On advection and diffusion of plankton in coarse resolution ocean models

There is a widespread use of general circulation models (GCMs) to study the importance of various physical and biological processes for the primary production, and to understand the structure of the oceanic food web. However, it is shown that low-resolution GCMs may produce too high advection speeds for a plankton bloom that invade an area with potentially high plankton growth rates. Under the prescribed situation, the advection and diffusion act to initiate an exponential growth of plankton in a large grid cell implying that the plankton signal is quickly carried over the entire grid cell. When a certain concentration is reached, it may seed the surrounding grid cells with phytoplankton, and the procedure is repeated. Accordingly, the phytoplankton bloom can travel faster in the low-resolution model than the ocean currents that carry them. Some numerical experiments show that the resolution of the model needs to be at least on the order of Δx U/γP or —whichever is largest—to describe the plankton dynamics in an adequate way. Here, U is the typical horizontal advection velocity, D is the horizontal diffusivity for plankton, and γP represents the growth rate of phytoplankton. With U=0.1 m/s and γP=1 day−1, it follows that the resolution needs to be at least 8 km to describe the phytoplankton dynamics in a reasonably correct way. Most GCM experiments use coarser resolution; accordingly, the results regarding the advection of plankton in these models should thus be viewed with some caution.