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

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 physicalbiogeochemical
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.