Laboratoire d’Océanographie Physique et Biogéochimique

Mesoscale eddies in the lee of Hawaii : closed or open systems ?

Francesco Nencioli, chercheur à University of California Santa Cruz présentera un séminaire le mardi 24 mars à 10h30 en salle de réunion du COM à Luminy :

“Mesoscale eddies in the lee of Hawaii : closed or open systems ?”

Mesoscale eddies are often treated as closed systems with respect to horizontal exchanges with the surrounding waters. Therefore, nutrient injections into the euphotic zone are assumed to occur only during the time of eddy formation. On March 2005, the cyclonic cold-core eddy Opal was sampled during the E-Flux III field experiment in the lee of Hawaii. Analysis of the physical data revealed a well developed, fairly circular eddy, characterized by an intense doming of isopycnal surfaces. The biogeochemical data show a shoaling of the nutricline and the Deep Chlorophyll Maximum Layer (DCML) within the eddy. Enhanced nutrient concentrations in the euphotic zone led to an increase of chlorophyll concentrations within the eddy’s core, as well as to a shift in the ecological community from small to large phytoplankton (prochlorococcus to diatoms). Vertical profiles of density show that the anomalies associated with the eddy extended to 700 m. However, Opal’s cyclonic circulation was primarily limited within the upper mixed layer. Analysis of the potential vorticity field suggests that the portion of Opal isolated from the surrounding waters was relatively shallow, and that radial exchanges of water might have occurred below this portion, during the eddy migration. We therefore hypothesize a conceptual model in which nutrient inputs are not limited to a single injection, but can occur more or less continuously during the eddy’s lifetime.