The Role of Geomagnetic Cues in Green Turtle Open Sea Navigation

Background: Laboratory and field experiments have provided evidence that sea turtles use geomagnetic cues to navigate in the open sea. For instance, green turtles (Chelonia mydas) displaced 100 km away from their nesting site were impaired in returning home when carrying a strong magnet glued on the head. However, the actual role of geomagnetic cues remains unclear, since magnetically treated green turtles can perform large scale (.2000 km) post-nesting migrations no differently from controls. Methodology/Principal Findings: In the present homing experiment, 24 green turtles were displaced 200 km away from their nesting site on an oceanic island, and tracked, for the first time in this type of experiment, with Global Positioning System (GPS), which is able to provide much more frequent and accurate locations than previously used tracking methods. Eight turtles were magnetically treated for 24–48 h on the nesting beach prior to displacement, and another eight turtles had a magnet glued on the head at the release site. The last eight turtles were used as controls. Detailed analyses of water masses-related (i.e., current-corrected) homing paths showed that magnetically treated turtles were able to navigate toward their nesting site as efficiently as controls, but those carrying magnets were significantly impaired once they arrived within 50 km of home. Conclusions/Significance: While green turtles do not seem to need geomagnetic cues to navigate far from the goal, these cues become necessary when turtles get closer to home. As the very last part of the homing trip (within a few kilometers of home) likely depends on non-magnetic cues, our results suggest that magnetic cues play a key role in sea turtle navigation at an intermediate scale by bridging the gap between large and small scale navigational processes, which both appear to depend on non-magnetic cues

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Effect of wind stress forcing on ocean dynamics at Air-Sea Interface

International audienceSurface currents in air-sea interaction are of crucial importance because they transport heat from low to high latitudes. At first order, oceanic currents are generated by the balance of Coriolis and pressure gradient (geostrophic current) and the balance of Coriolis and the frictional force dominated by wind stress in the surface ocean (Ekman current). The GEKCO product is a daily 1/4 degree resolution product which permits the computation of two kinds of vector fields: geostrophy with and without wind stress forcing. We aim at studying the difference in term of turbulent hydrodynamics carried by the wind forcing at the air-sea interface. We explore the statistical properties of singularity spectra computed from velocity norms and vorticity data, notably in relation with kurtosis information to underline differences in the turbulent regimes associated with both kinds of velocity fields. This study is conducted over 1 year of daily data and demonstrates the differences in terms of turbulent property of wind forcing

Multi-sensor data fusion through multiscale model: a method for high resolution oriented geostrophy

International audienceObtention of the orientation of the goestrophic vector field from satellite data and the Microcanonical Multiscale Formalis

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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

Ocean Turbulent Dynamics at Superresolution From Optimal Multiresolution Analysis and Multiplicative Cascade

International audienceThe synoptic determination of ocean circulation using the data acquired from space, with a coherent depiction of its turbulent characteristics, remains a fundamental challenge in oceanography. This determination has the potential of revealing all aspects of the ocean dynamic variability on a wide range of spatiotemporal scales and will enhance our understanding of ocean–atmosphere exchanges at superresolution, as required in the present context of climate change. Here, we show a four-year time series of spatial superresolution (4 km) turbulent ocean dynamics generated from satellite data using emerging ideas in signal processing coming from nonlinear physics, low-resolution dynamics, and superresolution oceanic sea surface temperature data acquired from optical sensors. The method at its core consists in propagating across the scales the low-resolution dynamics in a multiresolution analysis computed on adimensional critical transition information

High-Resolution Ocean Dynamics from Microcanonical Formulations in Nonlinear Complex Signal Analysis

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