Modeling the ocean thermohaline circulation
:
analysis of its interdecadal variability.
The thermohaline circulation and its potential role in climate variability is studied
through coarse-resolution oceanic numerical models. First, we analyze the influence of
various parameterizations for momentum dissipation and associated boundary conditions.
The later, inducing variations of almost an order of magnitude in the upwelling of cold
water beneath the western boundary current, are critical. "Classical" models (no-slip
boundary conditions) produce through the continuity equations huge vertical transports
along the boundaries, hence large meridional and zonal overturning : insufficiently cold
(sharp) bottom waters (thermocline) result in a weak poleward heat transport. "Original"
models, allowing tangential velocities along the boundaries, reduce vertical velocities and
vorticity dissipation in the lateral boundary layers: a better agreement between downwelling
regions and deep-convection areas, cooler bottom waters, sharper thermocline and stronger
heat transport are achieved.
Under fixed flux surface boundary conditions, these models produce large interdecadal
oscillations, robust to various sub-grid-scale parameterizations and forcing (wind,
"realistic" restoring boundary conditions). An extensive parameter sensitivity study reveals
a geostrophic three-dimensional mechanism, principally damped by horizontal diffusion
(critical diffusion coefficients are in the traditional range of values used for such
resolution). In contradiction with various mechanisms already proposed, the variability is
found to be driven by the baroclinic instability in the western boundary current (close to the
separation region): growth rate agrees well with critical damping terms, and the minimal
model embedding this instability (2 1/2 layer) reproduces some variability. In spite of the
crude representation of important oceanic physical processes, various features of the
observed variability in the North Atlantic are reproduced here.