Bruegel : The Harvesters (1565 )

Bruegel : The Harvesters (1565)

Mechanisms for European summer

temperature response to solar

forcing over the last millenniumSwingedouw D., Terray L., Servonnat J., Guiot J.

Background IPCC 2007: large increase in temp.

and drought in the Med. region, while increase in precip. in Northern Europe

« Transition zone » governed by evapotranspiration (Boé and Terray 2008)

Evapotranspiration was also playing a crucial role in 2003 heat wave (Fischer et a. 2007)

Transition zone

IPCC 2007, Chap. 11, Fig. 2

Last millennium: a case-test for European summer response to

radiative changes?

A recent reconstruction now available (Guiot et al. 2010) goes back to 600

Use lots of pollen data which allow a better representation of low frequency

Use of this dataset to test a climate model response to solar forcing

Climate simulation with complex land-surface scheme: CNRM-CM3 using SECHIBA (Swingedouw et al. 2011)

Solar, GHG and volcanic forcings

Last millennium simulation

Solar forcing fingerprints

Low but significant correlation with solar forcing

both in data and model

Summer European temperature

Regression over solar variations : 90%: 95%: 99%

Solar forcing impact in the model

Schar et al. 1999

Large-scale response

Sea ice is responding to solar changes by decreasing its cover in the Nordic Seas

The large-scale atmospheric circulation does not exhibit significant changes over the North Atlantic sector in summer

Surface radiative changes in the model

Evapotranspiration driver

Evapotranspiration over Central Europe is highly correlated in summer with water content in the soil

It is therefore the loading in water of the soil in winter that is mainly driving changes in evapotranspiration

Indeed winter precipitation slighty increases with solar forcing due to Clausius-Clapeyron relationship (not shown)

1950-2006 (Alkama et al. 2010)

Summary of the mechanism

Loading of soil water reservoir in winter

Evapotranspiration increases in summer

More evaporation in the tropics

Summary of the mechanism

55°N25°N 75°N

Summary of the mechanism

55°N25°N 75°N

Temperature

Conclusions Solar forcing over last millennium has small but significant

inprints over Europe in summer with similar latitudinal pattern in reconstruction and model

Long time scales useful to increase signal to noise ratio Evapotranspiration responds strongly to solar forcing in

Central Europe and damps temperature signal through cloud feedback

Sea-ice also responds strongly and enhances solar forcing signal in Northern Europe

Too large amplitude of TSI variations in CNRM-CM3 simulation?

Thank you!

Didier.Swingedouw@lsce.ipsl.fr

Swingedouw D., Terray L., Servonnat J., Guiot J. Mechanisms for European summer temperature response to solar forcing over the last

millennium. Climate of the past 8, 1487-1495, 2012

TSI

Winter precipitation

Early spring soil moisture

Summer Evapotranspiration

Convective clouds

Surface SW

Surface temperature

Convective precipitation

Non convective precipitationSoil

moisture+

+

+

+

+

-

+

+

+

++

Schär et al. (1999) positive feedback

loop

Mechanisms to explain Central Europe response to solar forcing