Mechanisms controlling primary and new production in a global ecosystem model ? Part II: The role of the upper ocean short-term periodic and episodic mixing events

E. E. Popova (1) , A. C. Coward (1) , G. A. Nurser (1) , B. de Cuevas (1) , T. R. Anderson (1)

1 NOC - National Oceanography Centre

The use of 6 h, daily, weekly and monthly atmospheric forcing resulted in dramatically different predictions of plankton productivity in a global 3-D coupled physical-biogeochemical model.

Resolving the diurnal cycle of atmospheric variability by use of 6 h forcing, and hence also diurnal variability in UML depth, produced the largest difference, reducing predicted global primary and new production by 25% and 10% respectively relative to that predicted with daily and weekly forcing. This decrease varied regionally, being a 30% reduction in equatorial areas primarily because of increased light limitation resulting from deepening of the mixed layer overnight as well as enhanced storm activity, and 25% at moderate and high latitudes primarily due to increased grazing pressure resulting from late winter stratification events. Mini-blooms of phytoplankton and zooplankton occur in the model during these events, leading to zooplankton populations being sufficiently well developed to suppress the progress of phytoplankton blooms. A 10% increase in primary production was predicted in the peripheries of the oligotrophic gyres due to increased storm-induced nutrient supply end enhanced winter production during the short term stratification events that are resolved in the run forced by 6 h meteorological fields.

By resolving the diurnal cycle, model performance was significantly improved with respect to several common problems: underestimated primary production in the oligotrophic gyres; overestimated primary production in the Southern Ocean; overestimated magnitude of the spring bloom in the subarctic Pacific Ocean, and overestimated primary production in equatorial areas. The result of using 6 h forcing on predicted ecosystem dynamics was profound, the effects persisting far beyond the hourly timescale, and having major consequences for predicted global and new production on an annual basis.

Domaines: Océan, Atmosphère

E. E. Popova, A. C. Coward, G. A. Nurser, B. de Cuevas, T. R. Anderson. Mechanisms controlling primary and new production in a global ecosystem model ? Part II: The role of the upper ocean short-term periodic and episodic mixing events. Ocean Science, 2006, 2 (2), pp.267-279. ⟨hal-00298302⟩