Chaotic behaviour of an ocean ecosystem model under seasonal external forcing

A four-component ecosystem model of the oceanic upper mixedlayer (UML) forced by the annual cycle of UML depth, solar irradiationand dissolved inorganic nitrogen (DIN) entrainment from theseasonal pycnocline is presented. The model solution demonstratesthe following types of temporal variability: a periodical regimewith the frequency of the external forcing, a regime with aperiod of more than 1 year, quasi-periodic, and chaotic motion.The model results suggest that the last three types describingthe interannual variability can occur only at low latitudesin regions of strong upwelling where the DIN concentration inthe seasonal pycnocline is high. However, the range of externalforcing parameters in which such behaviour takes place is sonarrow that it is unlikely to be a common phenomenon in theocean. The quasi-periodic or chaotic variability of the modelecosystem is very sensitive to the initial conditions, and thereforeany exact prediction of model behaviour is impossible. Nevertheless,a prediction of model ecosystem behaviour can be obtained interms of a probability density. The annual cycle of the modelcomponents calculated in this way shows that the dispersionof the trajectories during the winter period is markedly smallerthan during the summer. It implies that the dynamics of themodel ecosystem during the summer period is less predictable.