2. Introduction
What are tropical cyclones and how do they form?
• SSTs > 26°C or 80°F
• disturbance waves (1000-3000 km)
and large cyclonic vorticity
• vertical shear < 10 ms
-1
• atmosphere potentially unstable to moist convection
• large Coriolis parameter
• large RH
3. Motivation
• greenhouse gases forcing leads to a warmer climate (disequilibreated energy balance,
increased SSTs)
• is a change in TC activity from climate with CO2 change similar to change from climate with
altered solar constant?
hence, can geoengineering (through solar radiation management) be a solution to climate
changes? can natural or anthropogenic aerosols forcing cancel CO2 forcing?
• which environmental variables are the best predictor for change in TC activity in altered climate?
Radar image of Hurricane Floyd (1999) making landfall on North Carolina.
4. Literature review
• Decrease in TCs frequency
weakening of upward mass flux
weakening of tropical circulation
weakening of deep convection in the
tropics
increase in saturation deficit of the
mid-troposphere
(Knutson et al. 2010 ; IPCC 2013)
VIIRS Captured Tropical Cyclone Nathan (March 2015)
• Increase in hurricanes frequency
increased SSTs
increased atmospheric water vapor
content
• Intensification of TCs
increase SSTs over the main
development region
correlation with power dissipation
index
5. Held and Zhao (2011) & Zhao (2013) have
separate the forcing of the CO2 from SSTs
• SSTs are increased by 2k (P2K) - CO2 is
unchanged
• SSTs are unchanged - CO2 concentration
is double (2xCO2)
• SSTs are increased and CO2
concentration is double (BOTH)
They found :
1.decrease in BOTH experiment (≈ 20%)
2.decrease in 2xCO2 and P2K experiments
(≈ 10% each)
=> suggessing linear relation
3.increase in global TCs intensity in P2K
experiment only
Held and Zhao (2011)
Direct effect of an increase in CO2 concentration on the
global TC frequency?
6. Method
Model description :
• high resolution general circulation model developped by the Geophysical Fluid
Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM)
=> HIRAM captures and simulates well tropical cyclones climatology,
frequency, seasonal cycle and interannual varibabiliiy
• 50-km horizontal resolution
• 180x180 grid points in each face of the cube-sphere
• 32 vertical levels
• insolation time-independant
• no aerosols or GHG (others than CO2)
• fixed heat flux convergence
• constant surface albedo (0.08)
• simulation with prescribed SSTs from slab ocean
7. • Minimum of sea level pressure
• Anomaly of 850-hPa relative vorticity (warm-core vortices)
higher than 3.5x10-5 s-1
• Local maximum values of 10-m surface wind speed
at least 17 ms-1
• Last for 6 hours within a 400 km distance of the preceding area
• Hurricanes : 10-m surface wind speed exceed 29.5 ms-1
Storms Tracker description :
Global Tropical Cyclone Tracks between 1985 and 2005
8. • maximum sustainable winds and minimum sustainable centra pressure of TCs
• TCs are considered as close system (Carnot engine)
• isothermal inflow (warmed by pressure drop but cooled by evaporation)
• moist adiabatic upward motion
• outflow below tropopause
• cooled air sink and return adiabatically to TC environment (to close the
system)
Potential intensity :
9. Simulations description :
Difference with reference simulation of the global mean net radiation at the TOA
-100 -50 0 50 100
Latitude
-150
-100
-50
0
50
100
R
TOA
net
[Wm
-
2
]
Slab-Ocean
S1400
S1450
4xCO
2
11. Frequency sensitivity of TCs to different forcing:
(others)
(1) (2) (3)
(1) & (3) are taken from Merlis et al. (2013) from slab-ocean experiments
(2), the normalized % change of TCs frequency per unit forcing of net radiation at
the TOA Wm−2
, is computed from prescribed SSTs experiments only :
(2)CO2
(2)So
15. Relative importance of environmental variables on TCs frequency
Fractional change of mean TCs genesis weighted precipitation (P), minimal potential pressure (Pmin), potential intensity
(PI), environmental shear (S), vertical wind at 500-hPa (ω500), relative vorticity at 850-hPa (ζ850) and global number of
TCs per year (TC).
P P
min
PI S ω
500
ζ
850
TC
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
Fractionalchange Fixed-SST*
S1350
S1400
S1425
S1450
4xCO2
17. The maximum value of the streamfunction at 600-hPa is :
• decreasing as the net radiative forcing increases in all experiments
=> weakening of tropical circulation between 5°N and 10°N
• shifting northward in slab-ocean experiment
= > weakening and northward shift of the Hadley circulation
14 15 16 17 18 19
Max Latitude
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
Maxstreamfunctionvalue
×1011
Slab-Ocean
Fixed-SST*
S1350
S1400
S1425
S1450
4xCO2
18. Conclusions
• TCs frequency response to 4xCO2 forcing is
a strong direct decrease
but a strong temperature dependent increase
• solar forcing ≠ CO2 forcing
• direct changes ≠ indirect (temperature dependent) changes
• Environmental variables :
1. Robust ↓ of the potential intensity :
when ↑ radative forcing (biggest ↓ for CO2 forcing) in prescribe SSTs simulations
when ↓ radiative forcing in slab-ocean simulation
2. Magnitude of (upward vertical wind) ω is ↓ in all simulations
3. Strong direct and indirect weakening of the circulation in planet radiatively forced by
1200 ppm CO2
=> Geoengineering is probably not a good solution (nor forcing a volcanic eruption!)
19. Future work
• Simulations with prescribed sea surface temperatures from :
1. 4xCO2 slab ocean simulation
2. 1450 Wm-2 slab ocean simulation
representing (as in Held and Zhao (2011) & Zhao (2013))
SSTs increased by 2k, while CO2 is unchanged
=> to understand why solar and CO2 forcing lead to
different responses for environmental variables when the
radiative forcing is the same
• Find which environmental variable is most important, thus
best predictor, for the TC changes
