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The Ocean and Climate Change: Variations in Ocean Circulation

Xavier Giraud
p. 46

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Source: La lettre, no. 34, July 2012

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1The oceans cover over 70% of the Earth's surface. Along with the atmosphere, they redistribute the energy that our planet receives from the sun.

Figure credit: A. Biastoch GEOMAR-Kiel

2The joint action of winds and the Coriolis force induce surface currents, most of which have been known to sailors for centuries. However, sailors did not suspect the existence of a much larger system, on a global scale, linked to the circulation of the intermediate and deep layers of the ocean. The ocean can therefore play a key part in the climate system, involving far more than a passive attenuation of atmospheric variations.

3Has the ocean circulation varied in the past and how will it behave in the future with likely changes in temperatures, rain and winds? With a view to exploring these questions, a symposium brought oceanographers from several countries together at the Collège de France. In his introductory address, Édouard Bard pointed out the fundamental role of the oceans in the climate’s machinery, before illustrating variations in ocean circulation as part of long-term trends on the scale of centuries and millennia.

4Harry Bryden (National Oceanography Centre, University of Southampton) reviewed the research on current variability in the North Atlantic, particularly the Gulf Stream and the southward deep western boundary current. The methods used range from the analysis of hydrographic data on over half a century, to direct measurements of water flows on an instrumented section at 26.5° N between Florida and Morocco. A complementary approach consists in studying the water masses in the North Atlantic, at a higher latitude, to follow their sinking at depth in the Nordic Seas and the Labrador Sea. Monika Rhein (Institute of Environmental Physics, University of Bremen) described recent research based on instrumented mooring lines, and on the penetration of che­mical tracers like anthropogenic freons into the ocean. While the time series illustrate the short-term complexity linked to the North Atlantic Oscillation (NAO), they are still too short to distinguish a long-term trend. Gilles Reverdin (Oceanography and Climate Laboratory, LOCEAN, CNRS-IRD-UPMC Paris) showed how the combination of data from altimetric satellites and from drifters allows for the mapping of su rface ­currents, 00and for the description of a “supergyre” system in the ­southern hemisphere, connecting the three main oceans. Long-term projections can be made using numerical models of the ocean-atmosphere coupling, by simulating the highest frequency oceanic variability. Jochem Marotzke (Max Planck Institute for Meteorology, Hamburg) reviewed the possibility of improving climate forecasts by initializing an oceanic model with atmospheric observations. This modelling work shows the importance of taking the dynamic atmosphere-ocean coupling into account to forecast surface temperatures in the North Atlantic and Europe over the next decade.

5At the symposium, the importance and complexity of the ocean’s role in global heat transfers and interactions with the atmosphere were illustrated using the example of the Agulhas current, around the southern tip of Africa. Mathieu Rouault (Department of Oceanography, University of Cape Town) first described this current’s influence on regional meteorology, before showing how part of the current returns towards the Indian Ocean, while an element escapes towards the Atlantic through a series of mesoscale eddies. For the last 40 years, the transfer from one basin to the other has apparently increased significantly, affecting the hydrology and circulation of the South Atlantic. As Arne Biastoch (GEOMAR, Helmholtz Centre for Ocean Research, Kiel) then showed, numerical models of this link between the Agulhas current and the Atlantic circulation are being developed, with a constantly improving spatial resolution to simulate explicitly the transient eddies (see figure).

6The study of the data and numerical models clearly shows that the variability of Atlantic meridional circulation and of its climatic impact can be understood by taking into account the influence of the high latitudes of the two hemispheres, especially sinking in the Labrador Sea and the Nordic Seas, as well as the combined effects of multiple phenomena occurring in the Southern hemisphere, particularly water mass transfers in South of Africa, through the Drake Passage, South of America, and at the level of the systems of westerly winds blowing over the Southern Ocean.

• Programme and videos available online at www.college-de-france.fr

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Légende Figure credit: A. Biastoch GEOMAR-Kiel
URL http://lettre-cdf.revues.org/docannexe/image/2694/img-1.png
Fichier image/png, 2,0M
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Xavier Giraud, « The Ocean and Climate Change: Variations in Ocean Circulation », La lettre du Collège de France [En ligne], 7 | 2015, mis en ligne le 02 novembre 2015, consulté le 24 août 2017. URL : http://lettre-cdf.revues.org/2694

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