Jet Jumping: Low Frequency Variability in the Southern Ocean

Low-frequency variability of quasi-zonal jets near large topographic features is studied using satellite altimetry and an idealised, quasi-geostrophic ocean model. In particular, we investigate the phenomena of "jet jumping", a mode of variability by which two (or more) jets which pass near a topographic feature undergo anticorrelated changes in strength.

Using satellite altimetry we study in detail three topographic features in the Southern Ocean: The Southeast Indian Ridge; the Macquarie Ridge; and the East Pacific Rise. In each of these locations, two Southern Ocean jets interact with topography and exhibit the anticorrelated changes in strength that characterise jet jumping. Common to all regions are the formation of vortices in between the jets. These vortices change their strength (as measured by spatially averaged vorticity) but in general, do not change their locations which are set by topography.

We investigate the dynamics of this variability in an idealised setting: quasi--geostrophic flow over a meridionally oriented ridge with two "canyons" that act to steer the flow. In this configuration, jet jumping can be simulated. Using this idealised model, we related the variability in jet strength to the changes in strength of the topographic vortices that form in between the jets.

Finally, we argue that the results from the simple numerical experiments are relevant for explaining the dynamics of the jet jumping in the Southern Ocean.