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Global Warming and Hurricanes Thomas R. Knutson NOAA/Geophysical Fluid Dynamics Lab Princeton, New Jersey Acknowledgements: Dr. Chris Landsea (NOAA/NHC)

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Presentation on theme: "Global Warming and Hurricanes Thomas R. Knutson NOAA/Geophysical Fluid Dynamics Lab Princeton, New Jersey Acknowledgements: Dr. Chris Landsea (NOAA/NHC)"— Presentation transcript:

1 Global Warming and Hurricanes Thomas R. Knutson NOAA/Geophysical Fluid Dynamics Lab Princeton, New Jersey Acknowledgements: Dr. Chris Landsea (NOAA/NHC) Dr. Kerry Emanuel (MIT)

2 Global Warming and Hurricanes: Historical Perspective and Future Outlook Global warming and its potential impact on the built environment (overview) Historical observations: sea surface temperatures and hurricanes Future simulations of tropical sea surface temperatures and hurricane intensity Future research directions

3 Global Warming and Hurricanes: Historical Perspective and Future Outlook Global warming and its potential impact on the built environment (overview) Historical observations: sea surface temperatures and hurricanes Future simulations of tropical sea surface temperatures and hurricane intensity Future research directions

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5 Inactive Era (July Dec.1989) Active Era (Jan – Aug. 2005) Surface Temperature Anomalies during most recent Inactive and Active Atlantic Major Hurricane Eras Data Source: HadCRUT2v (Climatic Research Unit, U.K.)

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7 a) CM2.1 All Forcings b) CM2.1 Natural Forcings c) CM2.1 Anthropogenic Forcings Global Mean Surface Temperature Simulations: Krakatau eruption

8 CO 2 and temperature are highly correlated on glacial/interglacial time scales… Estimates of radiative forcing during last ice age: ~ -6.5 W/m 2 (Hansen 1993) are comparable in magnitude to future positive forcing over the next years under some IPCC scenarios… Source: Petit et al., Nature, Paleoclimate evidence…

9 Potential Impacts of Climate Change on the Built Environment Heat and thermal stress  Infrastructure sensitive to thermal stress; heat mortality; wildfire risk Floods and drought  Flood risk; agricultural impacts; wildfire risk Sea level rise  Exacerbate coastal flooding, including from storm surge Tropical cyclones  Increased wind damage, flooding from rains, and storm surge

10 # of models that predict increase in precipitation by 2100 in A1B scenario, out of 20 models used by IPCC/AR4 Source: Isaac Held, GFDL/NOAA Red = wetter Blue = drier

11 Modeled return period of 100-yr flood under 4xCO2 climate change Source: Milly et al. 2002, Nature, v. 415, p Based on one model (GFDL R30). Potential increasing risk of great floods under future climate change…

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13 Global Warming and Hurricanes: Historical Perspective and Future Outlook Global warming and its potential impact on the built environment (overview) Historical observations: sea surface temperatures and hurricanes Future simulations of tropical sea surface temperatures and hurricane intensity Future research directions

14 Trends in Tropical Atlantic “Main Development Region” SSTs… Main Development Region

15 Source: Chris Landsea (NOAA/National Hurricane Center)

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18 Why have Tropical Atlantic (MDR) SSTs warmed? GFDL CM2 coupled model historical simulations ( ); Aug-Oct season All Forcings (n=8) Natural Forcings Only (n=4) Anthropogenic Forcings Only (n=4) 5-yr running means

19 NW Pacific Basin: Intensity vs. SST Source: Baik and Paek, J. Meteor. Soc. Japan (1998). Used with permission. Minimum surface pressure (mb) Sea surface temperature (deg C) The most intense storms occur at high SSTs

20 Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial). Note that 1933 had 21 named storms N=28

21 Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial) N=7

22 Emanuel (2005) Original PDI from Emanuel (2005) Revised PDI from Landsea ( updated)

23 Emanuel’s Multi-basin Tropical Cyclone Power Dissipation Index (PDI) has increased substantially over past 50 years, along with tropical SSTs Source: Kerry Emanuel, MIT, SST anomaly (deg C) with arbitrary vertical offset. PDI scaled by constant.http://wind.mit.edu/~emanuel/anthro2.htm

24 Source: K. Emanuel, MIT, 2006 Storm-Maximum Power Dissipation Index – Atlantic Basin

25 Source: Kerry Emanuel, MIT Atlantic Basin: SSTs vs number of tropical cyclones

26 Webster et al.: The percentage of hurricanes which reach Category 4-5 has increased in all basins, comparing two recent 15-year periods… Question: Are the historical data adequate for this conclusion? Source: Adapted from Webster et al., Science, Sept

27 Source: Knaff and Sampson, AMS Hurricanes Conference Proceedings, 2006 Knaff and Sampson’s reanalysis of NW Pacific max intensities produces a reduced Cat 4-5 trend, relative to “best track”

28 Sriver and Huber’s PDI from reanalysis, although weaker, is well-correlated after 1978 with Emanuel’s PDI from “best track” data (Atlantic + NW Pacific) Source: Sriver and Huber, Geophysical Research Letters, in press.

29 Source: Chris Landsea, NOAA/NHC

30 Source: Chris Landsea (NOAA/National Hurricane Center)

31 Georgia-New England Major Hurricanes Source: Chris Landsea (NOAA/National Hurricane Center)

32 Source: Chris Landsea, NOAA/NHC

33 Conditions Associated With the Active 2004 Atlantic Hurricane Season Source: Chris Landsea (NOAA/National Hurricane Center)

34 Category A tale of three seasons… 1995…very active… but few strong U.S. landfalls 2004…active… four Florida hurricanes 2005… very active… four major U.S. Gulf Coast hurricanes

35 A tale of three seasons… 1995…very active… but few strong U.S. landfalls 2004…active… four Florida hurricanes 2005… very active… four major U.S. Gulf Coast hurricanes Plots: September 500mb geopotential heights

36 Global Warming and Hurricanes: Historical Perspective and Future Outlook Global warming and its potential impact on the built environment (overview) Historical observations: sea surface temperatures and hurricanes Future simulations of tropical sea surface temperatures and hurricane intensity Future research directions

37 Hurricane–region SSTs in the 21 st Century

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39 GFDL Simulations: Hurricanes are more intense for warmer climate conditions …(~4% per deg C) Note: Min. central pressures are averages over day 5 of integrations. Source: Knutson and Tuleya, J. Climate, 2004.

40 GFDL Simulations: Hurricanes have significantly more near-storm rainfall for warmer climate conditions …(~12% per deg C) Average rainfall in a 32,700 km 2 region of highest 6-hour accumulation (equivalent to 100km radius region).

41 So What’s the Problem?

42 Reality check: comparing models with observations… GFDL Model wind speed intensity, V (and hurricane theory) vs SST: ~4-5% per o C Emanuel (2005) for Atl, NW Pac, NE Pac: V 3 increases 50% for 0.5 o C, so V: ~30% per o C Emanuel (2006) for Atlantic only: Century-scale data: V increases ~10% per o C; Data since 1980 only: ~20% per o C Factor of 2 to 6 discrepancy in sensitivity… Implications for future projections??

43 Resolving the Discrepancy? 1.Past trend of intensity over-estimated? (i.e.: the data is wrong) 2.Hurricane model/theory not sensitive enough to SST change? (i.e., the models are wrong) 3.Other factors besides SST which can affect potential intensity are playing a role? (i.e., our simple analysis is wrong) Possibilities:

44 Global Warming and Hurricanes: Historical Perspective and Future Outlook Global warming and its potential impact on the built environment (overview) Historical observations: sea surface temperatures and hurricanes Future simulations of tropical sea surface temperatures and hurricane intensity Future research directions

45 Where are we going from here? Simulations of Atlantic hurricane frequency using nested regional models…

46 GFDL Zetac Nonhydrostatic Regional Model: 18km Tropical N. Atlantic Simulation Simulated hurricanes

47 Observed vs Simulated Atlantic Tropical Storm Tracks 1982: Inactive year 1995: Active year GFDL Zetac regional model, 18km resolution, large-scale nudged toward observed large scale Observed (Aug-Oct) Simulation 2 (48-hr nudging) Category

48 Note: Uses large-scale interior nudging

49 Note: Winds derived from pressure.

50 Tropical Storm Formation Tropical Storm Occurrence Hurricane Occurrence Observed Observed Observed Model: GFDL Zetac Regional Model with large-scale interior nudging to NCEP Reanalysis Period of Simulation: August-October seasons, Simulated Simulated Simulated

51 Summary of Main Points: Global Warming and Hurricanes Tropical SSTs (including tropical North Atlantic):  Substantial warming (~0.6 o C) occurred in 20 th century, roughly tracking global mean temperature  Substantially greater 21 st century warming (~2 o C) is anticipated due to anthropogenic forcing (greenhouse gas emissions, etc.) Historical hurricane observations give conflicting information on past trends:  Several Atlantic hurricane activity measures are dominated by multi-decadal “cycles” or noise (e.g., landfalling PDI)—not trends. Some basin-wide indices show unprecedented levels in recent years, correlated with rising SSTs. Data quality issues remain unresolved at this time.  Hurricane intensity sensitivity implied by some studies greatly exceeds that of current model simulation and theory, a discrepancy that remains unresolved at this time. GFDL hurricane model future simulations:  Maximum intensities increase (roughly ½ category or 7% -- per 100 yr)  Near-hurricane precipitation increases (roughly 20% per 100yr) Ongoing work at GFDL: high-resolution seasonal Atlantic simulations  Future frequency changes? highly uncertain  Futurte regionally specific effects? highly uncertain


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