Ocean deoxygenation: everyone’s problem. Causes, impacts, consequences and solutions

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Square wave voltammetry measurements of low concentrations of nitrate using Au/AgNPs electrode in chloride solutions

International audienceThe aim of this work is to highlight the potential of using a modified gold electrode with controlled quantity of silver nanoparticles as a working electrode to detect low concentrations of nitrate in chloride solutions. Optimal charge for silver deposition has been determined to obtain the highest signal for the nitrate reduction as the electrocatalytic properties of the bimetallic electrode were directly influenced by its composition. According to the Volcano plot obtained the charge chosen was -52 µC for a 3 mm diameter electrode, corresponding to 4.6 x 1015 Ag atoms cm-2. It has been shown that dioxygen did not participate to the nitrate reduction mechanism. In order to decrease the limit of quantification, square wave voltammetry was preferred to less sensitive cyclic voltammetry. Nitrate was quantified in chloride solutions in the concentration range found in the open ocean, i.e. 0.39-50 µmol L-1 with a good linear regression (R2=0.9969). The stability of the bimetallic Au-Ag systems has been evaluated and showed almost no difference on the signal recorded over a 26 days period which is suitable to consider an in situ sensor development for marine applications

Inferring fluid motion at submesoscale from high resolution remotely sensed data

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How to obtain ocean turbulent dynamics at super resolution from optimal multiresolution analysis and multiplicative cascade?

International audienceA fundamental challenge in oceanography is the synoptic determination of ocean circulation using the data acquired from space, with a coherent depiction of its turbulent characteristics. This determination has the potential of revealing all aspects of the ocean dynamic variability on a wide range of spatio-temporal scales and will enhance our understanding of ocean–atmosphere exchanges at super resolution, as required in the present context of climate change. A method to obtain ocean dynamics products at different super resolutions is presented here, using an approximation of the energy cascade, expressed in a microcanonical formulation, and associated to turbulent signals provided by different products of Sea Surface Temperature (SST). The basics idea is to propagate across the scales motion information at lower resolution coming from GEKCO product [Sudre et al., 2013] in a multiresolution analysis computed on adimensional critical transition informations [Su-dre et al., 2015].REFERENCESSudre J., Maes C., and Garc ̧on V., 2013, On the global estimates of geostrophic and Ekman surface currents, Limnology and Oceanography: Fluids and Environments, vol. 3, pp. 1–20, DOI 10.1215/21573689-2071927.Sudre J., Yahia H., Pont O. and Garc ̧on V., 2015, Ocean turbulent dynamics at Superresolution from optimal multiresolution analysis and multiplicative cascade, IEEE transaction on geoscience and remote sensing, vol. 53, NO. 11, DOI 10.1109/TGRS.2015.2436431

ENSO diversity driving low-frequency change in mesoscale activity off Peru and Chile

International audienceTransient mesoscale oceanic eddies in Eastern Boundary Upwelling Systems are thought to strongly affect key regional scale processes such as ocean heat transport, coastal upwelling and productivity. Understanding how these can be modulated at low-frequency is thus critical to infer their role in the climate system. Here we use 26 years of satellite altimeter data and regional oceanic modeling to investigate the modulation of eddy kinetic energy (EKE) off Peru and Chile by ENSO, the main mode of natural variability in the tropical Pacific. We show that EKE tends to increase during strong Eastern Pacific (EP) El Ninõ events along the Peruvian coast up to northern Chile and decreases off central Chile, while it is hardly changed during Central Pacific El Niño and La Niña events. However the magnitude of the EKE changes during strong EP El Niño events is not proportional to their strength, with in particular the 1972/1973 El Niño event standing out as an extreme event in terms of EKE increase off Peru reaching an amplitude three times as large as that during the 1997/1998 El Niño event, and the 2015/2016 El Niño having instead a weak impact on EKE. This produces decadal changes in EKE, with a similar pattern than that of strong EP El Niño events, resulting in a significant negative (positive) long-term trend off Peru (central Chile)

Water mass analysis of the Coral Sea through an Optimum Multiparameter method

A water mass analysis of the Coral Sea thermocline waters provides a description of their distribution, pathways and mixture based on recent oceanographic cruises in this region of strong western boundary currents. The Optimum Multiparameter method is used to determine the relative contribution of core water masses based on their measured temperature, salinity and dissolved oxygen. The thermocline waters, carried by the broad South Equatorial Current (SEC), are essentially composed of four core water masses of different origins. Coming from the south, the South Pacic Tropical Water South (SPTWS, σ=25.3 kg.m−3) and the Western South Pa-cic Central Water (WSPCW, σ=26.3 kg.m−3) enter the Coral Sea by the channel between the island of New Caledonia and the Vanuatu archipelago. Coming from the north, the South Pacic Tropical Water North (SPTWN, σ=24.5 kg.m−3) and the Pacic Equatorial Water (PEW, σ=26.3 kg.m−3) flow north of Vanuatu. The upper thermocline water that exits the Coral Sea equatorward, is mainly composed of SPTWN carried by the New Guinea Coastal Undercurrent. In contrast, upper thermocline waters exiting the Coral Sea poleward, in the East Australian Current, is dominated by SPTWS. The relative contributions are different in the lower thermocline where WSPCW dominates both western boundary currents. This refined description is consistent with the dynamics of the main currents, with a very strong depth dependence in the partitioning of incoming SEC waters

Comparison of two remote sensing methods for the evaluation of ocean dynamics

International audienceThe dynamics of the ocean flow are governed by the cascade process where the energy is injected in the system at the largest scale and dissipated at viscous scale. In between, the energy crosses all the intermediary scales and is responsible for the creation of oceanographic structures at different scales. To obtain the motion field of ocean dynamics [1] at the high resolution of Sea Surface Temperature (SST) Modis data (spatial resolution : 4 km) using one single image, we associate the Microcanonical Multiscale Formalism (MMF) [2] and the Microcanonical Cascade (MC) which represent these processes where the energy is transferred from scale to scale [3]. To link two different scales (or, in a remote sensing context, the same image at two different resolutions), the MC requires the exact determination of an optimal wavelet which separates completely the Multiscale Operator and the wavelet transform at the highest resolution. This exact determination of the optimal wavelet for a given dataset is difficult and there is no systematic algorithm to find it. Our goal is to best approximate the optimal wavelet for SST data by using Q-test [4]. References [1] H. Yahia, J. Sudre, C. Pottier and V. Garçon, 2010, Motion analysis in oceanographic satellite images using multiscale methods and the energy cascade, Pattern Recognition, DOI: 10.1016/J.patcog.2010.04.011 [2] A. Turiel, H. Yahia and C. Perez-Vicente, 2008 Microcanonical Multifractal Formalism: a geometrical approach to multifractal systems. Part I: Singularity Analysis, Journal of Physics A 41:015501. [3] C. Pottier, A. Turiel, V. Garçon, 2008, Inferring missing data in satellite chlorophyll maps using turbulent cascading, Remote Sensing of Environment, 112,4242-4260, 10.1016/j.rse.2008.07.010 [4] O. Pont, A Microcanonical cascade formalism for multifractal systems ans its application to data inference and forecasting. PhD thesis, Dept. of Fundamental Physics, University of Barcelona, 200

Evidencing of multiplicative cascading and intermittency in real/synthetic oceanographic signals: application to the evaluation of ocean dynamics

International audienceMost existing methods in analyzing motion in Computer Vision do not take into account key features of ocean dynamics: turbulence and intermittency, which contribute importantly to the shape and motion of observed and acquired coherent structures. Besides, the specificities of these remotely sensed acquisitions (Sea Surface Temperature, chlorophyll a concentration) lead to question the pertinence of existing Computer Vision approaches to analyze motion in these types of image sequences. To compute the motion field at high resolution associating the Microcanonical Multiscale Formalism (MMF) [1] and the Microcanonical Cascade (MC) [2], one needs to give prominence to the intermittency and cascade properties observed in SST Modis data and/or synthetic signals from an ocean model. We propose here to evaluate the ocean dynamics with respect to intermittency/cascading properties evaluation for data acquired by satellites or synthetic data coming from a simulation model. We then apply the combination of MMF and MC to obtain high resolution (4km) motion fields [3]. References [1] A. Turiel, H. Yahia and C. Perez-Vicente, 2008 Microcanonical Multifractal Formalism: a geometrical approach to multifractal systems. Part I: Singularity Analysis Journal of Physics A 41:015501. [2] C. Pottier, A. Turiel, V. Garçon, 2008, Inferring missing data in satellite chlorophyll maps using turbulent cascading, Remote Sensing of Environnement, 112,4242-4260, 10.1016/j.rse.2008.07.010 [3] H. Yahia, J. Sudre, C. Pottier and V. Garçon, 2010, Motion analysis in oceanographic satellite images using multiscale methods and the energy cascade, Pattern Recognition, DOI: 10.1016/J.patcog.2010.04.01