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

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?

Black Dots: Annual values Red-line: decadal filter

Atlantic Basin

ACE ∑ Vmax2/104 PDI ∫Vmax3 dt / 1010

Atlantic Tropical Cyclones: Normalized Annual Means

Year

Annual Means

Filtered to remove timescalesless then 14 years.

Atlantic Tropical Cyclones: Normalized Annual Means

Year

Entire Basin

Main Development Region20 - 80W 10-20N

Rest of Basin

● 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.

Low Pass of the Data (timescales > 13 years)

Year

Low Pass of the Data (timescales > 13 years)

Year

Largest

difference

in recent

period.

Vorticity Terms

Wind ShearPotential Intensity/ Heat Content+Stability

Relative Humidity

Contribution of each term

SGP

GPI

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)

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

Year Year

Potential Intensity/ Heat Content+Stability

Correlations with SST

PDI

SGP

GPI

Named Storms

Why is potential Intensity Decreasing even though SSTs are increasing?

Correlation of GPI and SST.

Global Mean SST removed

Strong anti correlation with Indian Ocean SST.

SST over Main Development Region

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.

Atlantic MDR SST from IPCC model’s (ensemble mean)

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

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”