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| Titel |
Natural forcing of climate during the last millennium: Fingerprint of solar variability |
| VerfasserIn |
Didier Swingedouw, Laurent Terray, Christophe Cassou, Aurore VOLDOIRE, David Salas-Melia, Jerome Servonnat |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035767
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| Zusammenfassung |
| The variability of the climate during the last millennium is partly forced by changes in total
solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that
recent reconstruction data suggest that low frequency variations in the North Atlantic
Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North
Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study
we use a state-of-the-art climate model to simulate the last millennium. We find that modelled
variations of surface temperature in the Northern Hemisphere are coherent with
existing reconstructions. Moreover, in the model, the low frequency variability
of this mean hemispheric temperature is found to be correlated at 0.74 with the
solar forcing for the period 1001-1860. Then, we focus on the regional climatic
fingerprint of solar forcing in winter and find a significant relationship between the low
frequency TSI forcing and the NAO with a time lag of more than 40 years for the
response of the NAO. Such a lag is larger than the around 20-yr lag suggested in other
studies. We argue that this lag is due, in the model, to a northward shift of the tropical
atmospheric convection in the Pacific Ocean, which is maximum more than 4 decades
after the solar forcing increase. This shift then forces a positive NAO through an
atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical
convection is due to the persistence of anomalous warm SST behind the anomalous
precipitation, associated with the advection of warm SST by the North Pacific subtropical
gyre in a few decades. Finally, we analyse the response of the Atlantic meridional
overturning circulation to solar forcing and find that the former is weakened when
the latter increases. Changes in wind stress, notably due to the NAO, modify the
barotropic streamfunction in the Atlantic 50 years after solar variations. This implies a
wind-driven modification of the oceanic circulation in the Atlantic sector in response to
changes in solar forcing, in addition to the variations of the thermohaline circulation. |
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