2004 LOM Workshop

Monday 9:55 - 10:15 a.m.

The T/S/rho conservation conundrum in isopycnic models

Rainer Bleck
Los Alamos National Laboratory
bleck@lanl.gov

ABSTRACT

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Temperature and salinity fronts in the ocean tend to adjust so as to minimize their joint impact on the buoyancy field. This phenomenon, known as T/S compensation, renders fronts dynamically invisible and long-lived. Numerically imperfect T/S advection in a circulation model can interfere with this T/S compensation. In an isopycnic model, lateral T/S transport errors are particularly troubling because they lead to nonphysical perturbations in layer thickness as the model struggles to restore the target density in affected layers. In MICOM and HYCOM, T/S compensation has traditionally been "built in" by diagnosing T from S & rho, rather than solving a prognostic equation for T. Due to the nonlinearity of the equation of state, this approach implies a small heat gain whenever S diffuses laterally, be it for numerical or physical reasons. Thermodynamic variables in a circulation model are subject to both transport and mixing. While enforcing T/S compensation during transport has clear physical advantages, our past strategy to treat T as a diagnostic variable during lateral mixing was strictly a matter of computational convenience. Experiments to reduce the internal heat source by retaining S and rho as the variables being transported, but evaluating lateral mixing in terms of T and S, have been successful. The resulting blend of S/rho advection and T/S mixing greatly improves the performance of MICOM and HYCOM in long-term climate simulations.

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LOM Users' Workshop, February 9-11, 2004