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 ( ) / ( ) 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 ( ) 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) years in the data (Waple et al. 2002) and with the reconstruction from Luterbacher (2001)  Low NAO phase at the end of the XVIII th century: delayed response of the Maunder Minimum(?) Solar leadsNAO 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 Decade s 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 1 st 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 leadsAMOC leads

Solar forcing and the subtropical gyre  The 2 nd 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=0.46 correlation=0.28