Abstract
Global warming may modify submesoscale activity in the ocean through
changes in the mixed layer depth and lateral buoyancy gradients. As a
case study we consider a region in the Northeast Atlantic under present
and future climate conditions, using a time-slice method and global and
nested regional ocean models. The high resolution regional model
reproduces the strong seasonal cycle in submesoscale activity observed
under present-day conditions. In the future, with a reduction in the
mixed layer depth, there is a substantial reduction in submesoscale
activity and an associated decrease in kinetic energy at the mesoscale.
The vertical buoyancy flux induced by submesoscale activity is reduced
by a factor of 2. When submesoscale activity is suppressed, by
increasing the parameterized lateral mixing in the model, the climate
change induces a larger reduction in winter mixed layer depths while
there is less of a change in kinetic energy at the mesoscale. A scaling
for the vertical buoyancy flux proposed by Fox-Kemper
et.\ al.\, based on the properties of
mixed layer instability (MLI), is found to capture much of the seasonal
and future changes to the flux in terms of regional averages as well as
the spatial structure, although it over predicts the reduction in the
flux in the winter months. The vertical buoyancy flux when the mixed
layer is relatively shallow is significantly greater than that given by
the scaling based on MLI, suggesting during these times other processes
(besides MLI) may dominate submesoscale buoyancy fluxes.