Speaker: Dr. Navid Constantinou from Scripps
Title: "Eddy saturation in a barotropic model"
Abstract: "Eddy saturation’ refers to a regime in which the total zonal volume transport of an ocean current is insensitive to the wind stress strength; the Antarctic Circumpolar Current (ACC) is the most prominent example. Baroclinicity is currently believed to be key to the development of an eddy-saturated state. Here, we show that eddy saturation can occur in a barotropic ﬂow over topography, without baroclinic-ity. We demonstrate that in this model the main factor controlling the appearance or not of eddy saturated states is the structure of geostrophic contours, that is the contours of f/H, with f the Coriolis parame-ter and H the depth. Eddy saturated states occur when the geostrophic contours are open, that is when the geostrophic contours span the whole zonal extent of the domain. We demonstrate this minimal re-quirement for eddy saturated states in a scenario relevant to the ACC using numerical integrations of a single-layer quasi-geostrophic ﬂow over two different topographies characterized by either open or closed geostrophic contours. With open geostrophic contours we ﬁnd a turbulent regime with impres-sive eddy saturation: only 4-fold transport increase over 60-fold wind stress increase. Furthermore, in the eddy saturation regime (i) the eddy kinetic energy increases linearly with wind stress and also (ii) the mean transport increases with increasing Ekman drag. Both these feature are further ‘symptoms’ of eddy saturation seen in comprehensive models of the ACC. Finally, we extend the stability method of Hart [J. Atmos. Sci., 36, 1736–1746, (1979)] to explain the ﬂow transitions that occur in the solutions with open geostrophic contours and provide insight on the mechanism underlying eddy saturation in this barotropic setting."