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Accueil > Actualités > Séminaires > Séminaires 2014

Lundi 30 juin 2014 à 14h30 en salle K118

Eric FIRING, Université de Hawaii, USA

Titre/Title :
Mean flows generated by waves on a beta plane : the role of dissipation.

Contact :
Chantal Staquet

Résumé/Abstract :
Conservation of potential vorticity strongly constrains geophysical
flows ; Lagrangian mean flows with water parcels crossing mean PV
contours can occur only to the extent that dissipative processes
modify their PV. For example, the dissipation of an equatorial mixed
Rossby-gravity wave beam, propagating downward and to the east away
from its source near the surface, can sustain deep mean zonal
circulation between the beam and the western boundary. This essential
role of dissipation raises the question of whether we can find a
regime in which increasing the explicit diffusivity of momentum
increases mean zonal flows more than it damps them. The problem is
approached with a variation on the classic Haidvogel and Rhines (1983)
experiment. Instead of a barotropic model, we use a 3-layer numerical
model with periodic forcing of the upper layer, bottom friction in the
bottom layer, and Laplacian lateral friction as the sole dissipative
process in the middle layer. The middle layer is analyzed using the
Generalized Lagrangian Mean (GLM) framework of Andrews and McIntyre
(1978) ; this works well so long as the motion is primarily wave-like,
but becomes problematic as the diffusivity is decreased to the point
where the motion becomes more irregular.