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