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Par Webmaster Legos Dernière modification 07/02/2012 11:59

Jeudi 17 Janvier - OVIDE-A25, a biennial hydrographic transect across the North Atlantic Subpolar Gyre since 2002: Overview of the main scientific findings about the variability of the Atlantic Meridional Overturning Circulation and its impact on the CO2

Par SEMSOU Dernière modification 11/12/2018 15:22
Quand ? Le 17/01/2019,
de 11:00 à 12:00
Où ? salle Jules Verne
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Pascale Lherminier

Laboratoire d'Océanographie Physique et Spatiale (LOPS, Brest)

 

OVIDE-A25, a biennial hydrographic transect across the North Atlantic Subpolar Gyre since 2002: Overview of the main scientific findings about the variability of the Atlantic Meridional Overturning Circulation and its impact on the CO2 physical pump

 

Abstract: Important processes regulate our climate in the northern North Atlantic: this is presently the place where waters transported northwards in the upper limb gain density and eventually sink into the southward flowing lower limb, driving the Meridional Overturning Circulation, and where the anthropogenic carbon storage rate is the highest. The variability of the subpolar gyre circulation, the MOC and heat transport were quantified from an inversion of hydrographic and velocity data from nine repeats of the Greenland to Portugal OVIDE-A25 section covering 1997-2016. The obtained circulation patterns revealed remarkable transport changes in the whole water column and evidenced large variations in the magnitude of the MOC computed in density coordinates (MOCσ). The heat transport estimated from the repeated hydrographic OVIDE sections was found linearly related to the MOC strength. The extent and timescales of the MOCσ variability in 1993–2016 were then evaluated using a monthly MOCσ index built upon altimetry and ARGO data at the OVIDE section location. The MOCσ index, validated by the good agreement with the in situ estimates, shows a large variability on monthly to decadal time scales that will be analyzed. The heat transport estimated from the repeated hydrographic OVIDE sections is linearly related to the MOCσ intensity. The uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean is also strongly impacted by the variability of the MOCσ. We found that the uptake of anthropogenic carbon dioxide occurred almost exclusively in the subtropical gyre. In contrast, natural carbon dioxide uptake dominated in the subpolar gyre. We attributed the weakening of total carbon dioxide uptake observed between 1997 and 2006 in the subpolar North Atlantic to  the transitory slowdown of the MOCσ, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic carbon dioxide storage in subpolar waters. Since 2014, we observed a drastic change in the subpolar gyre, with an enhanced MOC and deep convection reaching 1500m in the Irminger and Labrador seas. As a consequence, the acidification signal penetrated deeper and is susceptible to affect the deep coral reefs in the North Atlantic.

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Lundi 7 janvier - Mass balance of Icelandic glaciers in variable climate

Par SEMSOU Dernière modification 10/12/2018 11:57
Quand ? Le 07/01/2019,
de 14:00 à 15:00
Où ? salle Coriolis
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Joaquin Belart

Docteur LEGOS - Université d'Islande

 

Mass balance of Icelandic glaciers in variable climate

 

 

Abstract : This thesis focuses on remote sensing techniques to measure geodetic mass balance of Icelandic glaciers from seasonal to decadal time spans and the relationship of mass balance to climate. The data used consists on frame stereo photographs (1945-2000), lidar (2008-2013) and satellite images (2000-present). At seasonal time scales, the winter mass balance of Drangajökull was measured from satellite sub-meter stereo images at the beginning, middle and end of the 2014–2015 winter using data from the Pléiades and WorldView-2 satellites. The results were complemented with in situ snow density measurements and validated with snow thickness measurements. The study concludes that images from the sensors mentioned above may now be used to monitor seasonal mass balance without tedious field logistics. In decadal time scales Eyjafjallajökull was used as test area to develop semi-automated processing chains based on open-source software for processing the stereo imagery, resulting in a 70-year time series of geodetic mass balances. Simple linear regression indicates that most mass balance variations can be related to changes in summer temperature and winter precipitation. It also allows infering the sensitivities of glacier mass balance to these two climatic variables. This processing chain was then applied to 14 Icelandic glaciers and ice caps spatially distributed in all quarters of Iceland. The glaciers of the south and west coasts exhibited the highest decadal variability. This study improves the knowledge of the Icelandic glaciers prior to the 1990‘s and how subsequent warming altered their mass balance.

 

The presentation consists in a remake of Joaquin's PhD defense and will be given in english.

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Jeudi 13 Décembre - Réponse biologique au phénomène d'upwelling: phytoplancton, zooplancton et écosystèmes

Par SEMSOU Dernière modification 23/11/2018 13:02
Quand ? Le 13/12/2018,
de 11:00 à 12:00
Où ? salle Lyot
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Monique Messié

Institut Méditerranéen d'océanologie (MIO, Marseille)

 

Réponse biologique au phénomène d'upwelling: phytoplancton, zooplancton et écosystèmes

 

Résumé : Dans les grands systèmes d'upwelling de bord est (EBUS : Californie, Pérou/Chili, Afrique nord-ouest et Benguela), les vents parallèles à la côte provoquent des upwellings, remontées d'eaux profondes et riches en nutriments qui stimulent une très forte productivité biologique. Ainsi, même si ces écosystèmes représentent moins de 1% de l'océan mondial, ils contribuent à plus de 3% de la production primaire océanique et 20% des pêches mondiales. Le flux de nutriments entraînés par l'upwelling dans la couche de surface (Nsupply pour « nitrate supply ») peut être calculé à partir de vents satellites et mesures de nitrate in situ, et utilisé pour étudier la réponse biologique au phénomène d'upwelling. Cette présentation explorera : (1) la régulation de la production primaire, en comparant directement Nsupply avec un produit satellite de production primaire; (2) la façon dont Nsupply et courants de surface structurent les "hotspots" de zooplancton (krill et copepods); et (3) comment la variabilité haute fréquence de l'upwelling se transmet à travers la chaîne trophique, depuis la surface jusqu'au fond de l'océan via la zone mésopélagique.

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