Accueil > Actualités > Séminaires > Seminaire de Svein Østerhus


Titre : Is the high latitude limb of the Bipolar Atlantic Thermohaline Circulation changing?
Nom du conférencier : Svein Østerhus
Son affiliation : Uni. Bergen
Laboratoire organisateur : LOCEAN
Date et heure : 12-11-2014 12h15
Lieu : UPMC - 4 place jussieu - Paris 5e - LOCEAN - tour 45-55 - 4e étage - salle de réunion
Résumé :

Most climate models predict a reduction in the Atlantic thermohaline circulation (THC). This presentation will address the question whether the weakening has already been initiated. Results from long-term monitoring of the Arctic Mediterranean Exchanges and for the Ice Shelf Water Overflow in Antarctica will be presented.

The flow of Atlantic water (Atlantic inflow) across the Greenland-Scotland Ridge (GSR) is critical for conditions in the Nordic Seas and Arctic Ocean by importing heat and salt. All three branches crossing the GSR have been monitored since the mid-1990s and the transports of water and heat have been estimated. The Atlantic inflow, that forms the surface part of the thermohaline circulation in the North Atlantic, is affected by wind forcing and freshwater input but the most important driving force appear to be the cooling of the ocean by the atmosphere in the subarctic seas and the increase salinity in the Arctic Ocean through freezing of seawater. This results in the sinking of surface waters that subsequently flow out of the area close to the bottom over the GSR. This removal of water from the Arctic region by the overflow generates sea level slopes that drive a northward transport of water and heat. With global climate change, the Arctic atmosphere is expected to warm and freshwater input to the Arctic to increase, both of which may act to slow down the mechanism that drives these flows.

The coldest and most oxygen-rich bottom waters in the Southern Ocean have their origin in the Weddell Sea. Cold, dense shelf waters are formed on the shallow shelves by cooling and ice formations during winter. This High Salinity Shelf Water (HSSW) penetrates below the floating glaciers and contributes to their melting from below. Because this melting takes place at high pressures (500 - 1500 m) the melt product, called Ice Shelf Water (ISW) is easy to identify in the ocean because it is supercooled with respect to the surface freezing point. On its way northwards, crossing the sill of the Filchner Depression and cascading towards the ocean abyss the ISW interacts with HSSW and offshore water masses at the continental shelf break and contributes to the formation of Antarctic Bottom Water (AABW), which feeds the global thermohaline circulation. Long term observations of the flow of dense waters from their area of formation to the abyss of the World Ocean, and the return flow of warm waters, are central to climate research. For the Weddell Sea an important component of such a system entails monitoring the formation of High Salinity Shelf Water on the continental shelf north of Ronne Ice Front, the transformation to Ice Shelf Water beneath the floating Filchner-Ronne ice shelf, and the flux of ISW overflowing the shelf break to the deep Weddell Sea. Equally important is the return flow of warm water towards the Filchner-Ronne Ice Shelf system. Modell results indicate an increased heat transport in this return flow with increased basal melting of the Filchner-Ronne Ice Shelf and changes in the production of deep water as a consequence.