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Remote Cloud Influences on the
Double ITCZ Problem
DARGAN M. W. FRIERSON
UNIVERSITY OF WASHINGTON
YEN-TING HWANG
NATIONAL TAIWAN UNIVERSITY
GRAND CHALLENGE WORKSHOP, 3/24/14
Cloud Radiative Effects
 Often discussed in terms of how they affect global
climate sensitivity, or local climate
 But patterns of heating have a strong influence on
atmospheric dynamics
 Today I’ll mostly focus on how cloud biases over the
Southern Ocean spread all the way to the ITCZ
ITCZ Response to High Latitudes
 Pioneering work by Chiang and Bitz (2005) showed
strong sensitivity of ITCZ to high latitude sea ice and
land ice in Last Glacial Maximum conditions
ITCZ shifts away from
Drying cooled NH
Paleoclimate data is
consistent with such a shift
Moistening
From Chiang and Bitz (2005)
See also simulations by
Zhang and Delworth 2005,
Broccoli et al, etc
Extratropical Influences on ITCZ
 Sarah Kang’s thesis (2009):
 Two aquaplanet GCMs w/ idealized forcing only in the
extratropics:
Forcing
SH warming
Think glaciers + sea ice in NH,
plus warming in SH (to keep global
mean temperature the same)
NH cooling
From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (2009, JAS)
ITCZ Changes
 In response to forcing, ITCZ precipitation shifts
towards warmed hemisphere
Tropical precip in aquaplanet GCM
With strong forcing,
ITCZ shifts up to
18 degrees
Control case: ITCZ
located on the equator
Maximum amplitude of
forcing = 0, 10, 30, 60
W/m2
From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (2009, JAS)
Mechanism for Energy Transport Change
 Eddies modify fluxes in midlatitudes
 Quasi-diffusively: moist static energy transport proportional to
moist static energy gradient
 See also Hwang & Fri.
(2012), Hwang, Fri. &
Kay (2013)
 Anomalous Hadley
circulation
modifies fluxes in
tropics
See Kang, Held, Fri., & Zhao (2008, J Clim) & Kang, Fri. & Held (2009, JAS) for more
Role of Cloud Responses
 ITCZ shift is hugely sensitive to cloud feedbacks!

Factor of 2 difference in response
even for the same forcing!

Varied Tokioka entrainment
limiter in Relaxed ArakawaSchubert convection scheme
Caused large SW CRF differences
primarily in midlatitudes & subtropics

Kang et al (2008, J Clim)
Theoretical Framework Based on Energetics
 We predict tropical rain will shift towards the
hemisphere with more heating

Heating not just at the surface though – also SW/LW effects
of clouds (or clear-sky effects)
 In CMIP3 slab runs, model spread in ITCZ shift
projections is due to extratropical cloud
feedbacks (Fri. & Hwang 2012)
 Let’s apply to the double ITCZ problem next…
GPCP Annual Mean Precipitation
1985~2004
Black: GPCP
Tropical Precipitation Biases:
(1) Precip minimizes too much at
the EQ
CMIP5 Ensemble Mean
(2) Too much tropical precip in the
SH compared with the NH
(3) SPCZ too horizontal (not tilted)
Focus: GCMs do not
simulate the hemispheric
asymmetry in tropical
circulation in observations
500
1000
1500
2000
2500
3000 mm/year
GPCP Annual Mean Precipitation
1985~2004
Zonal Mean
(each line is one GCM)
NP
Black: GPCP
with its standard
deviation of year-toyear varibility
50N
30N
15N
EQ
CMIP5 Ensemble Mean
15S
30S
50S
SP
mm/yr
500
1000
1500
2000
2500
3000 mm/year
Cross-EQ Atmospheric Energy Transport
(PW)
Too much heating of the
SH atmosphere
Zonal Mean
(each line is one GCM)
NP
0.4
Black: GPCP &
ERA-I reanalysis
Gray lines: standard
deviation of interannual variability
0.2
0
Black: GPCP
50N
30N
15N
-0.2
EQ
-0.4
15S
R=-0.88
-0.2
30S
-0.1
0
0.1
0.2
0.3
Precipitation Asymmetry Index
(EQ~20N - EQ~20S) /
20S~20N
Too much precipitation in SH tropics
(compare with NH)
50S
SP
mm/yr
Biases in Shortwave Cloud
Radiative Effect
Black: CERES
CMIP5 Ensemble Mean Biases in SWCRE
(compare with CERES satellite observation
2000~2011)
30
20
10
0
10
20
30
W/m2
See also Trenberth and Fasullo 2010 for
CMIP3
2
SW CRF: CERES and Biases
SW CRF in CERES
Multi-model Mean Bias
in SW CRF
Too little reflection from marine stratocumulus regions,
Southern Ocean storm track, N. Pacific storm track
S. Ocean bias: Williams et al 2013, Bodas-Salcedo et al 2012
2
Surface temperature in SH is affected all the
way to the tropics
Red Models:
Larger northward cross-EQ
atmos. energy transport
NH minus SH
NH wetter
2
More precipitation in the SH
tropics
SH wetter
NH less cooling
Less cooling from clouds in
SH mid-to-high latitudes
SH less cooling (reflection)
Anomalously warm in SH
mid-to-high latitudes
(compare with NH)
NH warmer
SH warmer
Cross-EQ Energy
Transport (PW)
Interhemispheric
Temperature Gradient (K)
R=-0.88
NH wetter
Precip Index
SH wetter
R=0.73
Precip Index
SWCRF 20N~NP 20S~SP
W/m 2
NH minus SH
NH less cooling
SH less cooling (reflection)
NH warmer
R=0.64
SH warmer
Cross-EQ Energy
Transport (PW)
Interhemispheric
Temperature Gradient (K)
20N~NP - 20S~SP
W/m2
20
R=-0.88
15
Precip Index
10
5
R=0.73
0
Precip Index
SWCRF 20N~NP 20S~SP
W/m 2
black cross:
CERES observations
-5
-10
R=0.64
R=0.80 without the
open circle
-15
SW CRF explains
most of the GCMs
spread and
biases
Same cloud biases are correlated
with mean jet latitudes
SH Jet Latitude vs.
SW Cloud Radiative
Forcing
-44
Too much solar  poleward
shifted storm track
Anomalous warming in midlats
shifts baroclinicity poleward,
results in poleward shifted jet
Jet Latitude
-46
-48
Obs are not on the best fit
line though – there must be
additional problems
-50
-52
-100
-90 -80 -70 -60 -50
2
SW cloud forcing (W/m )
Ceppi, Hwang, Frierson, and Hartmann
Clouds cause the jet latitude changes…
 Examine interannual variability of jet latitudes &
shortwave cloud forcing in extreme models
In each GCM, all years have
similar cloud forcing, despite
very different jet latitude
Imposed heating simulations
confirmed that heating over the
storm track causes a poleward
shift
Ceppi et al 2012
Other Uses of this Attribution Procedure
 Ocean heat transport by the MOC is why the ITCZ
sits in the Northern Hemisphere in the current
climate (Frierson et al 2013)

Net radiation into SH is greater than NH! Ocean compensates
 Sulfate aerosols and a southward shift of precip in
the late 20th century (Hwang et al 2013)

Sahel drought linked to N American/European aerosols
Conclusions
 Clouds affect circulation: energy transports, ITCZ
shift

Moist static energy diffusion is a good approx for how energy
spreads in the atmosphere
 Southern Ocean clouds cause part of the double ITCZ
problem

One of many examples of extratropical heating affecting
tropical rainfall
How Much Bias Does this Effect Cause?
Multi-model mean bias
Correlation coeff b/w
asymm index and precip bias
Equatorial minimum, ITCZs too far off-equator will not be fixed
Top of the Atmosphere
Radiation Budget
90
SH receives more energy
2
than NH (1.5W/m)!
-50
Sahara
0
The
reflects SW, and
radiates more easily to
space.
W/m 2
50
50
30
NH
EQ
SH
-150
-50
50 W/m2
Ocean circ heats NH more
Upward Surface Flux
(ERA-I MSE Divergence minus CERES TOA
Budget)
NP
50N
30N
EQ
30S
50S
-60
-50
0
50
W/m 2
0
40
W/m 2
Removing the ocean heat
divergence from a GCM shifts
ITCZ to SH
• Experiments with full and
symmetrized surface heat flux
Control
Symmetrized
Structure of Drought
 Rain gauge data: 1971-1990 minus 1931-1950
Drying on northern side of the tropics, moistening on southern side
Structure of Simulated Drought
 CMIP3/5 data (28 models): 1971-90 minus 1931-50
Modeled drought is not limited to the Sahel! It’s a global southward shift
Correlation of precip shift w/ energy
flux
Well-correlated
with cross-eq
atmospheric flux
(hemispheric
difference in
heating)
•
Sulfate aerosols
are most important
for S’ward ITCZ shift
Atmospheric feedbacks
cause a lot of spread
though…
Hard to say how much of
the observed shift was from
aerosols
 Southward
•
Northward 
Attribution of Multi-Model Mean Shift
The Effect of Cloud Biases over
Southern Ocean on ITCZ in
GCMs
I
T
C
Z
colder
30S
EQ
30N
warmer
The Effect of Cloud Biases over
Southern Ocean on ITCZ in GCMs
I
T
C
Z
too much incoming
solar radiation
due to deficient
clouds
(or too little reflection)
too
30S
high SST
EQ
30N
warmer
The Effect of Cloud Biases over
Southern Ocean on ITCZ in GCMs
weaker SH
Hadley Cell
I
T
C
Z
weaker poleward
energy transport
too
30S
high SST
EQ
30N
warmer
The Effect of Cloud Biases over
Southern Ocean on ITCZ in GCMs
weaker poleward
energy transport
double ITCZ is
more persistent
(Mar. ~ Oct.)
than in the real I
world
T
(only Mar.) C
Z
too
30S
high SST
I
T
C
Z
EQ
30N
warmer