LEGOS IRD

Toulouse, France

LEGOS IRD

Toulouse, France

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Vazquez-Cuervo J.,Jet Propulsion Laboratory | Dewitte B.,LEGOS IRD | Chin T.M.,Jet Propulsion Laboratory | Armstrong E.M.,Jet Propulsion Laboratory | And 2 more authors.
Remote Sensing of Environment | Year: 2013

The Peruvian Coastal Upwelling System (PCUS) is one of the most productive fisheries in the world. Upwelling events are associated with changes in the magnitude and location of frontal structures. SST gradients from four different data sets, NCDC, REMSS, OSTIA, and MUR are compared in two test areas off the PCUS: Païta (5°S) and Pisco (14°S). In both areas gradients derived from the MUR data set show greater magnitudes, as well as larger seasonal cycles. Off Pisco, the magnitude of the seasonal cycle of 2.2. °C/100. km in MUR is larger than the one derived from the lower resolution data sets. All data sets at Pisco exhibit a seasonal cycle that peaks in late Austral summer and early fall. Hovmöller diagrams calculated at 5.5°S, 10.5°S, and 14.5°S show clearly defined offshore maxima in the cross-shore gradients for all the data sets. Upwelling scales determined by the distance to the first maxima vary depending on the data set used. At 5.5°S upwelling scales vary from 10. km for MUR to 50. km for NCDC. At 14.5°S the scales vary from 20. km for MUR to 40. km for OSTIA. All four data sets show similar large-scale structures associated with the Peruvian upwelling. However, MUR shows finer scale structures that are most likely due to submesoscale to mesoscale eddies. Sub-sampled MUR 1. km data at the 25. km, 9. km, and 4. km resolutions compare well in magnitude and phase with the lower resolution products. Agreement in gradient magnitude between the lower resolution data sets and the MUR sub-sampled at their respective resolutions implies that the pixel-to-pixel analysis noise in MUR is at a similar level as the other data sets. © 2012.


Dewitte B.,LEGOS IRD | Vazquez-Cuervo J.,NASA | Goubanova K.,LEGOS IRD | Goubanova K.,French National Center for Space Studies | And 11 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2012

The tropical Pacific variability has experienced changes in its characteristics over the last decades. In particular, there is some evidence of an increased occurrence of El Niño events in the central Pacific (a.k.a. 'Central Pacific El Niño' (CP El Niño) or 'El Niño Modoki'), in contrast with the cold tongue or Eastern Pacific (EP) El Niño which develops in the eastern Pacific. Here we show that the different flavours of El Niño imply a contrasted Equatorial Kelvin Wave (EKW) characteristic and that their rectification on the mean upwelling condition off Peru through oceanic teleconnection is changed when the CP El Niño frequency of occurrence increases. The Simple Ocean Data Assimilation (SODA) reanalysis product is first used to document the seasonal evolution of the EKW during CP and EP El Niño. It is shown that the strong positive asymmetry of ENSO (El Niño Southern Oscillation) is mostly reflected into the EKW activity of the EP El Niño whereas during CP El Niño, the EKW is negatively skewed in the eastern Pacific. Along with slightly cooler conditions off Peru (shallow thermocline) during CP El Niño, this is favourable for the accumulation of cooler SST anomalies along the coast by the remotely forced coastal Kelvin wave. Such a process is observed in a high-resolution regional model of the Humboldt Current system using the SODA outputs as boundary conditions. In particular the model simulates a cooling trend of the SST off Peru although the wind stress forcing has no trend. The model is further used to document the vertical structure along the coast during the two types of El Niño. It is suggested that the increased occurrence of the CP El Niño may also lead to a reduction of mesoscale activity off Peru. © 2012 Elsevier Ltd.


Lebel T.,Joseph Fourier University | Parker D.J.,University of Leeds | Flamant C.,French National Center for Scientific Research | Holler H.,German Aerospace Center | And 14 more authors.
Atmospheric Science Letters | Year: 2011

The AMMA (African Monsoon Multidiscplinary Analysis) field programme aimed at documenting the West African Monsoon (WAM) climate system, in all its geophysical components. It also includes an important socio-economic component. Encompassing a wide range of spatial and time scales, AMMA is one of the most ambitious such programmes ever set up. While the key accomplishments of AMMA are summarised in this paper, a few lessons of broad interest are also drawn both as a tribute to the extraordinary efforts made by a community of several hundreds of people and as possible guidelines for ensuring a long lasting future to integrated climate and environmental studies in West Africa. Copyright © 2011 Royal Meteorological Society.


Monteiro P.M.S.,South African Council for Scientific and Industrial Research | Monteiro P.M.S.,University of Cape Town | Dewitte B.,LEGOS IRD | Dewitte B.,Instituto Del Mar Del Peru IMARPE | And 4 more authors.
Environmental Research Letters | Year: 2011

In this study we investigate the possible reasons for the widespread differences between the seasonal cycles of carbon production and export compared to those of hypoxia in eastern boundary upwelling systems. An idealized model is proposed that qualitatively characterizes the relative roles of physics and biogeochemical fluxes. The model is tested on three contrasting upwelling systems: the Benguela (from relatively aerated to interannual anoxic), the Humboldt (sub-oxic and interannually anoxic) and the Cariaco (permanently anoxic). Overall we propose that shelf hypoxia variability can be explained on the basis of the interaction between ventilation by ocean boundary forcing through ocean-shelf exchange and the role of shelf geometry in the retention of shelf-based particulate organic carbon (POC) fluxes. We aim to identify the hypoxia regimes associated with low ventilation - wide-shelf systems and high ventilation - narrow-shelf systems, considering them as extremes of conditions controlled by the two factors. We propose that this may help to explain differences in the seasonal cycles of the biogeochemical drivers and responses as well as difference between upwelling systems and within individual upwelling systems. It is suggested that when seasonal hypoxia emerges it does so preferentially at a wide-shelf part of a system. © 2011 IOP Publishing Ltd.


Zhang J.,East China Normal University | Gilbert D.,Maurice Lamontagne Institute | Gooday A.J.,UK National Oceanography Center | Levin L.,University of California at San Diego | And 17 more authors.
Biogeosciences | Year: 2010

Hypoxia has become a world-wide phenomenon in the global coastal ocean and causes a deterioration of the structure and function of ecosystems. Based on the collective contributions of members of SCOR Working Group #128, the present study provides an overview of the major aspects of coastal hypoxia in different biogeochemical provinces, including estuaries, coastal waters, upwelling areas, fjords and semi-enclosed basins, with various external forcings, ecosystem responses, feedbacks and potential impact on the sustainability of the fishery and economics. The obvious external forcings include freshwater runoff and other factors contributing to stratification, organic matter and nutrient loadings, as well as exchange between coastal and open ocean water masses. Their different interactions set up mechanisms that drive the system towards hypoxia. Coastal systems also vary in their relative susceptibility to hypoxia depending on their physical and geographic settings. It is understood that coastal hypoxia has a profound impact on the sustainability of ecosystems, which can be seen, for example, by the change in the food-web structure and system function; other influences include compression and loss of habitat, as well as changes in organism life cycles and reproduction. In most cases, the ecosystem responds to the low dissolved oxygen in non-linear ways with pronounced feedbacks to other compartments of the Earth System, including those that affect human society. Our knowledge and previous experiences illustrate that there is a need to develop new observational tools and models to support integrated research of biogeochemical dynamics and ecosystem behavior that will improve confidence in remediation management strategies for coastal hypoxia. © Author(s) 2010. CC Attribution 3.0 License.


Dupouy C.,IRD Montpellier | Savranski T.,IRD Montpellier | Lefevre J.,IRD Montpellier | Despinoy M.,Center IRD | And 8 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

We present data collected as part of VALidation HYperspectral of a BIOgeochemical model (ValHyBio), a PNTSsponsored program dedicated to chlorophyll satellite imaging and validation as affected by bathymetry in the South Western Tropical Lagoon of New Caledonia. The specific goals of ValHyBio are to: - examine time-dependent oceanic reflectance in relation to dynamic surface processes, - construct field/satellite reflectance-based chlorophyll models, - investigate the feasibility of inverting the model to yield surface chlorophyll and turbidity, and - validate the biogeochemical model with field/satellite observations. The in situ bio-optical parameters include absorption coefficients by CDOM and particles, Secchi disk depth, backscattering coefficient, pigment concentration, suspended matter concentration, and K-dPAR. They are measured every month at 5 stations, of contrasted bathymetry and bottom reflectance, as well as at a reference station situated 4 miles offshore, and at a station over coral reefs. Remote sensing reflectance is calculated from the absorption and backscattering coefficients and compared with satellite data. SeaWIFS and MODIS Aqua match-ups collected over the period 1997-2010 (ValHySat database) are used. Satellite retrievals are examined as a function of bathymetry and for the special case of Trichodesmium red tides. The feasibility of a long-term monitoring program of water optical properties with satellite remote sensing techniques is examined in the frame of the GOPS (South Pacific Integrated Observatory). © 2010 SPIE.


Dupouy C.,IRD Montpellier | Savranski T.,IRD Montpellier | Lefevre J.,IRD Montpellier | Despinoy M.,Center of Noumea | And 7 more authors.
34th International Symposium on Remote Sensing of Environment - The GEOSS Era: Towards Operational Environmental Monitoring | Year: 2011

We present data collected as part of VALidation HYperspectral of a BIOgeochemical model (ValHyBio), a PNTS-sponsored program dedicated to chlorophyll satellite imaging and validation as affected by bathymetry in the South Western Tropical Lagoon of New Caledonia. MODIS Aqua match-ups collected over the period 1997-2010 (ValHySat database) are examined for two algorithms OC3 and OC5.


Dewitte B.,LEGOS IRD | Choi J.,Yonsei University | An S.-I.,Yonsei University | Thual S.,LEGOS IRD
Climate Dynamics | Year: 2012

Recent studies report that two types of El Niño events have been observed. One is the cold tongue El Niño or Eastern Pacific El Niño (EP El Niño), which is characterized by relatively large sea surface temperature (SST) anomalies in the eastern Pacific, and the other is the warm pool El Niño (a.k.a. 'Central Pacific El Niño' (CP El Niño) or 'El Niño Modoki'), in which SST anomalies are confined to the central Pacific. Here the vertical structure variability of the periods during EP and CP is investigated based on the GFDL_CM2. 1 model in order to explain the difference in equatorial wave dynamics and associated negative feedback mechanisms. It is shown that the mean stratification in the vicinity of the thermocline of the central Pacific is reduced during CP El Niño, which favours the contribution of the gravest baroclinic mode relatively to the higher-order slower baroclinic mode. Energetic Kelvin and first-meridional Rossby wave are evidenced during the CP El Niño with distinctive amplitude and propagating characteristics according to their vertical structure (mostly first and second baroclinic modes). In particular, the first baroclinic mode during CP El Niño is associated to the ocean basin mode and participates to the recharge process during the whole El Niño cycle, whereas the second baroclinic mode is mostly driving the discharge process through the delayed oscillator mechanism. This may explain that the phase transition from warm to neutral/cold conditions during the CP El Niño is delayed and/or disrupted compared to the EP El Niño. Our results have implications for the interpretation of the variability during periods of high CP El Niño occurrence like the last decade. © 2011 Springer-Verlag.


Palanisamy H.,French National Center for Space Studies | Cazenave A.,French National Center for Space Studies | Delcroix T.,LEGOS IRD | Meyssignac B.,French National Center for Space Studies
Ocean Dynamics | Year: 2015

This study investigates the spatial trend patterns and variability of observed sea level and upper ocean thermal structure in the Pacific Ocean during the altimetry era (1993–2012), and the role of thermocline depth changes. The observed sea level trend pattern in this region results from the superposition of two main signals: (1) a strong broad-scale V-shaped positive trend anomaly extending to mid-latitudes in the central Pacific and (2) another very strong positive trend anomaly located in the western tropical Pacific within about 120° E–160° E and 20° S–20° N latitude. In this study, we focus on the tropical Pacific (20° N–20° S) where the strongest trends in sea level are observed. By making use of in situ observational data, we study the impact of thermocline depth changes on steric sea level between the surface and 700 m and its relation with the altimetry-based observed sea level changes. This is done by calculating the time-varying thermocline depth (using the 20 °C isotherm depth as a proxy) and estimating the sea level trend patterns of the thermocline-attributed individual steric components. We show that it is essentially the vertical movement of the thermocline that governs most of the observed sea level changes and trends in the tropical Pacific. Furthermore, we also show that in the equatorial band, the changes in the upper ocean thermal structure are in direct response to the zonal wind stress. Away from the equatorial band (say, within 5°–15° latitude), the changes in the upper ocean thermal structure are consistent with the wind stress-generated Rossby waves. We also estimate the contribution of the Interdecadal Pacific Oscillation (IPO) on the vertical thermal structure of the tropical Pacific Ocean. Removing the IPO contribution to the upper layer steric sea level provides a non-negligible residual pattern, suggesting that IPO-related internal ocean variability alone cannot account for the observed trend patterns in the Pacific sea level. It is likely that the residual signal may also reflect non-linear interactions between different natural modes like El Niño Southern Oscillation (ENSO), IPO, etc. © 2015, Springer-Verlag Berlin Heidelberg.

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