SORCE,
Keystone,
CO, May,
December
2001
2010
Temperature Responses to Spectral Solar Variability
On Decadal Timescales
Robert F. Cahalan, NASA/Goddard
Guoyong Wen, NASA/Goddard & UMBC
Peter Pilewskie, U. Colorado – LASP
Jerry Harder, U. Colorado – LASP
Observations of Solar Spectral Irradiance (SSI)
from SIM, an Alternative 11-yr Solar Forcing Scenario
Radiative-Convective Model (RCM)
RCM description, experiments, results
GISS-GCM – results from Alternative 11-year
Summary & Plans
Robert Cahalan, Page 1
SORCE,
Keystone,
CO, May,
December
2001
2010
History of Solar Irradiance Observations

TSI
» 28 year history – continuous observations over ~ 3 solar cycles

SSI
» 6 year history commenced with SORCE SIM
» Half a single solar cycle observed thus far
» Until SORCE, comparisons have previously been made
between individual reference spectra, not full time series at UV,
VIS and IR wavelengths.
Robert Cahalan, Page 2
UMD
– Apr 2001
-2010
December
A Contrast in Spectral Variability
Brightening
with
decreasing
solar
activityvity
Dimming with
decreasing
solar activity
Robert Cahalan, Page 3
SORCE Observations of Spectral Solar
Irradiance
(SSI)
SORCE,
Keystone,
CO, May,
December
2001
2010
Harder et al, 2009: GRL 36, L0701, doi:10.1029/2008GL036797
Instrument
λ Range (nm)
∆λ (nm)
TIM: Total Irradiance Monitor
TSI (all)
-
SIM: Spectral Irradiance Monitor
200-2700
1-30
SOLSTICE: Solar Stellar Irradiance
Comparison Experiment
115-320
0.1
XPS: XUV Photometer System
0.1-27, 121.6
7-10
TSI & SSI do not vary in-phase
–
–
Before Sorce, models fix the spectral shape
T<Tave brightening, T>Tave dimming
Near-UV and Visible Are Compensating
–
UV variability much larger, but offset by the
Visible
NIR below 970 nm compensates that above
–
Peak ~1600 nm due to H– (Wildt, 1939)
Obs & Models have similar ∆TSI, not ∆SSI
–
–
Pre-industrial TSI and SSI poorly known
climate implications await further research
Robert Cahalan, Page 4
SORCE,
Keystone,
CO, May,
December
2001
2010
Absorption of Solar Radiation
O3
Hartley bands
Huggins bands
Chappuis bands
O2-A band
H2O
0.72µm
0.82µm
0.94µm
1.14µm
1.38µm
1.87µm
175nm
1
2
3
2 4 5 6
------UV-C
UV
UV-B
7
8
9
10
11
10µm
UV-A
VIS
NIR
Robert Cahalan, Page 5
SORCE,
Keystone,
CO, May,
December
2001
2010
Radiative Convective Model (RCM)


Tropical Atmosphere: Convective adjustment with water vapor feedback (Manabe
and Weatherald, 1967) with 18 vertical layers (0.2 hPa ≈ 61 km)

Set Cloud Fraction = 0.0, tune Horizontal Flux to get T(Sfc) = 300 K
Ocean mixed layer: coupled with atmosphere. Latent heat flux computed from
atmospheric water vapor.
feedback ∆Teq = f∆To
ρ w Cw H
dTm (t)
= −ρa ⋅ CD⋅ V {L[qs (Tm (t)) − q(Ts (t))]+ C p [Tm (t) − Ts (t)]}
dt
SH
LH
+ Fr (t) + Pr(t)
Rad Flux
g = ∆Tfeedback /∆Teq , f =
gain
∆Teq =
Precipitation
1
1− g
∆To
1− ∑ gi
i
ρw
water density
Cw
specific heat of water
Solar Irradiance at 11 bands as a function of time
Cp
speciifc heat of air
Surface albedo
H
depth of ocean mixed layer
ρa
surface air density
CD
drag coefficient
CO2,O3,…
Lw
latent heat of vaporization
Cloud Fraction, LWP/IWP, heights
qs
water vapor sat. mixing ratio
q
surface mixing ratio
V
surface wind speed
Fr
surface net radiation flux
Pr
precipitation
Model Inputs
Water vapor property (fixed relative or specific humidity)
Aerosol optical thickness
Surface albedo, Net horizontal flux
Robert Cahalan, Page 6
SORCE,
Keystone,
CO, May,
December
2001
2010
RCM Response to Instantaneous CO2 Doubling
TOA Forcing
Stratospheric dT Response
Stratospheric dT from Microwave
Longwave
Shortwave
Net
Temperature response
40 km
25 km
Surface Forcing
Surface T Response
Longwave
Shortwave
Net
Temperature response
Mixed Layer
Surface Air
Robert Cahalan, Page 7
SORCE,
Keystone,
CO, May,
December
2001
2010
2 Experiments with 11-year Solar Forcing
∆TSI / TSI ≈ 0.1%

Experiment 1: as in Lean (2000)
» UV, VIS and NIR vary in phase with TSI
» UV, VIS, NIR amplitudes proportional to band energies
– Amp (UV, VIS, NIR) = 0.28, 0.47, 0.25 X Amp (TSI)

Experiment 2: as in Harder et al. (2009)
» VIS varies out-of-phase with TSI
» UV amplitude much larger, compensated by out-of-phase VIS
– Amp (UV, VIS, NIR) = 1.67, –0.80, 0.13 X Amp (TSI)

Both Experiments: °
» TSI variations (peak to peak) ≈ 0.09% X TSI ≈ 1.2 W/m2
» TSI top-of-atmosphere (TOA) forcing: [cos (55°) / 2] X TSI ≈ 0.34 W/m2
Robert Cahalan, Page 8
SORCE,
Keystone,
CO, May,
December
2001
2010
Solar Variations (Lean 2000)
And RCM Response
Solar Variations (Harder et al 2009)
And RCM Response
TSI
VIS
UV
IR
TSI
UV IR
40km
40km
25km
25km
To
Ta
Robert Cahalan, Page 9
SORCE,
Keystone,
CO, May,
December
2001
2010
Remsberg, 2008:
“Response of HALOE Stratospheric O3
and T to 11-year Solar Cycle Forcing”
Solar Variations (Harder et al 2009)
And RCM Response
There is a “significant maximum minus minimum
response on the order of 1 K for temperature
between 15 S and 15 N and from 2 to 0.7 hPa.“
TSI
UV IR
40km
25km
To
Ta
Robert Cahalan, Page 10
SORCE,
Keystone,
CO, May,
December
2001
2010
Remsberg, 2008:
“Response of HALOE Stratospheric O3
and T to 11-year Solar Cycle Forcing”
GCM Response to Solar Variations
Harder vs Lean
There is a “significant maximum minus minimum
response on the order of 1 K for temperature
between 15 S and 15 N and from 2 to 0.7 hPa.“
Harder et
al (2009)
1.0
(48 km)
1.0
(48 km)
2.0
(43 km)
2.0
(43 km)
1.0
(48 km)
2.0
(43 km)
Lean
(2000)
1.0
(48 km)
2.0
(43 km)
Robert Cahalan, Page 11
SORCE,
Keystone,
CO, May,
December
2001
2010
GCM Response to Solar Variations
Harder vs Lean @Surface
20S-20N average ~0.02K
20S-20N average ~0.002K
Robert Cahalan, Page 12
UMD
– Apr 2001
-2010
December
Summary
Direct Responses to SIM-based Alternative Forcing Scenario


NUV in phase with TSI, larger amplitude, explains observed ~1 K amplitude @40 km.
VIS and NIR out of phase with TSI.
»

TSI, the integral over the spectrum, comprised of spectral bands with compensating effects.
Surface radiative forcing very small, direct surface response < 0.1 K in 11-year
»
»
Climate Impacts of SSI:
Cahalan et al, 2010: Geophys. Res. Lett., 37, L07705, doi:10.1029/2009GL041898.
Next Steps





Include stratospheric chemistry & circulation (see Haigh, also Stolarski)
Reconsider stratosphere-troposphere coupling mechanisms with alternative forcing
Consider alternative cloud-aerosol feedbacks
Consider alternative forcing scenarios for centennial timescales
 Requires coupling to deep ocean
Search for proxies with sensitivity to UV, VIS and NIR multi-decadal trends.
Robert Cahalan, Page 13