Didier Swingedouw, Laurent Terray, Christophe Cassou, Aurore Voldoire,
David Salas-Mélia, Jérôme Servonnat
CERFACS, France
ESCARSEL project
Natural forcing of climate during
the last millennium: Fingerprint of
solar variability
Trouet et al., Science, 2009
The last millennium
MCA / LIA
(850-1250) / (1400-1800)
Mann et al., Science, 2009
Nina / Nino
NAO+ / NAO-
Solar variability
Volcanic eruptions
GHG variations
Natural variability of the ocean (low frequency)
Lots of debate!
Mechanism of climate variability during the last
millennium
Shapiro et al., Astronomy & Astrpohysics , 2011
Forcing of the little ice age (1/2)
Solar forcing?
A very slight forcing
Need for amplifying mechanism: (Schindell et al., 2001)
Decrease in solar irradiance leads to a negative NAO (and ozone response)
20 years delay at least for this low NAO trend => origin?
Regression / TSI, lag 20 years filter 40 years (1780-1680)
Proxy Mann
Simulation ModelE
Another potential amplifying mechanism (Lund et al., 2006)
Multi-secular variability of the Gulf-Stream
Related to a change in thermohaline circulation or wind forcing?
Gulf stream transport reconstruction
Forcing of the little ice age (2/2)
Can the solar forcing explain the low frequency of the last millennium climate variability (before 1850)?
What are the fingerprints of solar forcing?
What are the main amplifiers of this forcing? (Nino, NAO, AMOC…)
Questions
Experimental design
CNRM-CM3 coupled GCM (atmosphere ARPEGE T63 = 2.8°, L31 , ocean ORCA2 =2°, L31)
External forcing:
Solar: Crowley 2000 (0.25% changes between Maunder Minimum and present day
GHG and aerosols
Volcanoes (Ammann et al. 2007)
Main response
Northern hemisphere variations in agreement with reconstructions
Strong correlation(>0.7) between solar forcing and temperature
Regression on solar forcing (filtering at 13 years cut-off)
Solar forcing and NAO
NAO and solar forcing exhibit a significant correlation with a more than 40 years time lag
This is not far from the (at least) 20-30 years in the data (Waple et al. 2002) and with the reconstruction from Luterbacher (2001)
Low NAO phase at the end of the XVIIIth century: delayed response of the Maunder Minimum(?)
Solar leads NAO leads
Changes in stationary
waves
The signal is reminiscent of the Arctic Oscillation (better correlation with this index)
The largest positive signal is found in the Pacific Basin
Tropical-extratropical teleconnection
DecadeDecade
ss
PSL+
PSL-
PSL+
: PrecipitationDJF SST,
NAO in model and reconstructions
NAO in model and observations
Solar forcing and AMOC
1. Change in NAO can modify convection in the Labrador Sea and the AMOC: Solar forcing + => NAO+ => convection Labrador + => AMOC +
1. Direct radiative effect of solar forcing can also affect the convection sites: Solar forcing + => SST+ => convection - => AMOC -
Which effect is the largest?
Quadfasel et al. 2005
Convection and AMOC in the model
Winter Mixed layer depth in CTRL
Convection sites correctly represented in this model.
Impact of the NAO on the Labrador sea is also correctly represented
The AMOC is of 21 Sv at 26.5°N in agreement with RAPID array
Solar forcing and AMOC
Principal component of 1st EOF of the AMOC is well correlated with solar forcing at lag 10 years
This corresponds to a weakening of the AMOC when solar forcing increases
Thermal effect (SST increase) due to radiative forcing dominates
Solar leads AMOC leads
Solar forcing and the
subtropical gyre
The 2nd EOF of the barotropic streamfunction exhibits a correlation with solar forcing
This is related with changes in NAO and winds
Effect on the Gulf Stream is unclear maybe due to low resolution of the ocean model
Conclusions
Solar forcing affects the low frequency of the NAO in this model with a 40 years lag
This is due to a mechanism implying the tropical Pacific Ocean response to solar forcing and a Rossby wave teleconnection
The change in tropical Pacific mean state, when solar is high, resembles la Nina State as in data from Mann et al. (2009) for the Medieval Warm Period
The AMOC is weakened when solar forcing increases in this model
To explain the Lund et al. (2006) modifications in the Gulf Stream, changes in wind stress is the best hypothesis in this model.
Thank you
Swingedouw et al.Natural forcing of climate during the last millennium: Fingerprint of solar variability.
Climate Dynamics, published online, 2010
NAO in model and reconstructions
correlation=0.46correlation=0.28