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PhD Defense

Guillaume Gastineau (LOCEAN)

Title : Decadal climate variability: Mechanisms and impacts

Date and time : The 18-01-2019 at 14h45

Type : HDR

Université qui délivre le diplôme : Sorbonne Université

Location : Salle de conférence de l'UFR TEB (tour 46-56, 2e étage), Campus Jussieu, 4 place Jussieu, 75005 Paris
Members of jury :

Masa Kageyama (LSCE) : Rapporteur

Michel Déqué (CNRM) : Rapporteur

Rowan Sutton (University of Reading) : Rapporteur

Pascale Bouruet-Aubertot (LOCEAN) : Présidente

Didier Swingedouw (EPOC) : Examinateur

Gwendal Rivière (LMD) : Examinateur

Summary :

I will summarize my studies dealing with the decadal to multidecadal climate variability. The decadal to multidecadal climate variability accounts for an important part of the near-term (next 30 years) climate evolution, with dominant effects at the regional scale. Over the North Atlantic Ocean, the Atlantic Multidecadal variability (AMV) is the dominant mode at low frequencies, with a signature on surface temperature and salinity. The AMV represents the effects of both external forcings and internal climate variability. In climate models, the AMV is also closely linked to the variability of the Atlantic Meridional Overturning circulation (AMOC). In IPSL-CM5A-LR, the AMOC variability and AMV are linked to a subpolar gyre variability with a 20-yr period. This variability is governed by an oceanic mode amplified by the atmosphere-ocean coupling.
Observation and model analyses show that the AMV and AMOC have some climate impacts during the cold-season. Most models and observations show in winter a negative NAO-like response to an intensified AMOC and to a warming of the subpolar gyre. This atmospheric response is induced by the meridional sea surface temperature gradient changes in the region of eddy growth, off the coast of Newfoundland and along the North Atlantic Current. The mechanisms responsible for the AMOC variability are not robust among models so that the atmospheric response is also model dependent and may show some non-stationarity.
The variability of the Arctic sea-ice may also modify the atmospheric circulation in the Northern Hemisphere. A reduction of the sea-ice in the Barents and Kara Seas and an increase of the Siberian snow cover in November may lead to a weakening of the stratospheric polar vortex, evolving into a negative North Atlantic Oscillation pattern in winter. This effect is largely underestimated in coupled climate models, or in atmosphere-only experiments. Lastly, we emphasize the important role of internal climate variability for the air temperature trends found over the recent decades.

Contact :
Guillaume Gastineau, LOCEAN/IPSL, guillaume.gastineau@upmc.fr
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