Subtropical gyres are the ocean’s deserts. Classically this is believed to result from surface wind forcing driving convergent surface currents and downwelling, preventing the supply of essential nutrients from the cold, nutrient-rich beneath. Nevertheless modest biological activity is found, at odds with this simple paradigm.
We have just published a paper in the Journal of Physical Oceanography which shows that turbulent eddies oppose the wind-driven downwelling. The following is a movie produced by Ed Doddridge, my former graduate student and lead author on the paper.
Particles tracked for ten model years. The small lines represent individual particles and the large circles show the centroid of the particles. The green particles move with the Eulerian-mean velocities, while the purple ones move with the full, eddying velocities. The background colours show the potential temperature on a meridional slice through the model domain. These tracked particles provide a method for estimating the “Generalised Lagrangian-mean velocity” of the subtropical mode water. The results demonstrate that the eddy transport opposes the Eulerian-mean downwelling.
We are currently writing a follow-up paper to explore the implications for biological activity in the subtropical oceans.
(thanks to Ed Doddridge for his post on which this is based)