Coupled physical and biological modelling of the spring bloom in the North Atlantic (II): three dimensional bloom and post-bloom processes

A set of three dimensional coupled physical and biological models is used to ascertain the importance of mesoscale dynamical activity during the 1989 JGOFS North Atlantic Bloom Experiment. First, various physical processes causing nutrient flux are studied with a simplified one compartment “nutrient model” utilized in an isolated vortex. A hierarchy of vertical transport processes is described for this case, ranging from the relatively minor fluxes caused by vortex stretching in the interior to the moderate transport resulting from the interaction of the interior and wind driven motions to the rather vigorous (3 m per day) effective transport caused by the propagation of the vortex. With these flux mechanisms in hand, the tuned four compartment biological model described in Part 1 is used to simulate the Small eddy in isolation. Together the propagation flux and the lifting of density surfaces caused by vortex evolution result in a twofold increase in mixed layer nitrate over that predicted by a one dimensional model. Enhanced phytoplankton and heterotroph production is associated with the elevated nutrient concentrations. Finally the three eddy configuration ovserved during the experiment is simulated. While all the previous vertical transports are active in this case, they are overshadowed by intense vertical motions associated with eddy-eddy interactions. Nutrient enhancements of up to an order of magnitude occur in the simulations that substantially increase both plant and animal production in localized regions for periods of weeks.