Fronts are regions of strong horizontal density gradients in the ocean. The strong horizontal pressure gradients generated at the front are often balanced by the Coriolis force, leading to strong along-front currents. When these two forces are not perfectly balanced, vertical circulations can be generated at the front. These vertical circulations have the potential of bringing deep, nutrient-rich waters into the well-lit surface waters, and stimulating phytoplankton growth. Fronts are typically the site of subsurface patches of phytoplankton, however, we do not have a good understanding of the physical and biological processes that maintain these patches.
One of the forcings known to have a strong influence on fronts is the wind. We have developed a mixed-layer/primitive-equation/ecosystem model to study the effects of wind on phytoplankton growth at fronts. This model showed that wind can stimulate the formation of phytoplankton patches, and that the wind direction relative to the front strongly determines the resulting patch structure.
Ekman flux to cold
side of front. Phytoplankton and temperature after the first wind
event.
Ekman flux to warm
side of front. Phytoplankton and temperature after the first wind
event.
An
animation of the
cross-frontal velocities, temperature structure, and mixed-layer depth over
20 days, with 2 wind events. Ekman flux to cold side of front.An
animation of the
evolution of the phytoplankton and temperature for the same case
as above.Animations in fli/flc format can be found here
This work was funded in part by UCAR and by U.S. GLOBEC
Franks, P.J.S. 1992. Phytoplankton blooms at fronts: patterns,
scales and
physical forcing
mechanisms. Reviews in Aquatic Sciences 6:121-137.
Franks, P.J.S. 1992. Sink or swim: accumulation of biomass at
fronts.
Marine Ecology Progress Series
82:1-12.
Franks, P.J.S. and L.J. Walstad. 1997. Phytoplankton patches at fronts:
a model of formation and response to transient wind events.
Journal of Marine Research 55:1-30.
Click here to download a Postscript version of the Franks and Walstad manuscript and its figures.
E-mail below for a reprint