Chaigneau A.,Laboratoire Doceanographie Et Of Climatologie Experimentation Et Analyse Numerique |
Chaigneau A.,Instituto Del Mar Of Peru |
Chaigneau A.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Le Texier M.,CNRS Institute of Fluid Mechanics of Toulouse |
And 3 more authors.
Journal of Geophysical Research: Oceans | Year: 2011
The mean vertical structure of mesoscale eddies in the Peru-Chile Current System is investigated by combining the historical records of Argo float profiles and satellite altimetry data. A composite average of 420 (526) profiles acquired by Argo floats that surfaced into cyclonic (anticyclonic) mesoscale eddies allowed constructing the mean three-dimensional eddy structure of the eastern South Pacific Ocean. Key differences in their thermohaline vertical structure were revealed. The core of cyclonic eddies (CEs) is centered at ∼150 m depth within the 25.2-26.0 kg m-3 potential density layer corresponding to the thermocline. In contrast, the core of the anticyclonic eddies (AEs) is located below the thermocline at ∼400 m depth impacting the 26.0-26.8 kg m-3 density layer. This difference was attributed to the mechanisms involved in the eddy formation. While intrathermocline CEs would be formed by instabilities of the surface equatorward coastal currents, the subthermocline AEs are likely to be shed by the subsurface poleward Peru-Chile Undercurrent. In the eddy core, maximum temperature and salinity anomalies are of ±1°C and ±0.1, with positive (negative) values for AEs (CEs). This study also provides new insight into the potential impact of mesoscale eddies for the cross-shore transport of heat and salt in the eastern South Pacific. Considering only the fraction of the water column associated with the fluid trapped within the eddies, each CE and AE has a typical volume anomaly flux of ∼0.1 Sv and yields to a heat and salt transport anomaly of ±1-3 × 1011 W and ±3-8 × 103 kg s-1, respectively. Copyright 2011 by the American Geophysical Union.
Neetu S.,National Institute of Oceanography of India |
Lengaigne M.,National Institute of Oceanography of India |
Lengaigne M.,Institute Pierre Simon Laplace |
Vincent E.M.,Institute Pierre Simon Laplace |
And 6 more authors.
Journal of Geophysical Research: Oceans | Year: 2012
Surface cooling induced by tropical cyclones (TCs) is about three times larger during premonsoon than during postmonsoon season in the Bay of Bengal. We investigate processes responsible for this seasonal contrast using an ocean general circulation model. The model is forced by TC winds prescribed from an analytic vortex using observed TC tracks and intensities during 1978-2007. The simulation accurately captures the seasonal cycle of salinity, temperature, and barrier layer in this region, with fresher waters, deeper upper-ocean stratification, and thicker barrier layers during postmonsoon season. It also reproduces the three times larger TC-induced cooling during premonsoon than during postmonsoon season. This difference is essentially related to seasonal changes in oceanic stratification rather than to differences in TC wind energy input. During the postmonsoon season, a deeper thermal stratification combined with a considerable upper-ocean freshening strongly inhibits surface cooling induced by vertical mixing underneath TCs. On average, thermal stratification accounts for ∼60% of this cooling reduction during postmonsoon season, while haline stratification accounts for the remaining 40%. Their respective contributions however strongly vary within the Bay: haline stratification explains a large part of the TC-induced cooling inhibition offshore of northern rim of the Bay (Bangladesh-Myanmar-east coast of India), where salinity seasonal changes are the strongest, while thermal stratification explains all the cooling inhibition in the southwestern Bay. This study hence advocates for an improved representation of upper-ocean salinity and temperature effects in statistical and dynamical TCs forecasts that could lead to significant improvements of TC intensity prediction skill. © 2012. American Geophysical Union. All Rights Reserved.
Peucker-Ehrenbrink B.,Woods Hole Oceanographic Institution |
Peucker-Ehrenbrink B.,Laboratoire DEtudes en Geophysique et Oceanographie Spatiale |
Miller M.W.,BenchmarkGIS Services |
Arsouze T.,Laboratoire DEtudes en Geophysique et Oceanographie Spatiale |
Jeandel C.,Laboratoire DEtudes en Geophysique et Oceanographie Spatiale
Geochemistry, Geophysics, Geosystems | Year: 2010
Realistic models of past climate and ocean chemistry depend on reconstructions of the Earth's surface environments in the geologic past. Among the critical parameters is the geologic makeup of continental drainage. Here we show, for the present, that the isotope composition of dissolved strontium in rivers increases linearly with the age of bedrock in drainage basins, with the notable exception of the drainage area of Arabia, India, and Southeast Asia that is affected by unusually radiogenic dissolved Sr from the Himalaya. We also demonstrate that the neodymium isotope compositions of suspended matter in rivers as well as clastic sediments deposited along the ocean margins decrease linearly with the bedrock ages of river drainage basins and large-scale continental drainage regions, as determined from digital geologic maps. These correlations are used to calculate the present-day input of dissolved Sr (4.7 × 1010 mol yr-1, 87Sr/86Sr of ∼0.7111) and particulate Nd isotopes (εNd of approximately -7.3 ± 2.2) to the oceans. The fact that the regionally averaged εNd of the global detrital input to the global coastal ocean is identical to globally averaged seawater (εNd of -7.2 ± 0.5) lends credence to the importance of "boundary exchange" for the Nd isotope composition of water masses. Regional biases in source areas of detrital matter and runoff are reflected by the observation that the average age of global bedrock, weighted according to the riverine suspended sediment flux, is significantly younger (∼336 Myr) than the age of global bedrock weighted according to water discharge (394 Myr), which is younger than the average bedrock age of the nonglaciated, exorheic portions of the continents (453 Myr). The observation that the bedrock age weighted according to Sr flux is younger (339 Myr) than that weighted according to water flux reflects the disproportionate contribution from young sedimentary and volcanic rocks to the dissolved Sr load. Neither the isotope composition of the dissolved nor the particulate continental inputs to the ocean provide unbiased perspectives of the lithologic makeup of the Earth's surface. Temporal changes in bedrock geology as well as the shifting focal points of physical erosion and water discharge will undoubtedly have exerted strong controls on temporal and spatial changes in the isotope chemistry of past global runoff and thus seawater. Copyright 2010 by the American Geophysical Union.
Copard K.,Laboratoire Des Interactions Et Dynamique Des Environnements Of Surface |
Copard K.,CEA Saclay Nuclear Research Center |
Colin C.,Laboratoire Des Interactions Et Dynamique Des Environnements Of Surface |
Frank N.,CEA Saclay Nuclear Research Center |
And 5 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2011
Nd isotopic compositions (εNd) of seawater profiles and deep-sea corals collected off the coast of Iberia and from the Bay of Biscay were measured (1) to constrain the Nd isotopic composition of water masses along the southwest European margin, (2) to track the Mediterranean Outflow Water (MOW) during its northward propagation, and (3) to establish hydrological changes during the last 1500 years. The Eastern North Atlantic Central Water (ENACW) is characterized by Nd isotopic composition of around -12.0. Mediterranean Sea Water (MSW) is collected from 800 and 1200 m depth and is characterized by εNd values ranging from -10.9, off the coast of Iberia, to -11.6 in the Bay of Biscay. These εNd results suggest a strong dilution of the pure MOW at the Strait of Gibraltar (εNd -9.4) of approximately 40% and 30% along its northward circulation pathway essentially with a contribution from ENACW. At around 2000 m depth, εNd water profiles display the occurrence of a nonradiogenic water mass (εNd -13), originating from the Labrador Sea (Labrador Sea Water). Fossil deep-sea corals, dated between 84 and 1500 years, display Nd isotopic compositions that vary moderately from present-day seawater values, suggesting a weaker influence of MOW in the formation of MSW during the Dark Ages and the Little Ice Age. These recent cold events seem to be associated with a reduction in the northward penetration of MSW, which may result from a greater eastward extension of the middepth subpolar gyre and/or a reduction of MSW formation, likely tied to a variation in deep Mediterranean water production. © 2011 by the American Geophysical Union.
Meyssignac B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Becker M.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Becker M.,IRD Montpellier |
Llovel W.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
And 2 more authors.
Surveys in Geophysics | Year: 2012
We compare different past sea level reconstructions over the 1950-2009 time span using the Empirical Orthogonal Function (EOF) approach. The reconstructions are based on 91 long (up to 60 years) but sparsely distributed tide-gauge records and gridded sea level data from two numerical ocean models over 1958-2007 (the DRAKKAR/NEMO model without data assimilation and the simple ocean data assimilation ocean reanalysis-SODA-) and satellite altimetry data over 1993-2009. We find that the reconstructed global mean sea level computed over the ~60-year-long time span little depends on the input spatial grids. This is unlike the regional variability maps that appear very sensitive to the considered input spatial grids. Using the DRAKKAR/NEMO model, we test the influence of the period covered by the input spatial grids and the number of EOFs modes used to reconstruct sea level. Comparing with tide-gauge records not used in the reconstruction, we determine optimal values for these two parameters. As suggested by previous studies, the longer the time span covered by the spatial grids, the better the fit with unused tide gauges. Comparison of the reconstructed regional trends over 1950-2009 based on the two ocean models and satellite altimetry grids shows good agreement in the tropics and substantial differences in the mid and high latitude regions, and in western boundary current areas as well. The reconstructed spatial variability seems very sensitive to the input spatial information. No clear best case emerges. Thus, using the longest available model-based spatial functions will not necessarily give the most realistic results as it will be much dependent on the quality of the model (and its associated forcing). Altimetry-based reconstructions (with 17-year long input grids) give results somewhat similar to cases with longer model grids. It is likely that better representation of the sea level regional variability by satellite altimetry compensates the shorter input grids length. While waiting for much longer altimetry records, improved past sea level reconstructions may be obtained by averaging an ensemble of different model-based reconstructions, as classically done in climate modelling. Here, we present such a 'mean' reconstruction (with associated uncertainty) based on averaging the three individual reconstructions discussed above. © 2012 Springer Science+Business Media B.V.
Dencausse G.,French National Center for Scientific Research |
Dencausse G.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Arhan M.,French National Center for Scientific Research |
Speich S.,French National Center for Scientific Research
Journal of Geophysical Research: Oceans | Year: 2011
A 14.3 year series of weekly absolute sea surface height (SSH) and associated geostrophic velocities is used for a study of the subtropical-to-subantarctic frontal system in the region south of Africa. Detecting the fronts from surface velocity maxima confirms a two-stepped transition in both the southeastern Atlantic Ocean (the Northern and Southern Subtropical fronts (NSTF, SSTF)) and southwestern Indian Ocean (the Agulhas Front and SSTF), as proposed previously from hydrographic data. An additional front associated with westward flow north of the NSTF is indicative of a partial eastern closure of the South Atlantic subtropical gyre between 11°E and 17.5°E. The role of northwestward propagating Agulhas rings in connecting the two fronts explains the varying location. The SSTF, which marks the southern limit of subtropical waters, is found continuous from the Atlantic to the Indian Ocean in the time-averaged SSH field. In the weekly fields, however, the velocity maximum criterion defining the front is often met at the southern flanks of Agulhas rings or Agulhas eddies at 12°E-23°E, indicating front disruptions. Such discontinuities suggest that no South Atlantic Central Water is directly advected into the Indian Ocean. Instead, the eastward limb of the Indo-Atlantic "super gyre" encompassing the subtropical gyres of both oceans would rest, at these longitudes, on diffusive processes at the upper levels, and possibly be enhanced at depth. A schematic diagram of the fronts and their relations to Agulhas rings and eddies is proposed. Copyright © 2011 by the American Geophysical Union.
Renault L.,Sistema dObservacio i Prediccio Costaner de les Illes Balears |
Dewitte B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Marchesiello P.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Illig S.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
And 6 more authors.
Journal of Geophysical Research: Oceans | Year: 2012
The spatial and temporal variability of nearshore winds in eastern boundary current systems affect the oceanic heat balance that drives sea surface temperature changes. In this study, regional atmospheric and oceanic simulations are used to document such processes during an atmospheric coastal jet event off central Chile. The event is well reproduced by the atmospheric model and is associated with the migration of an anomalous anticyclone in the southeastern Pacific region during October 2000. A robust feature of the simulation is a sharp coastal wind dropoff, which is insensitive to model resolution. As expected, the simulated oceanic response is a significant sea surface cooling. A surface heat budget analysis shows that vertical mixing is a major contributor to the cooling tendency both in the jet core area and in the nearshore zone where the magnitude of this term is comparable to the magnitude of vertical advection. Sensitivity experiments show that the oceanic response in the coastal area is sensitive to wind dropoff representation. This is because total upwelling, i.e., the sum of coastal upwelling and Ekman pumping, depends on the scale of wind dropoff. Because the latter is much larger than the upwelling scale, coastal wind dropoff has only a weak positive effect on vertical velocities driven by Ekman pumping but has a strong negative effect on coastal upwelling. Interestingly though, the weakening of coastal winds in the dropoff zone has a larger effect on vertical mixing than on vertical advection, with both effects contributing to a reduction of cooling. Copyright 2012 by the American Geophysical Union.
Thual S.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Thual S.,Instituto Geofisico del Peru |
Dewitte B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Dewitte B.,Instituto Geofisico del Peru |
And 2 more authors.
Journal of Climate | Year: 2011
El Niño-Southern Oscillation (ENSO) is driven by large-scale ocean-atmosphere interactions in the equatorial Pacific and is sensitive to change in the mean state. Whereas conceptual models of ENSO usually consider the depth of the thermocline to be influential on the stability of ENSO, the observed changes in the depth of the 208C isotherm are rather weak, on the order of approximately 5 m over the last decades. Conversely, change in stratification that affects both the intensity and sharpness of the thermocline can be pronounced. Here, the two-strip conceptual model of An and Jin is extended to include three parameters (i.e., the contribution of the first three baroclinic modes) that account for the main characteristics of the mean thermocline vertical structure. A stability analysis of the model is carried out that indicates that the model sustains a lower ENSO mode when the high-order baroclinic modes (M2 and M3) are considered. The sensitivity of the model solution to the coupling efficiency further indicates that, in the weak coupling regime, the model allows for several ocean basin modes at low frequency. The latter can eventually merge into a low-frequency and unstable mode representative of ENSO as the coupling efficiency increases. Also, higher baroclinic modes project more energy onto the ocean dynamics for the same input of wind forcing. Therefore, in this study's model, a shallower, yet more intense mean thermocline may still sustain a strong (i.e., unstable) and low-frequency ENSO mode. Sensitivity tests to the strength of the two dominant feedbacks (thermocline vs zonal advection) indicate that the presence of high-order baroclinic modes favors the bifurcation from a low-frequency regime to a higher-frequency regime when the zonal advective feedback is enhanced. It is suggested that the proposed formalism can be used to interpret and measure the sensitivity of coupled general circulation models to climate change. © 2011 American Meteorological Society.
Gille S.T.,University of California at San Diego |
Carranza M.M.,University of California at San Diego |
Cambra R.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale |
Cambra R.,Institute Pierre Simon Laplace |
Morrow R.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale
Biogeosciences | Year: 2014
In contrast to most of the Southern Ocean, the Kerguelen Plateau supports an unusually strong spring chlorophyll (Chl ia) bloom, likely because the euphotic zone in the region is supplied with higher iron concentrations. This study uses satellite wind, sea surface temperature (SST), and ocean color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the euphotic zone. Results show that, in the Kerguelen region, cold SSTs correlate with high wind speeds, implying that wind-mixing leads to enhanced vertical mixing. Cold SSTs also correlate with negative wind-stress curl, implying that Ekman pumping can further enhance upwelling. In the moderate to high eddy kinetic energy (EKE) regions surrounding Kerguelen, we find evidence of coupling between winds and SST gradients associated with mesoscale eddies, which can locally modulate the wind-stress curl. This coupling introduces persistent wind-stress curl patterns and Ekman pumping around these long-lived eddies, which may modulate the evolution of Chl ia in the downstream plume far offshore. Close to the plateau, this eddy coupling breaks down. Kerguelen has a significant wind shadow on its downwind side, which changes position depending on the prevailing wind and which generates a wind-stress curl dipole that shifts location depending on wind direction. This leads to locally enhanced Ekman pumping for a few hundred kilometers downstream from the Kerguelen Plateau; Chl ia values tend to be more elevated in places where wind-stress curl induces Ekman upwelling than in locations of downwelling, although the estimated upwelling rates are too small for this relationship to derive from direct effects on upward iron supply, and thus other processes, which remain to be determined, must also be involved in the establishment of these correlations. During the October and November (2011) KErguelen Ocean and Plateau compared Study (KEOPS-2) field program, wind conditions were fairly typical for the region, with enhanced Ekman upwelling expected to the north of the Kerguelen Islands. © Author(s) 2014.
Gushchina D.,Moscow State University |
Dewitte B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale
Central European Journal of Geosciences | Year: 2011
The intraseasonal tropical variability (ITV) patterns in the tropical troposphere are documented using double space-time Fourier analysis. Madden and Julian oscillations (MJO) as well as equatorial coupled waves (Kelvin and Rossby) are investigated based on the NCEP/NCAR Reanalysis data for the 1977-2006 period and the outputs of an intermediate ocean-atmosphere coupled model named LODCA-OTCM. A strong seasonal dependence of the ITV/ENSO relationship is evidenced. The leading relationship for equatorial Rossby waves (with the correlation of the same order than for the MJO) is documented; namely, it is shown that intensification of Rossby waves in the central Pacific during boreal summer precedes by half a year the peak of El Niño. The fact that MJO activity in spring-summer is associated to the strength of subsequent El Niño is confirmed. It is shown that LODCA-QTCM is capable of simulating the convectively coupled equatorial waves in outgoing long wave radiation and zonal wind at 850 hPa fields with skill comparable to other Coupled General Circulation Models. The ITV/ENSO relationship is modulated at low frequency. In particular the periods of low ENSO amplitude are associated with weaker MJO activity and a cancellation of MJO at the ENSO development phase. In opposition, during the decaying phase, MJO signal is strong. The periods of strong ENSO activity are associated with a marked coupling between MJO, Kelvin and equatorially Rossby waves and ENSO; the precursor signal of MJO (Rossby waves) in the western (central) Pacific is obvious. The results provide material for the observed change in ENSO characteristics in recent years and question whether the characteristics of the ITV/ENSO relationship may be sensitive to the observed warming in the central tropical Pacific. © 2011 © Versita Warsaw and Springer-Verlag Wien.