RSS Feed HIGH RESOLUTION OCEAN MODELING
Matt O'Keefe, A. Sawdey, U of Minnesota, R. Bleck, E. Chassignet and L. Smith
Scientific Significance:
The goal is a high resolution simulation of the North Atlantic circulation with the Miami Isopycnic Coordinate Ocean Model (MICOM). The trait common to so-called "layer'' models such as MICOM is that they reverse the traditional roles of a pair of dependent and independent variables -- depth and density -- in the prognostic equations. The argument has been made that of all numerical schemes used for transporting thermodynamic variables in 3-dimensional ocean models, those relying on isopycnic discretization (constant density layers) will come closest to accomplishing the transport without introducing spurious mixing of constant chemistry density layers.
The current work at the PSC has led to an 11 layer version of the model (0.225 degree mesh or 512x512x11 grid points), covering the Atlantic between 25 degS and 65 degN and forced by wind stress, air temperature and humidity from the Comprehensive Ocean-Atmosphere Data Set (COADS) in combination with precipitation and net radiation from the Oberhuber atlas.
At the time of this writing, the model has been integrated for 4 model years on the Cray T3D in Pittsburgh. It takes a minimum of 10 model years for the wind-driven circulation to be in statistical equilibrium. This is approximately the time it takes for the first baroclinic Rossby wave to reach the western boundary.
Despite the short integration time, many realistic features are observed. The western boundary current, the Gulf Stream, separates from the North American continent at Cape Hatteras in agreement with observations and turns into a meandering free jet detectable at least halfway across the Atlantic basin. The dynamically unstable trade wind-driven westward flow north of the equator generates nice tropical waves. Vigorous eddies form off the North Brazilian coast, traverse the Caribbean basin and eventually lead to eddy shedding by the Loop Current in the Gulf of Mexico.
The seasonal cycle induced by the forcing functions is well illustrated by the advance/retreat of the mixed layer and the sea surface temperature (SST) field. In winter, a cold and windy atmosphere creates uniform densities in the subarctic ocean up to 3000 meters deep. This vertical homogenization is one facet of the slow meridional overturning cell in the Atlantic which has become known as the conveyor belt. In summer, the mixed layer depth is relatively shallow on the average (less than 100 meters). The seasonal SST changes are realistic and one can see the northward advection of warm temperatures from the Sargasso Sea by the western boundary current. Upwelling of cold water due to wind forcing is observed in the tropics and along the African coast.
This simulation is the first of its kind for MICOM and we are presently analysing in detail the large amount of data generated. We are looking forward to an even higher resolution experiment with the advent of the 512-node Cray T3D.
Numerical Approach and Performance:
The approach involves the solution of a hyperbolic (wave) equation. It is time-integrated in totally explicit fashion and this makes the model an attractive test object for massively parallel processors (MPPs). This has led to the installation of the model on two of the leading MPPs, Thinking Machines' CM-5 and Cray's T3D. Maintaining a still-evolving model code in several programming languages can be problematic as programs written in standard Fortran presently do not run well on MPPs.
Currently, one hour of computing time moves us ahead about 4 days.
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