Frictional secondary circulation near an upwelled thermocline

Abstract

A first order, linear analysis of thermocline upwelling shows that a quasigeostrophic baroclinic jet accompanies the upwelling. The secondary circulation induced in such a flow field in a meridional plane by turbulent friction is investigated. A double Ekman layer is found to form at the thermocline, of a few meters in thickness. The suction of this Ekman layer destroys the primary flow by the "spin-up" mechanism in a period of which the order of magnitude is tentatively estimated at 100 hrs. Thus the primary flow must be reestablished by successive, reasonably frequent wind-stress impulses to maintain a quasi-stationary upwelling. The lower side Ekman layer on the thermocline is fed from a free shear layer (turbulent "Stewartson layer") underneath the free surface -thermocline intersection, which thus becomes an upwelling zone. The width of the turbulent Stewartson layer is estimated to be the velocity difference divided by the Coriolis parameter, i.e. typically of the order of 1 km.