Summary: The Role of Oceanic Heat Transport and Wind Stress Forcing in Abrupt
Millennial-Scale Climate Transitions
OLIVIER ARZEL
Climate Change Research Centre, The University of New South Wales, Sydney, New South Wales, Australia
ALAIN COLIN DE VERDIE` RE
Laboratoire de Physique des Oce´ans, Universite´ de Bretagne Occidentale, Brest, France
MATTHEW H. ENGLAND
Climate Change Research Centre, The University of New South Wales, Sydney, New South Wales, Australia
(Manuscript received 27 April 2009, in final form 7 December 2009)
ABSTRACT
The last glacial period was punctuated by rapid climate shifts, known as Dansgaard­Oeschger events, with
strong imprint in the North Atlantic sector suggesting that they were linked with the Atlantic meridional
overturning circulation. Here an idealized single-hemisphere three-dimensional ocean­atmosphere­sea ice
coupled model is used to explore the possible origin of the instability driving these abrupt events and to
provide a plausible explanation for the relative stability of the Holocene. Focusing on the physics of noise-free
millennial oscillations under steady external (solar) forcing, it was shown that cold climates become unstable,
that is, exhibit abrupt millennial-scale transitions, for significantly lower freshwater fluxes than warm climates,
in agreement with previous studies making use of zonally averaged coupled models. This fundamental dif-
ference is a direct consequence of the weaker stratification of the glacial ocean, mainly caused by upper-ocean
cooling. Using a two-hemisphere configuration of a coupled climate model of intermediate complexity, it is

Source: Arzel, Olivier - Climate Change Research Centre, University of New South Wales

Collections: Geosciences