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PhD Defense

Jorge Alvarez-Solas (LSCE)

Title : Abrupt changes and rapid variability in different climatic contexts

Date and time : The 18-10-2010 at 14h30

Type : thèse

Université qui délivre le diplôme :

Location : Université Pierre et Marie Curie, 4 place Jussieu, Paris, Amphi 25, Tour 25
Members of jury :

Pr. Hervé Le Treut (Président)
Dr. Frédérique Rémy (Rapporteur)
Dr. Frank Pattyn (Rapporteur)
Dr. Catherine Ritz (Examinatrice)
Dr. Marisa Montoya (Examinatrice)
Dr. Sylvie Charbit (Directrice de thèse)
Dr. Gilles Ramstein (Co-directeur de thèse)

Summary :

Ice core data as well as marine and continental records reveal the existence of pronounced millennial time-scale variability in the Quaternary climate system. Such rapid climate variability appears to be stronger in glacial periods than during interglacials, but there is not yet a full consensus about its origin.

Firstly, the Dansgaard-Oeschger events are characterized by abrupt transitions occurring in a few decades, and by a period of a few thousand years. Two types of explanation have been suggested concerning its triggering mechanism : periodic external forcing and internal oscillations in the climate system, for which ocean circulation is a likely candidate.

On the other hand, six periods of extreme cooling in the Northern Hemisphere were marked by an enhanced discharge of icebergs into the North Atlantic Ocean, increasing the deposition of ice-rafted debris (known as Heinrich events). Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses, but recent observations suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions.

I tried here to bring new insights about the mechanisms responsible for the millennial glacial variability, more consistent with the present knowledge of the different Earth's components. This work is based on the use of a hierarchy of climate and ice-sheet models of different complexities.

We used a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resulting changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf.

Using a state-of-the-art tri-dimensionnal ice-sheet model we also explore the conditions leading to internal oscillations of geometrically idealised ice sheets. Our studies are then focalised on the Heinrich event 1, showing a new mechanism based on the effects of a subsurface warming on the ice shelves stability. We demonstrate that such ice-shelf break-up and the subsequent icestream acceleration should be considered as a likely candidate to generate the icebergs surge implicated in Heinrich event 1.

Leaving glacial period we finally focus on the present-day anthropically perturbed interglacial. We analyse with a fully coupled climate ice sheet model whether the shift into a warmer climate in the future could favor the occurrence of a new millennial-scale climate variability.

Contact :
Jorge Alvarez-Solas, LSCE 0169083197
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