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Sudre J.,French National Center for Scientific Research | Garcon V.,French National Center for Scientific Research | Provost C.,LOCEAN | Sennechael N.,LOCEAN | And 2 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2011

A multiparametric analysis is applied on the ANT-XXIII/3 bottle data to determine the spreading and mixing of water masses across Drake Passage during January-February 2006, focusing our interest on the flow of the deep water masses. Mixing proportions are quantified along both the southward and northward journeys between the tip of South America and the Antarctic Peninsula. The method is sensitive enough to detect a marked variability in the flow pathways of the Circumpolar Deep Water, the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water at the intersection of the Shackleton Fracture Zone (SFZ) and the West Scotia Ridge (WSR) between the southward and northward journeys. Indeed the presence of a sharp meander of the southern branch of the Polar Front (PFS) together with the displacement of the northern branch of the Southern Antarctic Circumpolar Current Front (SACCFN) above the complex bathymetric structure of the WSR induced a strong perturbation in the deep flow field during the southward journey. The height of the SFZ crests clearly sets the lower limit of the SPDW in the Ona Basin. An enhanced δ3He signature is confined to the north of the Polar Front between 55.5°S and 56.5°S in the 1500-2200m depth range. It clearly indicates the signature of the core of the Southeast Pacific Deep Slope Water. The entry pathway in the ACC of the SPDSW during ANT-XXIII/3 was farther to the south off the continental slope of South America than it was during the WOCE A21 expedition in 1990, probably due to the sharp changes in the slope orientation at the extreme tip of the South America. © 2011 Elsevier Ltd.

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.

Joseph S.,Indian Institute of Tropical Meteorology | Sahai A.K.,Indian Institute of Tropical Meteorology | Goswami B.N.,Indian Institute of Tropical Meteorology | Terray P.,LOCEAN | And 2 more authors.
Climate Dynamics | Year: 2012

Reasonably realistic climatology of atmospheric and oceanic parameters over the Asian monsoon region is a pre-requisite for models used for monsoon studies. The biases in representing these features lead to problems in representing the strength and variability of Indian summer monsoon (ISM). This study attempts to unravel the ability of a state-of-the-art coupled model, SINTEX-F2, in simulating these characteristics of ISM. The coupled model reproduces the precipitation and circulation climatology reasonably well. However, the mean ISM is weaker than observed, as evident from various monsoon indices. A wavenumber-frequency spectrum analysis reveals that the model intraseasonal oscillations are also weaker-than-observed. One possible reason for the weaker-than-observed ISM arises from the warm bias, over the tropical oceans, especially over the equatorial western Indian Ocean, inherent in the model. This warm bias is not only confined to the surface layers, but also extends through most of the troposphere. As a result of this warm bias, the coupled model has too weak meridional tropospheric temperature gradient to drive a realistic monsoon circulation. This in turn leads to a weakening of the moisture gradient as well as the vertical shear of easterlies required for sustained northward propagation of rain band, resulting in weak monsoon circulation. It is also noted that the recently documented interaction between the interannual and intraseasonal variabilities of ISM through very long breaks (VLBs) is poor in the model. This seems to be related to the inability of the model in simulating the eastward propagating Madden-Julian oscillation during VLBs. © 2011 Springer-Verlag.

Kim S.T.,CSIRO | Cai W.,CSIRO | Cai W.,Ocean University of China | Jin F.-F.,University of Hawaii at Manoa | And 5 more authors.
Nature Climate Change | Year: 2014

The destructive environmental and socio-economic impacts of the El Niño/Southern Oscillation (ENSO) demand an improved understanding of how ENSO will change under future greenhouse warming. Robust projected changes in certain aspects of ENSO have been recently established. However, there is as yet no consensus on the change in the magnitude of the associated sea surface temperature (SST) variability, commonly used to represent ENSO amplitude, despite its strong effects on marine ecosystems and rainfall worldwide. Here we show that the response of ENSO SST amplitude is time-varying, with an increasing trend in ENSO amplitude before 2040, followed by a decreasing trend thereafter. We attribute the previous lack of consensus to an expectation that the trend in ENSO amplitude over the entire twenty-first century is unidirectional, and to unrealistic model dynamics of tropical Pacific SST variability. We examine these complex processes across 22 models in the Coupled Model Intercomparison Project phase 5 (CMIP5) database, forced under historical and greenhouse warming conditions. The nine most realistic models identified show a strong consensus on the time-varying response and reveal that the non-unidirectional behaviour is linked to a longitudinal difference in the surface warming rate across the Indo-Pacific basin. Our results carry important implications for climate projections and climate adaptation pathways.© 2014 Macmillan Publishers Limited. All rights reserved.

P Sabin T.,Indian Institute of Tropical Meteorology | Krishnan R.,Indian Institute of Tropical Meteorology | Ghattas J.,Laboratoire Meteorologie Dynamique | Denvil S.,Laboratoire Meteorologie Dynamique | And 3 more authors.
Climate Dynamics | Year: 2013

This study examines the feasibility of using a variable resolution global general circulation model (GCM), with telescopic zooming and enhanced resolution (~35 km) over South Asia, to better understand regional aspects of the South Asian monsoon rainfall distribution and the interactions between monsoon circulation and precipitation. For this purpose, two sets of ten member realizations are produced with and without zooming using the LMDZ (Laboratoire Meteorologie Dynamique and Z stands for zoom) GCM. The simulations without zoom correspond to a uniform 1° × 1° grid with the same total number of grid points as in the zoom version. So the grid of the zoomed simulations is finer inside the region of interest but coarser outside. The use of these finer and coarser resolution ensemble members allows us to examine the impact of resolution on the overall quality of the simulated regional monsoon fields. It is found that the monsoon simulation with high-resolution zooming greatly improves the representation of the southwesterly monsoon flow and the heavy precipitation along the narrow orography of the Western Ghats, the northeastern mountain slopes and northern Bay of Bengal (BOB). A realistic Monsoon Trough (MT) is also noticed in the zoomed simulation, together with remarkable improvements in representing the associated precipitation and circulation features, as well as the large-scale organization of meso-scale convective systems over the MT region. Additionally, a more reasonable simulation of the monsoon synoptic disturbances (lows and disturbances) along the MT is noted in the high-resolution zoomed simulation. On the other hand, the no-zoom version has limitations in capturing the depressions and their movement, so that the MT zone is relatively dry in this case. Overall, the results from this work demonstrate the usefulness of the high-resolution variable resolution LMDZ model in realistically capturing the interactions among the monsoon large-scale dynamics, the synoptic systems and the meso-scale convective systems, which are essential elements of the South Asian monsoon system. © 2013 Springer-Verlag Berlin Heidelberg.

Pietri A.,Locean | Testor P.,Locean | Echevin V.,Locean | Chaigneau A.,Legos omp | And 3 more authors.
Journal of Physical Oceanography | Year: 2013

The upwelling system off southern Peru has been observed using an autonomous underwater vehicle (a Slocum glider) during October-November 2008. Nine cross-front sections have been carried out across an intense upwelling cell near 14°S. During almost two months, profiles of temperature, salinity, and fluorescence were collected at less than 1-km resolution, between the surface and 200-m depth. Estimates of alongshore absolute geostrophicvelocities were inferred from the density field and the glider drift between two surfacings. In the frontal region, salinity and biogeochemical fields displayed cross-shore submesoscale filamentary structures throughout the mission. Those features presented a width of 10-20 km, a vertical extent of ~150 m, and appeared to propagate toward the shore. They were steeper than isopycnals and kept an aspect ratio close to f/N, the inverse of the Prandtl ratio. These filamentary structures may be interpreted mainly as a manifestation ofsubmesoscale turbulence through stirring of the salinity gradients by the mesoscale eddy field. However, meandering of the front or cross-frontal wind-driven instabilities could also play a role in inducing vertical velocities. © 2013 American Meteorological Society.

Lefevre N.,IRD LOCEAN | Caniaux G.,Meteo - France | Janicot S.,IRD LOCEAN | Gueye A.K.,LOCEAN
Journal of Geophysical Research: Oceans | Year: 2013

The fugacity of CO2 (fCO2) has been measured underway during the voyages of a merchant ship sailing from France to Brazil since 2008. High fCO2 values are observed in 2010, between approximately 8°S and 8°N, and are particularly pronounced in boreal spring. These high values are explained by the anomalous situation occurring in the tropical Atlantic in 2010 after the 2009 El Niño in the Pacific. The weakening of the trade winds during the El Niño event contributes to an increase of sea surface temperatures in the tropical Atlantic and a northward shift of the intertropical convergence zone. The anomalous position of the intertropical convergence zone is accompanied by reduced precipitation in boreal spring and hence a higher than usual sea surface salinity. The year 2010 is also characterized by a strong positive Atlantic multidecadal oscillation index and a negative North Atlantic oscillation index that contribute to the tropical Atlantic warming. Positive anomalies of both surface temperature and salinity cause an increase of surface CO2, leading to a stronger outgassing of CO2 in 2010 compared with 2009 and 2011. The main factor responsible for the CO2 anomalies is the Pacific El Niño teleconnection. The Atlantic multidecadal oscillation index also contributes in the northern hemisphere, and the role of the North Atlantic oscillation is negligible. © 2013. American Geophysical Union. All Rights Reserved.

Brajard J.,MREN Inc | Santer R.,MREN Inc | Crepon M.,LOCEAN | Thiria S.,LOCEAN
Remote Sensing of Environment | Year: 2012

One of the difficulties in analyzing the ocean signal provided by satellite ocean color sensors is that it is strongly polluted by atmospheric contributions, which should be removed by an atmospheric correction process.We propose a new methodology, based on spectral optimization in the near-infrared, to simultaneously estimate the contributions generated by atmospheric signals and oceanic particles, which is valid for case-1 and case-2 waters. This approach, denoted NeuroVaria, combines a neural network to model the radiative transfer with a variational algorithm for the spectral inversion.NeuroVaria was applied to MERIS data recorded between August 2003 and September 2005 over the Adriatic Sea, off the Venice Lagoon, for which, in situ measurements of the water-leaving reflectance and aerosol optical thickness were available. We present comparisons between the results obtained using NeuroVaria and the MERIS second reprocessing (Megs7.4), and those derived from in situ measurements. We show that NeuroVaria achieves better estimations of the aerosol optical properties, and improves the atmospheric correction for case-2 waters. Using MERIS multi-spectral images, it was thus possible to detect typical features of the Po River discharge into the northern Adriatic, as well as suspended sediments due to the shoaling of wind waves on their approach to the seashore shallow waters. © 2012 Elsevier Inc.

Berg A.,LOCEAN | Sultan B.,LOCEAN | De Noblet-Ducoudre N.,LSCE
Geophysical Research Letters | Year: 2010

A large-scale crop model is forced by a range of climate datasets over West Africa to test the sensitivity of simulated yields to errors in input rainfall. The model skill, defined as the correlation between observed and simulated yield anomalies over 1968-1990 at the country scale, is used for assessment. We show that there are two essential rainfall features for the model to skillfully simulate interannual yield variability at the country scale: cumulative annual variability and frequency. At such a scale, providing additional information on intraseasonal variability, such as the chronology of rain events, does not improve the model skill. We suggest that such information is relevant at smaller spatial scales but is not spatially consistent enough to impact large-scale yield variability. Copyright © 2010 by the American Geophysical Union.

Renault A.,LOCEAN | Provost C.,LOCEAN | Sennechael N.,LOCEAN | Barre N.,LOCEAN | Kartavtseff A.,LOCEAN
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2011

In January/February 2006, a hydrographic section under Jason track 104 across Drake Passage (DP) was performed twice within 3 weeks, on board of the R/V Polarstern. Two LADCPs mounted on the CTD rosette provided the most comprehensive synoptic top-to-bottom observations of directly measured velocity available in the Drake Passage to date, with an estimated error of less than 3cms -1. Geostrophic velocities computed from the hydrographic data were referenced to LADCP velocities. The velocity structure of the Antarctic Circumpolar Current (ACC) along the track and its evolution within 3 weeks reflect the frontal branches presented by Barré et al. (2011) and Provost et al. (2011). The repeated section offers two independent transport estimates, computed from the LADCP velocities (145±8.8 and 137.9±10.5Sv, respectively) and from the adjusted geostrophic velocities (136.6±7 and 129±7Sv, respectively). Error bars of the transport estimates are small. Contributions of the fronts and eddies to the total transport are modified within 3 weeks but tend to compensate each other. The total transport is reduced by about 10% (15-17Sv) between the southward and northward journey. Total transport estimations are compared to previous results, in particular the ISOS (International Southern Ocean Study) data, from which the canonical value of 134Sv has been estimated. Within the uncertainties due to uneven data distribution, no trend is discernible neither in the baroclinic nor total transports from 1975 to 2006. © 2011 Elsevier Ltd.

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