Prof Joseph Henry Lacasce, Oslo

Affiliation: University of Oslo, Norway

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Title: Vortices over bathymetry

Abstract: In many regions of the ocean, the flow is dominated by surface-intensified vortices with scales of 10s of kilometers. These interact with bottom topography, often non-intuitive ways. It is possible to predict the mean flow generated by such “geostrophic turbulence” using a simple variational argument. With a single fluid layer, the predicted flow is correlated with the bathymetry, with cyclonic flow around basins, but with the small scales smoothed out. With two fluid layers, the flow is bottom-intensified, with the same sense of circulation. Interestingly, at larger energies the deep flow is strong enough that the deep potential vorticity is “homogenized”. In such cases, the topographic effect vanishes and the flow is effectively like that over a flat bottom. 

We examine a range of numerical turbulence simulations in light of the predictions. The basin-scale flow is as predicted, bottom-intensified with the correct circulation. But in addition, an oppositely-signed (anticyclonic) vortex is often observed at the surface. Such vortices are also seen in the ocean, notably a semi-permanent feature in the Lofoten Basin west of Norway. We consider possible explanations for this, and also how bottom friction modifies the different components of the flow. With the addition of the planetary beta effect, the resulting surface vortices closely resemble those in the ocean, as seen by satellite.