Have Humans Affected Atlantic Hurricane Climate?

Hurricane Katrina, Aug. 2005

GFDL model simulation of Atlantic hurricane activity

Tom Knutson

Geophysical Fluid Dynamics Lab/NOAAPrinceton, New Jersey

http://www.gfdl.noaa.gov/~tk

Regional model low-level winds (388 m); Red: >40 m/s (~“hurricane” strength)

GFDL Regional Model Simulation of the 2005 Atlantic Hurricane Season

Reference: T. Knutson et al., Bulletin of the American Meteorological Society, October 2007.

There is some recent evidence that overall Atlantic hurricane activity may have increased since in the 1950s and 60s in association with increasing sea surface temperatures…

Source: Kerry Emanuel, J. Climate (2007).

PDI is proportional to the time integral of the cube of the surface wind speeds accumulated across all storms over their entire life cycles.

~1950-2005

What are the implications of pronounced future warming for Atlantic Power Dissipation Index (PDI)?

Power Dissipation Index vs SST

~1950 - 2005

IPCC AR4 Climate ModelsObservations

Source: Chris Landsea, NOAA/NHC

A measure of annual U.S. landfalling hurricane activity shows no clear long-term trend since 1900…

Attribution of hurricane changes to human-induced climate change?

• Detection: is there an observed change that exceeds “internal variability”?

• Attribution: is the observed change consistent with expected anthropogenic influence? And inconsistent with alternative explanations?

• Models/theory must reconcile with observations

• Observations must be assessed for “false trends” based on evolving observational capabilities

What type of SST “anthropogenic signal” should we look for in the tropical Atlantic?

Anthropogenic forcing: Linear trend-like, but the caveat of no indirect aerosol forcing yet.

Sources: Vecchi and Knutson, J. Climate, accepted for publication. See also Knutson et al. J. Climate, 2006

Natural + Anthropogenic forcing: Volcanic/solar/anthropogenic forcing gives a closer fit to observations … Residual (blue – black) = AMO??

The frequency of recorded storms (low-pass filtered) in the Atlantic basin is well-correlated with tropical Atlantic SSTs

Source: Emanuel (2006); Mann and Emanuel (2006) EOS. See also Holland and Webster (2007) Phil. Trans. R. Soc. A

But is the storm record reliable enough for this?

Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)Trend from 1900-2006: Is significant at p=0.05 level

Source: Vecchi and Knutson, J. Climate, in press.

+1.60 storms/century (1878-2006)+4.39 storms/century (1900-2006)

Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)

Trend from 1900-2006: Is significant at p=0.05 level

Source: Vecchi and Knutson, J. Climate, in press.

+1.60 storms/century (1878-2006)+4.39 storms/century (1900-2006)

TK 3

U.S. Landfalling vs basin-wide activity

Global Mean Temperature

Tropical Atlantic Sea Surface Temp.

Atlantic Tropical Storm Counts (unadj.)

Adj. Atlantic Trop. Storm Counts (Vecchi/Knutson)

U.S. Landfalling Tropical Storms (unadj.)

U.S. Landfalling Hurricanes (unadj.)

Note: All time series are low-pass filtered (5-yr mean) and normalized to unit standard deviation (y-axis tic marks: 1 st. dev).

Source: Vecchi and Knutson, J. Climate, accepted for publication.

Storm Track Density Linear Trend (1878-2006)

Unadjusted Adjusted

TK 4

GFDL Zetac Model: A new high-resolution regional model for Atlantichurricane season simulations…

• The model runs for entire hurricane seasons.• The model generates its own sample of hurricanes during each season. • These experiments push the limits of available computing resources.

Prototype model for future global hurricane forecasting system…

Note: Model uses large-scale interior nudging to NCEP Reanalysis. Aug.-Oct. season only.

The model captures both the increase in hurricane activity since the 1980s and the year-by-year fluctuations….

Source: Updated from Knutson et al. 2007 (Bull. Amer. Meteor. Soc.)

Correlations: Model1: 0.72 Model2: 0.68 Ensem: 0.77

Correlations: Model1: 0.64 Model2: 0.51 Ensem: 0.69

Correlations: Model1: 0.70 Model2: 0.62 Ensem: 0.73

ACE:

PDI:

Cat 3-5:

Other hurricane metrics (ACE, PDI) are simulated fairly well, but major hurricanes are under-simulated in number (though still well-correlated)…

a) Observed b) Observed c) Observed

Tropical Storm Formation Tropical Storm Occurrence Hurricane Occurrence

d) Simulated e) Simulated f) Simulated

Zetac Regional Model Downscaling: geographical distribution of storms

Note: Model uses large-scale interior nudging to NCEP Reanalysis

The model also reproduces the observed reduction of Atlantic hurricane activity during El Niño events fairly well…

Source: Knutson et al. (2007) Bull. Amer. Meteor. Soc.

Large-scale tropical Atlantic climate changes projected for late 21st century by CMIP3 models (A1B scenario). Average SST change in MDR is 1.72oC with warming near 4oC in the upper troposphere.

Late 21st Century projections: increased vertical wind shear may lead to fewer Atlantic hurricanes

“storm-friendly” “storm-hostile”

Average of 18 models, Jun-Nov

Source: Vecchi and Soden, Geophys. Res. Lett., (2007)

Projected changes in Atlantic hurricane/tropical storm numbers:

Late 21st century; Zetac regional model downscaling of CMIP3 multi-model ensemble climate change signal.

Note: U.S. Landfalling hurricanes: -30%

Source: Knutson et al., 2008, submitted.

-27%

-18%

-8%

The 26.5oC “threshhold temperature” for tropical storm formation: a climate dependent threshhold…

The control model reproduces the observed close relationship between SST and hurricane frequency (1980-2006), but this statistical relationship does not hold for future human-caused warming in the model.

Hurricane frequency actually decreases by 18% in the warm climate case… although the model does not simulate hurricanes as intense as observed.

Lesson: Caution using correlations from the present climate to make future climate projections…

Source: Knutson et al., submitted (2008).

The new model simulates increased hurricane rainfall rates in the warmer climate (late 21st 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: 1.72oC

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 21st century, downscaled fromm 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, submitted.

Next Steps: Examining Model Dependence:

The regional details of future projections matter… (and these are not highly confident features of future climate projections).

Major hurricane count sensitivity varies (-8% to +~70% so far in very preliminary testing).

+70%Late 21st century

Current Climate

Changes in Atlantic hurricane activity as downscaled from GFDL CM2.1 climate model:

Hurricane Frequency: +3% Major Hurricane Frequency: +70%

Courtesy Morris Bender and Tim Marchok, NOAA/GFDL

Hurricane models project increasing hurricane intensities and rainfall rates with climate warming… …but probably not detectable at present.

Sources: Knutson and Tuleya, J. Climate, 2004 (left); Knutson and Tuleya (2008) Cambridge Univ Press (right).

See also Bengtsson et al. (Tellus 2007) and Oouchi et (J. Meteor. Soc. Japan, 2006); Walsh et al. (2004) Stowasser et al. (2007).

~Late 21st century

Current climate

Hurricane Intensity Hurricane Rainfall Rates

Current climate

~Late 21st century

6-hr accumulated rainfall [cm] within ~100 km of storm center.

Sensitivity: ~4% increase in wind speed per oC SST increase

Sensitivity: ~12% increase in near-storm rainfall per oC SST increase

Alternative Downscaling Approach: Emanuel et al. (2008)

Alternative Downscaling Approach: Emanuel et al. (2008)Emanuel “Obs.” Last 25 yr: +250% (Atl.) +47% (global)

KT04: +6%

K08: 2-3% (Atl.)

K08: -27% (? To -8%) Atl. Trop. Storms -18% (? To +7%) Atl. Hurricanes -8% (? To +70%) Major Hurricanes

K08: -25% (Atl., weak hurricanes)

(Tropical Storms)

…with some selected rough comparisons to other modeling studies

Projected 21st Century Changes in Thermodynamic Potential Intensity of Hurricanes

From Vecchi and Soden (2007, GRL)

Average of 18 models, Jun-Nov

Observations

Model/Theory

~8 m/s/oCEstimated Sensitivity of TC intensity to large-scale SST

How sensitive is hurricane intensity to greenhouse gas-induced warming?

Source for TC intensity/SST plots: Jim Kossin, personal communication, 2007.

Summarizing the Science At This Point…

1. Based on the current state of models and ongoing data concerns, it is not appropriate at this time to make a likelihood statement attributing past changes in hurricane activity to increasing greenhouse gases or other human-caused factors.

2. Some statistical studies, particularly using data from recent decades, suggest the possibility of a human influence on hurricanes. Atlantic SST and hurricane “power dissipation” are well-correlated on low frequencies since ~1950. Studies attribute part of the Atlantic SST warming since 1950 to increasing greenhouse gases.

3. However, longer (~100-128 yr) tropical cyclone records, including consideration of data problems, give conflicting indications on whether there have been significant increases in Atlantic tropical storm and hurricane numbers. U.S. landfalling hurricanes show no increase. Basin-wide statistics are much more uncertain for these longer records than for the more recent decades.

4. Models: The recent increase in Atlantic hurricane numbers (1980-2006) is reproduced by models, and is strongly correlated to SSTs in both models and observations. However, the much larger projected greenhouse warming (late 21st century, A1B scenario) leads to decreased hurricane numbers in our model. We infer that the detailed structure of a tropical SST changes (whether it be due to internal variability, greenhouse gases, aerosols, etc.) has a major influence on how those SST changes will influence hurricanes.

5. Model dependence: While the average model projection calls for a slight decrease (-8%) in major hurricane counts by the late 21st century, at least one model (GFDL CM2.1) projects a large increase (+70%) in major hurricane counts. This indicates that further studies are needed to narrow the uncertainty in important details of future regional SSTs which may impact Atlantic hurricane activity.

6. Intensity: models simulate increased average hurricane wind speeds and rainfall rates in response to a general greenhouse gas-induced warming of tropical SSTs. Rough range of sensitivity: 1 to 8%/ deg C for wind speed; ~12%/deg C for near-storm rainfall. Significant increases of intensity have not yet been detected in observations (but stay tuned…). Attribution?

What did we learn in our most recent study and what are the remaining uncertainties and questions?

• Tropical storm and hurricane frequency?– Results do not support the notion of a strong greenhouse gas driven increase in

Atlantic tropical storm or hurricane counts.– While no model is supporting a very strong sensitivity, there is considerable

range of results depending on what global model is downscaled.

• Intensities of the strongest storms? – New work reported at AMS/New Orleans (Elsner, Kossin, Jaeger) finds increases

globally in intensities of strongest storms since 1980 (a short, but global, homogeneous satellite-based record).

– Intense hurricanes are a weak aspect of the Zetac model and thus require further investigation with the GFDL hurricane model.

• What caused the dip in Atlantic hurricane activity in the 1970s and 80s?

– SST correlates with the dip in activity. What caused the SST dip?– Both internal variability (Atlantic Multidecadal Oscillation) and radiative forcing

(volcanic, solar, anthropogenic aerosols) may have contributed to the SST behavior (e.g., Zhang/Delworth et al.)

– Decadal prediction work should also help illuminate the causes of past multi-decadal variations

Climate Change and Tropical Cyclones:What are the Impediments to Scientific Understanding?

1. Inhomogeneities in climate records, insufficient length of climate records related to tropical cyclones

2. Uncertainties in global climate projections (emissions, transient sensitivity)

3. Uncertainties in regional projections of climate models (wind shear, lapse rate, SST patterns, easterly waves)

4. Uncertainties in response of tropical cyclones to regional changes in climate (frequency, intensity)

5. For decadal predictions (and hindcasts): Separating the radiatively forced response (Greenhouse gases, aerosols) from internal climate variability (AMO).

Hurricane/Climate Modeling Plans at GFDL

1. Zetac regional Atlantic model (18 km)– Target: Climate change uncertainty analysis, downscaling different

global models– Target: NW Pacific analysis (extension in collaboration with Taiwan)

2. GFDL hurricane prediction model (9 km)– Target: high intensity hurricanes and climate change (Cat 4-5)

3. New Global models (100, 50, 25 km resolution versions)– Target: Global tropical cyclone sensitivity to global warming (all basins)– Target: Reconstructions of 20th century hurricane activity– Target: Seasonal predictability investigation (persistence/forecast for

SSTs?)– Target: Decadal predictions of Atlantic hurricanes if SSTs can be

predicted.

THE END ---- THANK YOU!!

Model: Radiative forcing and ocean circulation changes both provide a plausible explanation for the fluctuations in 20th century Northern Hemisphere mean temperature (detrended)

Model: Some tentative evidence in support of a significant role for ocean circulation changes: surface and sub-surface fluctuations are out of phase in observations and modeled AMO-like change.

Sources: Zhang, Delworth and Held (2006) Zhang (2007) GRL papers.

Internal Climate Variability vs Radiative Forcing: Detrended NH Mean Temperatures

Source: Chris Landsea, NOAA/NHC

Atlantic Major Hurricane counts (basin-wide) since the mid-1940s: no long-term trend