| Abstract: Two different types of numerical
ocean circulation models are used in a classical idealized problem, the
thermally induced circulation in an ocean basin bounded by two meridians
to the east and west and by the equator and a line of constant
latitude. A simple scaling theory exists for predicting poleward
heat transport and the strength of meridional overturning as a function
of vertical diffusivity and other external factors. However,
previous studies have indicated conflicting results, and other scaling
laws have been proposed. Experiments with two widely used types of
numerical models, one based on depth coordinates and the other based on
isopycnal layers, provide insight into the discrepancies of previous
studies. In the numerical experiments vertical diffusivity is
varied over a range of 200. The source of the difficulty in
previous studies is in part traced to applying a fixed restoring
coefficient at the upper boundary and considering the buoyancy forcing
at the surface fixed irrespective of vertical diffusivity k.
Globally or zonally averaged results show a robust agreement between the
two models and support the simple scaling law in a flat-bottom basin and
a bowl-shaped basin, as long as the meridional circulation is estimated
along isopycnal surfaces and in situ rather than externally imposed
restoring density differences are used to estimate the geostrophic-scale
velocity. Over the thermocline the vertical mean of the zonally
averaged zonal baroclinic pressure gradient has constant ratio to the
vertical mean of the zonally averaged meridional baroclinic pressure
gradient, consistent with the scaling assumptions for a diffusive
thermocline. |