  In the recent past observational and modelling studies have shown that the
vertical displacement of water parcels, and therefore, phytoplankton particles
in regions of deep-reaching convection plays a key role in late winter/early
spring primary production. The underlying mechanism describes how convection
cells capture living phytoplankton cells and recurrently expose them to
sunlight. This study presents a parameterisation called `phytoconvection' which
focuses on the influence of convection on primary production. This
parameterisation was implemented into a three-dimensional
physical-biogeochemical model and applied to the Northwestern European
Continental Shelf and areas of the adjacent Northeast Atlantic. The simulation
was compared to a `conventional' parameterisation with respect to its influence
on phytoplankton concentrations during the annual cycle and its effect on the
carbon cycle. The simulation using the new parameterisation showed good
agreement with observation data recorded during winter, whereas the reference
simulation did not capture the observed phytoplankton concentrations. The new
parameterisation had a strong influence on the carbon export through the
sinking of particulate organic carbon. The carbon export during late
winter/early spring significantly exceeded the export of the reference run.
Furthermore, a non-hydrostatic convection model was used to evaluate the major
assumption of the presented parameterisation which implies the matching of the
mixed layer depth with the convective mixing depth. The applied mixed layer
depth criterion principally overestimates the actual convective mixing depth.
However, the results showed that this assumption is reasonable during late
winter, while indicating a mismatch during spring.
