Eddy mean-flow cancellation in subtropical gyres and implications for biological productivity

in

PO015: Mesoscale and submesoscale processes: Characterization, dynamics, and representation

An electronic copy of my poster can be found here.

This video shows the Lagrangian particle tracking experiments. Just like the poster, the purple particles are in the full time-varying flow, and the green ones are advected by the Eulerian-mean flow. In the video you can see when the particles get reseeded every three months.

The depth-density stream functions that I showed on my poster were calculated excluding the southern- and northernmost 10 degrees of latitude. This removed the basin-scale overturning circulation. If you’re curious, you can see the unmasked circulations here. The contour interval is two Sverdrups, the red lobes rotate clockwise, and the blue ones counterclockwise.

From the Eulerian-mean fields

From the instantaneous fields

It’s still possible to see the that the eddy-induced circulation has cancelled the Eulerian-mean downwelling, but it’s a bit harder with the basin-scale circulation superimposed over the top.

Eddy cancellation of Eulerian-mean downwelling may help to resolve the discrepancy between estimates of nutrient supply and biological productivity in the subtropical gyres. To show that the rectified effect of eddies is important we ran two biogeochemical simulations. In one we used the full model velocity fields, in the other we used the Eulerian-mean fields. Videos of these two simulations are shown below.

The pink contours are density, the black contours are phosphate, blue contours are sea surface height, and the green colour scale represents biological activity.

In the Eulerian-mean simulation the nutrients are downwelled and productivity is lower. In the eddying simulation the nutrients are kept in the gyre for longer and productivity is higher. These results are preliminary, but promising. They suggest that the rectified transport from transient eddies has a role to play in balancing the nutrient budget of the oligotrophic ocean and may help explain why correlations between Ekman pumping and biological activity are so low.

Eulerian mean velocities

Eddying velocity fields