An Assessment of Low Frequency Variability in Tropical Cyclones in the North Atlantic Basin.
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Transcript of An Assessment of Low Frequency Variability in Tropical Cyclones in the North Atlantic Basin.
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An Assessment of Low Frequency Variability in Tropical
Cyclones in the North Atlantic Basin.
Philip J. Pegion
GMAO Goddard Space Flight Center, SAIC
Siegfried D. Schubert, Max Suarez
GMAO Goddard Space Flight Center
Julio Bacmeister
GEST,GMAO
Kerry Emanuel
Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA
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Motivation
The hurricane Best Track Dataset shows an increase in the number of
storms over the 20th Century as well as an increase in the intensity of
these storms. The questions we have are:
● How much of the variability is due to decadal changes in Tropical Atlantic SSTs?
● How much of the variability is due to a Global Warming 'trend' or to differences in the observing system?
● Does an AGCM forced with observed SSTs capture this variability?
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Black Dots: Annual values Red-line: decadal filter
Atlantic Basin
ACE ∑ Vmax2/104 PDI ∫Vmax3 dt / 1010
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Atlantic Tropical Cyclones: Normalized Annual Means
Year
Annual Means
Filtered to remove timescalesless then 14 years.
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Atlantic Tropical Cyclones: Normalized Annual Means
Year
Entire Basin
Main Development Region20 - 80W 10-20N
Rest of Basin
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● Seasonal Genesis Parameter (Gray 1979)
SGP=Vorticity*Coriolis*Shear*Thermal energy*Moist stability *RH
● Genesis Potential Index (Emanuel and Nolan 2004)
GPI=Absolute vorticity*RH * Potential Intensity * Shear
Does a AGCM represent this variability?
Model: NASA NSIPP AGCM, 22 member ensemble run from 1902-2006.
Since simulations are run at 3x3.5 degrees. Model is not explicitly able to represent tropical cyclones, so we used an index to represent tropical cyclones.
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Low Pass of the Data (timescales > 13 years)
Year
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Low Pass of the Data (timescales > 13 years)
Year
Largest
difference
in recent
period.
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Vorticity Terms
Wind ShearPotential Intensity/ Heat Content+Stability
Relative Humidity
Contribution of each term
SGP
GPI
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Are the fluctuations natural variability, or a response to global
warming?
Data: IPCC 4th Net Assessment Climate of the 20th Century Simulations
NCAR ccm3 (6 ensemble members)
ECHAM4 (4 ensemble members)
GFDL cm2_0 and cm2_1 (3-members each)
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Year
Model's show a general increase in SGP over the century, GPI is flat. Lack multi-decadal variability (individual member's don't show it either)
Results from IPCC runs
SGP GPI
Year
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Year Year
Potential Intensity/ Heat Content+Stability
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Correlations with SST
PDI
SGP
GPI
Named Storms
Why is potential Intensity Decreasing even though SSTs are increasing?
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Correlation of GPI and SST.
Global Mean SST removed
Strong anti correlation with Indian Ocean SST.
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SST over Main Development Region
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Atlantic MDR SST from IPCC model’s
IPCC Models show a general warming of the Atlantic SST, but lack theDecadal variability indicated by the observations.
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Atlantic MDR SST from IPCC model’s (ensemble mean)
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Less Stable
More Stable
Air Temperature over Main Development Region
Wildly different answers from the different reanalyses. Only JRA-25 agrees with the models.
Warming of the upper troposphere is increasing stability over the Atlantic
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Summary● The changes of number of tropical cyclones matches the variability of SST over the
Atlantic Main Development Region, but the same is not true for power dissipation,
which only shows decadal variability.
● Atmospheric model derived indices show similar decadal variability, but differences
arise in the long term trend. The SGP, which defines heat content referenced to 26oC,
shows a trend, but the GPI, which calculates stability, shows only decadal variability
from the AGCM, and no change from the IPCC runs.
Looking forward to new reanalyses such as MERRA, CFSRR to get more confidence on
the nature of the changes in the modern period, and am very interested in getting data
from the “The 20th Century Reanalysis Project”