MOMB20 Figure Caption

Snapshot of the temperature field (red is warm ~25°C, blue is cold ~2°C) after 600 years of integration of a primitive equation ocean model (the GFDL Modular Ocean Model) at fairly high resolution (20 km).
One can easily picture the turbulent nature of the field, with mushroom-like structures and isolated vortices. In fact, even with no wind forcing, the circulation is almost as turbulent as the real ocean (as measured by turbulent kinetic energy or sea surface height variations).
Apart from this mesoscale turbulence, the model produces large-amplitude interdecadal variations of the meridional overturning (the thermohaline circulation). That was in fact the scientific rationale for these experiments, since such long-term oscillations were found at low resolution (160 km horizontal grid spacing) and explained as driven by unstable planetary waves. This longwave baroclinic instability relies on large eddy-mixing, such that its interactions with the most energetic traditional mesoscale baroclinic instability was not straightforward.
The domain geometry is a Cartesian beta-plane centered at 40°N, extending from 20°N to 60°N (4470 km), 5120 km wide, 4500 m deep. The forcing is purely thermal, consisting of prescribed zonally-uniform surface heat flux linearly varying with latitude from +45 W/m² at 20°N to -45 W/m² at 60°N. Sub-grid-scale parameterizations are uniform, with biharmonic diffusion of momentum and tracer on the horizontal (4 10^{10} m^4/s), and standard eddy-difusivity on the vertical (10^{-4} m²/s for temperature and 10^{-3} m²/s for momentum). The model was initialized at year 0 by an interpolation of a 40-km-resolution previous 1000-yr run, itself initialized from a 3000-yr 80-km-resolution run, itself initialized from a 6000-yr 160-km run, such that the thermohaline circulation can be safely considered as statiscally stationnary.

References

  • Huck, T., 1997: Modélisation de la circulation thermohaline: Analyse de sa variabilité interdécennale. Thèse de doctorat, Université de Bretagne Occidentale, Brest, France, 250 p.
  • Huck, T., G. K. Vallis, and A. Colin de Verdière, 2001: On the robustness of the interdecadal modes of the thermohaline circulation. Journal of Climate, 14, 940-963.