Eddy cancellation of the Ekman cell in subtropical gyres

Edward Doddridge, David Marshall & Andy Hogg
Journal of Physical Oceanography, 2016.
http://dx.doi.org/10.1175/JPO-D-16-0097.1

Abstract

The presence of large-scale Ekman pumping associated with the climatological wind stress curl is the textbook explanation for low biological activity in the subtropical gyres. Using an idealized eddy-resolving model it is shown that Eulerian-mean Ekman pumping may be opposed by an eddy-driven circulation, analogous to the way in which the atmospheric Ferrel cell and the Southern Ocean Deacon cell are opposed by eddy-driven circulations. Lagrangian particle tracking, potential vorticity fluxes, and depth-density streamfunctions are used to show that, in the model, the rectified effect of eddies acts to largely cancel the Eulerian-mean Ekman downwelling. To distinguish this effect from eddy compensation, it is proposed that the suppression of Eulerian-mean downwelling by eddies be called “eddy cancellation.”

Download pdf

Lagrangian particle tracking video

This video shows particles tracked for ten years of model time. The small lines represent individual particles, and the large circles show the centroid of each particle distribution. The green particles are advected by the Eulerian-mean velocities, while the purple ones are advected by the full velocities.

The background colours show the temperature on a meridional slice through the model domain.

Tracking particles like this provides a numerical method for estimating the Generalised Lagrangian-mean velocity of the mode water. These results show that the eddy transport opposes the Eulerian-mean downwelling from Ekman convergence.

Configuration, input, and analysis files

The configuration and input files for our MITgcm simulations, as well as the analysis scripts used to produce the results may be downloaded from here.

Python package for analysing MITgcm simulations

The analysis presented in this paper used this version of the python package described here.

 

Advertisements