Large scale signature of the last
millennium variability: challenges
for climate models
Didier Swingedouw
LSCE, France
The last millennium
 MCA / LIA
(950-1250) / (1400-1700)
 Nina / Nino
 NAO+ / NAO-
Trouet et al., Science, 2009
Mann et al., Science, 2009
Mechanism of climate variability
during the last millennium
 Solar variability
 Volcanic eruptions
 GHG variations
 Natural variability of the ocean (low frequency)
 Lots of debate!
Shapiro et al., Astronomy & Astrophysics , 2011
What does climate
CGCM reproduces?

Tropical Pacific signal hardly found

NAO signal is also very week

Except in CNRMCM3?
GonzalesRoucot et al.
2011
Solar forcing
and NAO
 CNRM-CM3 millennium
experiment (Swingedouw et al.
2010)
 NAO and solar forcing linked
with more than 40 years time lag
 Not inconsistent with
reconstructions
 Low NAO phase at the end of
the XVIIIth century: delayed
response of the Maunder
Minimum(?)
Solar leads
NAO leads
Tropical-extratropical teleconnection
DJF SST,
: Precipitation
SLP-
SLP+
Decade
s
SLP+
NAO in model and reconstructions
NAO in model and observations
Oceanic circulation changes in the Atlantic?
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?
Not found in many GCM.
 internal variability?
 Changes in wind stress (Cf.
CNRM-CM3)
Gulf stream transport reconstruction
Conclusions
 Large scale climate variability for the last millennium in the
reconstructions (Tropical Pacific, NAO, Atlantic circulation)
 AOGCMs hardly capture these features, raising debate
 Nevertheless, in CNRM-CM3, solar forcing affects the low frequency
of the NAO in the CNRM-CM3 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
Link with Greenland project
 Use data from Greenland ice core to evaluate the changes in
large scale circulation as compared to simulations
 For this purpose:
1. Relate large scale atmospheric dynamics and Greenland
climate (analogues and clustering techniques) for
instrumental era
2. Evaluate the changes in these large scale regimes in the
simulations
3. Evaluate any coherency with the data
Thank you
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
Questions
 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…)
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)
Forcing of the little ice age (1/2)
Regression / TSI, lag 20 years filter 40 years (17801680)
Proxy Mann
 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?
Simulation ModelE
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
Volcanoes forcing
 Ammann et al. (2007) :
 Latitude-time forcing (with
largest forcing in the
tropics)
 We isolate 20 eruptions
larger than Pinatubo (in
red peaks in Fig in the top)
T2M signature
 Composite on the 20
volcanoes > Pinatubo
 Large cooling in the
tropics
 Warming in Northern
Europe
 Zonally average: we follow
the forcing (1st order)
: T2m
: Epaisseur optique
Other
signatures
 NAO+ like-response
(agreement with Otera et al.
2008
using the same atm.
model)
 For temperature,
unclear
Persistence of
the signature
 Each “bar” correspond to one event
anomalies (from the 20 selected)
 1 year before the event, at the beginning
(year 0) and so on).
 Surface temperature in the tropics, SLP in
the NH > 65°N (the negative anomalies
 SST in nino3 boxes
 Errors bars on the right= 2sigma