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Gourcuff C.,French National Center for Scientific Research | Lherminier P.,French National Center for Scientific Research | Mercier H.,French National Center for Scientific Research | Le Traon P.Y.,Laboratoire dOceanographie Spatiale
Journal of Atmospheric and Oceanic Technology | Year: 2011

A method to estimate mass and heat transports across hydrographic sections using hydrography together with altimetry data in a geostrophic inverse box model is presented. Absolute surface velocities computed from Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) altimetry products made up of a combination of sea surface height measurements and geoid estimate are first compared to ship acoustic Doppler current profiler (S-ADCP) measurements of the Observatoire de la Variabilité Interannuelle et Décennale (OVIDE) project along hydrographic sections repeated every 2 yr in summer from Portugal to Greenland. The RMS difference between S-ADCP and altimetry velocities averaged on distances of about 100 km accounts for 3.3 cm s -1. Considering that the uncertainty of S-ADCP velocities is found at 1.5 cm s -1, altimetry errors are estimated at 3 cm s -1. Transports across OVIDE sections previously obtained using S-ADCP data to constrain the geostrophic inverse box model are used as a reference. The new method is found useful to estimate absolute transports across the sections, as well as part of their variability. Despite associated uncertainties that are about 50% larger than when S-ADCP is used, the results for the North Atlantic Current and heat transports, with uncertainties of 10%-15%, reproduce the already observed variability. The largest uncertainties are found in the estimates of the East Greenland Irminger Current (EGIC) transport (30%), induced by larger uncertainties associated with altimetry data at the western boundary. © 2011 American Meteorological Society. Source

Tournadre J.,Laboratoire dOceanographie Spatiale | Bouhier N.,Laboratoire dOceanographie Spatiale | Girard-Ardhuin F.,Laboratoire dOceanographie Spatiale | Remy F.,French National Center for Space Studies
Journal of Geophysical Research C: Oceans | Year: 2015

Large uncertainties exist on the volume of ice transported by the Southern Ocean large icebergs, a key parameter for climate studies, because of the paucity of information, especially on iceberg thickness. Using icebergs tracks from the National Ice Center (NIC) and Brigham Young University (BYU) databases to select altimeter data over icebergs and a method of analysis of altimeter waveforms, a database of 5366 icebergs freeboard elevation, length, and backscatter covering the 2002-2012 period has been created. The database is analyzed in terms of distributions of freeboard, length, and backscatter showing differences as a function of the iceberg's quadrant of origin. The database allows to analyze the temporal evolution of icebergs and to estimate a melt rate of 35-39 m·yr-1 (neglecting the firn compaction). The total daily volume of ice, estimated by combining the NIC and altimeter sizes and the altimeter freeboards, regularly decreases from 2.2 104km3 in 2002 to 0.9 104km3 in 2012. During this decade, the total loss of ice ( ∼1800 km3·yr-1) is twice as large as than the input ( ∼960 km3·yr-1) showing that the system is out of equilibrium after a very large input of ice between 1997 and 2002. Breaking into small icebergs represents 80% ( ∼1500 km3·yr-1) of the total ice loss while basal melting is only 18% ( ∼320 km3·yr-1). Small icebergs are thus the major vector of freshwater input in the Southern Ocean. © 2015. American Geophysical Union. Source

Tournadre J.,Laboratoire dOceanographie Spatiale | Bouhier N.,Laboratoire dOceanographie Spatiale | Girard-Ardhuin F.,Laboratoire dOceanographie Spatiale | Remy F.,CNRS Geophysical Research and Oceanographic Laboratory
Journal of Geophysical Research: Oceans | Year: 2016

Basal melting of floating ice shelves and iceberg calving constitute the two almost equal paths of freshwater flux between the Antarctic ice cap and the Southern Ocean. The largest icebergs (>100 km2) transport most of the ice volume but their basal melting is small compared to their breaking into smaller icebergs that constitute thus the major vector of freshwater. The archives of nine altimeters have been processed to create a database of small icebergs (<8 km2) within open water containing the positions, sizes, and volumes spanning the 1992-2014 period. The intercalibrated monthly ice volumes from the different altimeters have been merged in a homogeneous 23 year climatology. The iceberg size distribution, covering the 0.1-10,000 km2 range, estimated by combining small and large icebergs size measurements follows well a power law of slope -1.52±0.32 close to the -3/2 laws observed and modeled for brittle fragmentation. The global volume of ice and its distribution between the ocean basins present a very strong interannual variability only partially explained by the number of large icebergs. Indeed, vast zones of the Southern Ocean free of large icebergs are largely populated by small iceberg drifting over thousands of kilometers. The correlation between the global small and large icebergs volumes shows that small icebergs are mainly generated by large ones breaking. Drifting and trapping by sea ice can transport small icebergs for long period and distances. Small icebergs act as an ice diffuse process along large icebergs trajectories while sea ice trapping acts as a buffer delaying melting. © 2015. American Geophysical Union. Source

Fournier S.,Laboratoire dOceanographie Spatiale | Chapron B.,Laboratoire dOceanographie Spatiale | Salisbury J.,Ocean Processes Analysis LaboratorUniversity of New HampshireDurham | Vandemark D.,Ocean Processes Analysis LaboratorUniversity of New HampshireDurham | Reul N.,Laboratoire dOceanographie Spatiale
Journal of Geophysical Research C: Oceans | Year: 2015

Large rivers are key hydrologic components in oceanography, particularly regarding air-sea and land-sea exchanges and biogeochemistry. We enter now in a new era of Sea Surface Salinity (SSS) observing system from Space with the recent launches of the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius/Sac-D missions. With these new sensors, we are now in an excellent position to revisit SSS and ocean color investigations in the tropical northwest Atlantic using multiyear remote sensing time series and concurrent in situ observations. The Amazon is the world's largest river in terms of discharge. In its plume, SSS and upper water column optical properties such as the absorption coefficient of colored detrital matter (acdm) are strongly negatively correlated (<-0.7). Local quasi-linear relationships between SSS and acdm are derived for these plume waters over the period of 2010-2013 using new spaceborne SSS and ocean color measurements. Results allow unprecedented spatial and temporal resolution of this coupling. These relationships are then used to estimate SSS in the Amazon plume based on ocean color satellite data. This new product is validated against SMOS and in situ data and compared with previously developed SSS retrieval models. We demonstrate the potential to estimate tropical Atlantic SSS for the extended period from 1998 to 2010, prior to spaceborne SSS data collection. © 2015. American Geophysical Union. All Rights Reserved. Source

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