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Dieng H.B.,LEGOS | Cazenave A.,LEGOS | Meyssignac B.,LEGOS | Ablain M.,CLS Ramonville St Agne France
Geophysical Research Letters | Year: 2017

We revisit the global mean sea level (GMSL) budget during the whole altimetry era (January 1993 to December 2015) using a large number of data sets. The budget approach allows quantifying the TOPEX A altimeter drift (amounting 1.5 ± 0.5 mm/yr over 1993-1998). Accounting for this correction and using ensemble means for the GMSL and components lead to closure of the sea level budget (trend of the residual time series being 0.0 ± 0.22 mm/yr). The new GMSL rate over January 1993 to December 2015 is now close to 3.0 mm/yr. An important increase of the GMSL rate, of 0.8 mm/yr, is found during the second half of the altimetry era (2004-2015) compared to the 1993-2004 time span, mostly due to Greenland mass loss increase and also to slight increase of all other components of the budget. ©2017. American Geophysical Union.

Guinehut S.,CLS Space Oceanography Division | Dhomps A.-L.,LEGOS | Larnicol G.,CLS Space Oceanography Division | Le Traon P.-Y.,French Research Institute for Exploitation of the Sea
Ocean Science | Year: 2012

This paper describes an observation-based approach that efficiently combines the main components of the global ocean observing system using statistical methods. Accurate but sparse in situ temperature and salinity profiles (mainly from Argo for the last 10 yr) are merged with the lower accuracy but high-resolution synthetic data derived from satellite altimeter and sea surface temperature observations to provide global 3-D temperature and salinity fields at high temporal and spatial resolution. The first step of the method consists in deriving synthetic temperature fields from altimeter and sea surface temperature observations, and salinity fields from altimeter observations, through multiple/simple linear regression methods. The second step of the method consists in combining the synthetic fields with in situ temperature and salinity profiles using an optimal interpolation method. Results show the revolutionary nature of the Argo observing system. Argo observations now allow a global description of the statistical relationships that exist between surface and subsurface fields needed for step 1 of the method, and can constrain the large-scale temperature and mainly salinity fields during step 2 of the method. Compared to the use of climatological estimates, results indicate that up to 50% of the variance of the temperature fields can be reconstructed from altimeter and sea surface temperature observations and a statistical method. For salinity, only about 20 to 30% of the signal can be reconstructed from altimeter observations, making the in situ observing system essential for salinity estimates. The in situ observations (step 2 of the method) further reduce the differences between the gridded products and the observations by up to 20% for the temperature field in the mixed layer, and the main contribution is for salinity and the near surface layer with an improvement up to 30%. Compared to estimates derived using in situ observations only, the merged fields provide a better reconstruction of the high resolution temperature and salinity fields. This also holds for the large-scale and low-frequency fields thanks to a better reduction of the aliasing due to the mesoscale variability. Contribution of the merged fields is then illustrated to describe qualitatively the temperature variability patterns for the period from 1993 to 2009. © 2012 Author(s).

Durand F.,LEGOS | Papa F.,National Oceanic and Atmospheric Administration | Rahman A.,National Oceanic and Atmospheric Administration | Bala S.K.,Bangladesh University of Engineering and Technology
Journal of Earth System Science | Year: 2011

This study investigates the impact of monthly Ganges-Brahmaputra river discharge variations on Bay of Bengal salinity and temperature during the period 1992-1999. The Ganges-Brahmaputra river discharge is characterized by a well-defined seasonal cycle with strong interannual variations. The highest/lowest yearly peak discharge occurs in summer 1998/summer 1992, with 1998 value amounting to twice that of 1992. This river discharge is then used to force an ocean general circulation model. Our main result is that the impact of these rivers on the variability of Bay of Bengal sea surface salinity is strong in the northern part, with excess run-off forcing fresh anomalies, and vice versa. Most of the years, the influence of the interannual variability of river discharge on the Bay salinity does not extend south of ~10°N. This stands in contrast with the available observations and is probably linked to the relatively coarse resolution of our model. However, the extreme discharge anomaly of 1998 is exported through the southern boundary of the Bay and penetrates the south-eastern Arabian Sea a few months after the discharge peak. In response to the discharge anomalies, the model simulates significant mixed-layer temperature anomalies in the northern Bay of Bengal. This has the potential to influence the climate of the area. From our conclusions, it appears necessary to use a numerical model with higher resolution (both on the horizontal and vertical) to quantitatively investigate the upper Bay of Bengal salinity structure. © Indian Academy of Sciences.

Sprintall J.,University of California at San Diego | Gordon A.L.,Lamont Doherty Earth Observatory | Koch-Larrouy A.,LEGOS | Lee T.,Jet Propulsion Laboratory | And 4 more authors.
Nature Geoscience | Year: 2014

The Indonesian seas represent the only pathway that connects different ocean basins in the tropics, and therefore play a pivotal role in the coupled ocean and climate system. Here, water flows from the Pacific to the Indian Ocean through a series of narrow straits. The throughflow is characterized by strong velocities at water depths of about 100 m, with more minor contributions from surface flow than previously thought. A synthesis of observational data and model simulations indicates that the temperature, salinity and velocity depth profiles of the Indonesian throughflow are determined by intense vertical mixing within the Indonesian seas. This mixing results in the net upwelling of thermocline water in the Indonesian seas, which in turn lowers sea surface temperatures in this region by about 0.5 °C, with implications for precipitation and air-sea heat flux. Moreover, the depth and velocity of the core of the Indonesian throughflow has varied with the El Niño/Southern Oscillation and Indian Ocean Dipole on interannual to decadal timescales. Specifically, the throughflow slows and shoals during El Niño events. Changes in the Indonesian throughflow alter surface and subsurface heat content and sea level in the Indian Ocean between 10 and 15° S. We conclude that inter-ocean exchange through the Indonesian seas serves as a feedback modulating the regional precipitation and wind patterns. © 2014 Macmillan Publishers Limited.

Huret M.,French Research Institute for Exploitation of the Sea | Sourisseau M.,French Research Institute for Exploitation of the Sea | Petitgas P.,French Research Institute for Exploitation of the Sea | Struski C.,French Research Institute for Exploitation of the Sea | And 2 more authors.
Journal of Marine Systems | Year: 2013

Multiple year oceanographic simulations (hindcast) are identified as a priority oceanography product for fisheries and environment studies since they provide a unique continuous long-term dataset allowing integrated assessment of the ocean state and evolution. We performed a 37. year (1972-2008) hindcast run with a coupled physical-biogeochemical model in the Bay of Biscay. The coupled model and the hindcast configuration are described. A model skill assessment is performed with a large set of in-situ data. Average seasonal currents show major circulation patterns over the shelf. Among tracers, temperature and salinity have the best agreement, ahead of nitrates and silicates, chlorophyll, and finally phosphates and ammonium. For chlorophyll, improved pattern statistics are found when compared to monthly composites of satellite-derived chlorophyll data. From the hindcast, we derived indices related to mesoscale activity (eddies, plumes, fronts, stratification) and production (chlorophyll and primary production). They help characterise the evolution of the environment in a functional way, on both the seasonal and multi-decadal scales. From these indices, first, a multivariate analysis reveals an increasing number of years that deviate from the mean seasonal pattern. Second, we propose interpretations of the simulated increasing trends detected in several of them (temperature, thermocline depth and primary production). We also recommend further developments to confirm these simulated evolutions, from addition of open boundary forcing with a global circulation model, to the improvement of the dynamics of nutrient regeneration and of the seasonal variability of secondary production. As a perspective, we review the different applications made from our hindcast in relation to anchovy life cycle, a species of major interest in the Bay of Biscay. © 2012 Elsevier B.V.

Echevin V.,LOCEAN | Goubanova K.,LEGOS | Belmadani A.,LOCEAN | Belmadani A.,University of Hawaii at Manoa | Dewitte B.,LEGOS
Climate Dynamics | Year: 2012

The impact of climate warming on the seasonal variability of the Humboldt Current system ocean dynamics is investigated. The IPSL-CM4 large scale ocean circulation resulting from two contrasted climate scenarios, the so-called Preindustrial and quadrupling CO2, are downscaled using an eddy-resolving regional ocean circulation model. The intense surface heating by the atmosphere in the quadrupling CO2 scenario leads to a strong increase of the surface density stratification, a thinner coastal jet, an enhanced Peru-Chile undercurrent, and an intensification of nearshore turbulence. Upwelling rates respond quasi-linearly to the change in wind stress associated with anthropogenic forcing, and show a moderate decrease in summer off Peru and a strong increase off Chile. Results from sensitivity experiments show that a 50% wind stress increase does not compensate for the surface warming resulting from heat flux forcing and that the associated mesoscale turbulence increase is a robust feature. © 2011 Springer-Verlag.

Marcos M.,CSIC - Mediterranean Institute for Advanced Studies | Calafat F.M.,CSIC - Mediterranean Institute for Advanced Studies | Llovel W.,LEGOS | Gomis D.,CSIC - Mediterranean Institute for Advanced Studies | Meyssignac B.,LEGOS
Global and Planetary Change | Year: 2011

The contributing factors to regional sea level variability have been explored for the period 2004-2008 based on altimetry observations, hydrographic data and GRACE measurements. The regional averaged annual cycle of the mass contribution to sea level is shown to be highly unsteady. When compared with steric-corrected altimetry, both signals are coherent, though in some regions the coherence analysis is limited by the use of interpolated hydrographic data and in the equatorial regions it is limited by the low signal-to-noise ratio of GRACE data. The closure of regional sea level budgets depends mainly on the GIA correction chosen. A reconstructed global sea level field (with the atmospheric signal eliminated) spanning the second half of the 20th century together with historical hydrographic observations are used to infer the regional mass contribution to sea level rise for the last decades. Results indicate that mass addition from continental ice is the major contributor to regional mean sea level rise for the last decades. In addition, the spatial patterns of mass rates of change point at Greenland as the main source of fresh water input. © 2011 Elsevier B.V.

Prigent C.,Paris Observatory | Papa F.,LEGOS | Aires F.,Estellus | Jimenez C.,Paris Observatory | And 2 more authors.
Geophysical Research Letters | Year: 2012

We developed a remote sensing approach based on multi-satellite observations, which provides an unprecedented estimate of monthly distribution and area of land-surface open water over the whole globe. Results for 1993 to 2007 exhibit a large seasonal and inter-annual variability of the inundation extent with an overall decline in global average maximum inundated area of 6% during the fifteen-year period, primarily in tropical and subtropical South America and South Asia. The largest declines of open water are found where large increases in population have occurred over the last two decades, suggesting a global scale effect of human activities on continental surface freshwater: denser population can impact local hydrology by reducing freshwater extent, by draining marshes and wetlands, and by increasing water withdrawals. Copyright 2012 by the American Geophysical Union.

Aucan J.,LEGOS | Hoeke R.,CSIRO | Merrifield M.A.,University of Hawaii at Manoa
Geophysical Research Letters | Year: 2012

Sixty years of sea-level data collected in the interior lagoon of Midway Atoll in the northern Hawaiian Islands are used to examine short-term (<2 days), high amplitude (up to 1 m) sea level anomaly (SLA) events that occur during the winter months. A combination of wind and wave model hindcasts, satellite altimetry product and in-situ water level data confirms that these high SLA events are driven primarily by the arrival of energetic swell waves generated by storms in the North Pacific as breaking waves on the northwest side of the atoll lead to the setup of the lagoon. The number of high SLA events recorded in Midway during each winter season correlates well with the storminess in the Central North Pacific, defined as the mean seasonal wave height. This leads us to conclude that the seasonal number of high SLA events measured at this specific tide gauge in Midway can be used as an index of storminess in the Central North Pacific over climatic time-scales. Our Midway-based index of storminess correlates well with the Pacific Decadal Oscillation (PDO) index, and its observed increase over the past 60 years is attributed to variability associated with the PDO rather than a long-term trend. © 2012. American Geophysical Union. All Rights Reserved.

Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 931.42K | Year: 2014

Sea levels around the world are currently rising, threatening populations living near the coast with flooding and increased coastal erosion. Evaluating the future threat requires a better understanding of the physical processes responsible for driving changes in the Earths ice sheets. Recent observations show that in some key locations around the ice sheets margins, rapid thinning is currently contributing 1.3 mm/yr to global sea level rise, and that that number has risen dramatically in recent years. Most of the attention has been focussed on the Greenland and West Antarctic ice sheets, where the thinning is most widespread and rapid. It is generally assumed that the culprit is a warming of the ocean waters that come into contact with the ice sheet. Increased melting of the floating ice shelves and tidewater glaciers has caused them to thin, forcing the grounding line or calving front to retreat and allowing the inland ice to flow faster towards the coast. Although thinning of the East Antarctic Ice Sheet (EAIS) is currently much less widespread and dramatic than that observed in West Antarctica, a large sector of the EAIS is grounded below sea level and is thus potentially vulnerable to the same process of ice shelf thinning, grounding line retreat and ice stream acceleration. In addition, analogous ocean forcing to that in West Antarctica could influence the marine-based sector of the EAIS. In both regions the Antarctic Circumpolar Current brings warm Circumpolar Deep Water (CDW) close to the continental slope. While CDW may already be influencing Totten Glacier, which now shows the strongest thinning signature over the entire EAIS, other glaciers in the region, most notably Mertz Glacier, may be protected by the formation of dense, cold Shelf Water in local polynyas. However, our knowledge of the oceanography of the continental shelf and of the waters that circulate beneath and interact with the floating ice shelves is presently insufficient to understand what processes are driving the change on Totten Glacier and how vulnerable its near neighbours such as Mertz Glacier might be. Our ability to project the future behaviour of these outlet glacier systems is severely limited as a result. To address this deficiency, this project will make observations of the critical processes that take place beneath the floating ice shelves, to determine how the topography beneath the ice and the oceanographic forcing from beyond the cavity control the rate at which the ice shelves melt. The key tool with which the necessary observations will be made is an Autonomous Underwater Vehicle (Autosub3), configured and run in a manner analogous to that used for an earlier, highly successful campaign in which it completed 500 km of along-track observations beneath the 60-km long floating tongue of Pine Island Glacier in West Antarctica. We will use these data to validate a numerical model of ocean circulation beneath the ice shelves and use the computed melt rates to force a numerical model of ice flow, in order to investigate the response of the glaciers to a range of climate forcing. A detailed understanding of ocean circulation and melting beneath Totten and Mertz glaciers will generate insight into ocean-ice interactions that will be relevant to many other sites in Greenland and Antarctica, and will advance our developing knowledge of ice sheet discharge and its future effect on sea-level rise. This work forms part of an intensive observational campaign focused on ocean-ice shelf interactions in East Antarctica. The collaborative, interdisciplinary effort consists of coordinated ocean and glacier studies conducted by groups at Australian, French, UK and US institutions.

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