Ollitrault Michel

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Ollitrault
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Michel
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  • Preprint
    The mid-depth circulation of the northwestern tropical Atlantic observed by floats
    ( 2009-05-29) Lankhorst, Matthias ; Fratantoni, David M. ; Ollitrault, Michel ; Richardson, Philip L. ; Send, Uwe ; Zenk, Walter
    A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e. g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16° N between the Antilles islands and the Mid- Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADWeast of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.
  • Preprint
    Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic
    ( 2004-12-09) Nunez-Riboni, Ismael ; Boebel, Olaf ; Ollitrault, Michel ; You, Yuzhu ; Richardson, Philip L. ; Davis, Russ E.
    This study combines float data from different projects collected between 1991 and 2003 in the South Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). Velocity spacetime averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW layer). When judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a nominal cell size of 3º (latitude) by 4º (longitude) with cell shapes deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Results show an anticyclonic Subtropical Gyre centred near 36ºS and spanning from 23º±1°S to 46° ± 1ºS. The South Atlantic Current meanders from 33ºS to 46ºS and shows a mean speed of 9.6 ± 7.8 cm s-1 (8.5 Sv ± 3.5 Sv; 1 Sv = 1×106 m3 s-1). The northern branch of the Subtropical Gyre is located between 22ºS and 32ºS and flows westward with a mean speed of 4.7 ± 3.3 cm s-1 (9.3 Sv ± 3.4 Sv). Evidence of a cyclonic Tropical Gyre divided in two sub-cells is visible on the stream function.