tropical circulation changes across forcing agents · global hurricane (tc) frequency response!...
TRANSCRIPT
![Page 1: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/1.jpg)
Tropical Circulation Changes Across Forcing Agents
Timothy M. Merlis McGill University
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• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
Conclusions
![Page 3: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/3.jpg)
Seasonality is important for precip response to sulfate aerosol forcing in “continental” regime
• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
Conclusions
![Page 4: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/4.jpg)
• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
• ‘Direct’ response of circulation to forcing agents may differ.
Conclusions
Seasonality is important for precip response to sulfate aerosol forcing in “continental” regime
![Page 5: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/5.jpg)
• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
• ‘Direct’ response of circulation to forcing agents may differ.
Conclusions
Seasonality is important for precip response to sulfate aerosol forcing in “continental” regime
New mechanism for direct CO2 circulation weakening posits a key role for spatial pattern of forcing
![Page 6: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/6.jpg)
ITCZ Energetic FrameworkEnergetic perspective: ITCZ in hemisphere exporting energy
Kang et al. (2009)
1) Take annual-mean forcing/feedback
2) Diffuse energy in atmos to determine annual-mean δcirculation
3) Compute annual-mean change in water vapor flux to determine P shift
Frierson & Hwang (2012), Hwang et al. (2013), Bischoff & Schneider (2014)
Recipe:
Hwang et al. (2013)
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ITCZ Energetic FrameworkEnergetic perspective: ITCZ in hemisphere exporting energy
Kang et al. (2009)
1) Take annual-mean forcing/feedback
2) Diffuse energy in atmosphere to determine annual-mean δcirculation
3) Compute annual-mean change in water vapor flux to determine P shift
Frierson & Hwang (2012), Hwang et al. (2013), Bischoff & Schneider (2014)
Recipe:
Hwang et al. (2013)
Should we worry about the often unstated ‘annual-means’?
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Sulfate Aerosol Forcing…in an aquaplanet GCM!
2.2x anthropogenic perturbation Yoshimori & Broccoli (2008)
Merlis et al. (2013a)
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Sulfate Aerosol Forcing…in an aquaplanet GCM!
“Continental”: 5m slab ocean “Oceanic”: 20m slab ocean } infinite reservoir for
evaporation
Merlis et al. (2013a)
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“Oceanic” PrecipitationLa
titud
e
Precipitation: "Oceanic"
1 3 5 7 9 11−40
−20
0
20
40
3
9
15
21
Ann. mean
0 5 10
Time (month)
Latit
ude
Precipitation Change
1 3 5 7 9 11−40
−20
0
20
40
−6
−2
2
6
Ann. mean
(mm day−1)−2 −1 0 1
Southward shift throughout seasonal cycle
PE
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Latit
ude
Precipitation: "Continental"
1 3 5 7 9 11−40
−20
0
20
40
3
9
15
21
Ann. mean
0 5 10
Time (month)
Latit
ude
Precipitation Change
1 3 5 7 9 11−40
−20
0
20
40
−6
−2
2
6
Ann. mean
(mm day−1)−2 −1 0 1
“Continental” Precipitation
Southward shift strong in NH summer & annual-mean response reflects this.
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Latit
ude
Precipitation: "Continental"
1 3 5 7 9 11−40
−20
0
20
40
3
9
15
21
Ann. mean
0 5 10
Time (month)
Latit
ude
Precipitation Change
1 3 5 7 9 11−40
−20
0
20
40
−6
−2
2
6
Ann. mean
(mm day−1)−2 −1 0 1
“Continental” Precipitation
Southward shift strong in NH summer & annual-mean response reflects this.
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‘Dynamic’ P-E change
Normalized by ann. mean
Seasonal change in circulation (summer maximum) correlated with climatological time of high humidity.
/ �!
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This is a rectification mechanism similar in spirit to ‘thermodynamic’ precession mechanism:
‘Dynamic’ P-E change
Merlis et al. (2013c)
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~40% underestimate of annual-mean change
‘Dynamic’ P-E change
Neglecting climatological seasonality of
humidity
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‘Dynamic’ P-E change
Neglecting seasonality minimally underestimates annual-mean P-E change in “oceanic” regime
/ �!
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Seasonality of Earth’s humidity
Longitude
Latitude
0 180 0−40−30−20−10
010203040
0%10%20%30%40%50%60%70%80%
ERA Interim
Magnitude of seasonal cycle relative to annual mean:
q(t) ⇡ [q] + q0 cos(2⇡t yr�1+ �)
q0
[q]
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Seasonality of Earth’s humidity
Longitude
Latitude
0 180 0−40−30−20−10
010203040
0%10%20%30%40%50%60%70%80%
ERA Interim
If circulation change has a similar magnitude seasonality:
q0
[q]~25%
~50%
~5%
�!0
�[!]⇠ q0
[q] Error
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• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
Seasonality is important for precip response to sulfate aerosol forcing in “continental” regime
N.B. Energetics of seasonal circulation changes is a useful perspective, though energy storage is important: Chou & Neelin (2003), Merlis et al.
(2013b), Chamales et al. (2015)‘Recipe’ TBD…
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Direct vs. Temperature Mediated Climate Changes
dX
dCO2⇡ @X
@hTsi@hTsi@CO2
Many climate changes are proportional to the amount of global warming:
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Direct vs. Temperature Mediated Climate Changes
But radiative forcing agents can also directly change aspects of climate:
dX
dCO2⇡ @X
@hTsi@hTsi@CO2
+@X
@CO2
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Thermodynamic
Tropical precipitation change
Dynamic
Bony et al. (2013)
Increased CO2 “directly” weakens tropical circulations.
CMIP5 abrupt 4xCO2
�u q0u0 �q
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Circulation changes in fixed-SST simulations
• Fixed-SST aGCM circulations weaken when CO2 is increased.
• CMIP5 aquaplanet circulations also weaken (land-sea effects modulate rather than cause the changes).
Bony et al. (2013)
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Global hurricane (TC) frequency response from direct GHG circulation change
Held & Zhao (2011)
M2K GHG SST TOPO SUM−5
0
5
10
15
δ G
loba
l Hur
rican
e Fr
eq. (
%)
LGM LGM LGM LGM
Merlis (2015, in prep.)
Direct GHG change in hurricane frequency is robust and ~50% of the total change.
Warming climate changes Cooling climate changes (LGM)
![Page 25: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/25.jpg)
• Allows the circulation to be related to the energy sources & sinks (e.g., radiation) without explicit consideration of latent heating.
• Efficiency of circulation energy transport (gross moist stability) may change.
Held & Hou (1980), Neelin & Held (1987), Held (2001), Merlis et al. (2013a,b)
Moist energetics of direct response of tropical circulations to CO2
Analysis of moist static energy:
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• Allows the circulation to be related to the energy sources & sinks (e.g., radiation) without explicit consideration of latent heating.
• Efficiency of circulation energy transport (gross moist stability) may change.
Held & Hou (1980), Neelin & Held (1987), Held (2001), Merlis et al. (2013a,b)
Moist energetics of direct response of tropical circulations to CO2
Analysis of moist static energy:
What is the radiative forcing of doubling CO2?
Quiz!
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• Allows the circulation to be related to the energy sources & sinks (e.g., radiation) without explicit consideration of latent heating.
• Efficiency of circulation energy transport (gross moist stability) may change.
Held & Hou (1980), Neelin & Held (1987), Held (2001), Merlis et al. (2013a,b)
Moist energetics of direct response of tropical circulations to CO2
Analysis of moist static energy:
The spatial structure of CO2 radiative forcing (often ignored) leads to direct weakening of tropical circulations.
Conclusion from moist energetics:
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Spatial structure of CO2 radiative forcing
Zhang & Huang (2014)
Annual meanWm�2
Govindasamy & Caldeira (2000)
Wm�2
The climatological cloud distribution masks the CO2 radiative forcing in regions of mean ascent.
Zonal mean
{/T
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Surface radiation & fluxes also affect circulation energetics.
Sketch of cloud masking of CO2 radiative forcing
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Required atmospheric energy transport decreases.
Sketch of cloud masking of CO2 radiative forcing
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Forcing gradient also acts to oppose Walker circulation.
Sketch of cloud masking of CO2 radiative forcing
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Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Comprehensive rad: “Cloud off” rad: “Fixed RH” rad:
-3.9% -1.4% +0.1%I = !# � !", �I/I :
�!4⇥�1⇥CO
2!1⇥CO
2
GFDL’s AM2.1 direct circulation response to 4⨉CO2
Masking of forcing deactivated
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Direct CO2 weakening of tropical circulations decreases as masking is deactivated!
Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Latit
ude
ω (500 hPa, Annual mean)
(Pa s−1)
0 90 180 −90 0−30−20−10
0102030
−0.12−0.09−0.06−0.03
0.030.060.090.12
Longitude
Latit
ude
δ ω (500 hPa, Annual mean)
0 90 180 −90 0−30−20−10
0102030
−0.024−0.018−0.012−0.006
0.0060.0120.0180.024
Comprehensive rad: “Cloud off” rad: “Fixed RH” rad:
�!4⇥�1⇥CO
2!1⇥CO
2
GFDL’s AM2.1 direct circulation response to 4⨉CO2
![Page 34: Tropical Circulation Changes Across Forcing Agents · Global hurricane (TC) frequency response! from direct GHG circulation change Held & Zhao (2011) M2K GHG SST TOPO SUM −5 0 5](https://reader033.vdocuments.us/reader033/viewer/2022041611/5e37caa535d6af581e6c350b/html5/thumbnails/34.jpg)
Idealized Models
~2% direct weakening across model hierarchy
GCM from Merlis et al. (2013)
SLM from Sobel & Schneider (2009)
Prescribed cloud
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• We cannot straightforwardly apply the energetic ITCZ framework to monsoons.
• ‘Direct’ response of circulation to forcing agents may differ.
Conclusions
Seasonality is important for precip response to sulfate aerosol forcing in “continental” regime
New mechanism for direct CO2 circulation weakening posits a key role for spatial pattern of forcing