HiRAM ensemble mean shading: model uncertainty corr=0.83 corr=0.62 corr=0.52 Hurricane counts for each basin are normalized by a time-independent multiplicative factor Correlation for the South Pacific is ~0.3 and insignificant for the Indian Ocean Source: Zhao, Held, Lin, and Vecchi (J. Climate, 2009) 18

Projected Changes in Regional Hurricane Activity GFDL 50-km HIRAM, using four projections of late 21 st Century SSTs. Red/yellow = increase Blue/green = decrease Regional increases/decreases much larger than global-mean. Pattern depends on details of SST change. Source: Zhao, Held, Lin and Vecchi (J. Climate, 2009) 19 Unit: Number per year 18-model CMIP3 Ensemble GFDL CM2.1 HadCM3 ECHAM5

Global Model Tropical Cyclone Climate Change Experiments: Use A1B Scenario late 21 st century projected SST changes from several CMIP3 models GFDL CM2.1 HadCM3 ECHAM5CMIP3 18-model Ensemble Source: Zhao, Held, Lin, and Vecchi (J. Climate, in press) 20 Unit: Deg C

Conclusions: i)GFDL model late 21 st century (ensemble) projections suggest a decrease in the number of hurricanes in the Atlantic (-24% to -32%), but nearly a doubling in the frequency of very intense (Cat 4-5) hurricanes by Preciptation rates in hurricanes are also projected to increase (~20% within 100km of storm center). ii)Based on present understanding, we would not expect a 10%/decade increase in Cat 4-5 frequency, if it occurred, to be detectable for a number of decades. iii)Not all individual climate models (when downscaled) produce an increase in Cat 4-5 frequency as was simulated for the 18-model ensemble climate change signal. iv)Remaining caveats include model limitations for the projected SST patterns (e.g., clouds; indirect aerosols); limitations of intense hurricane simulations; and observed data concerns (i.e., cat 4-5 record). v)Future work: expanding TC downscaling work to other basins; statistical approaches to downscaling are also being pursued (Villarini/Smith et al.; Held and Zhao) 21