On the Application and Evaluation of the Relocatable Diecast Ocean Circulation Model in Coastal and Semi-Enclosed Seas

Abstract

ADDENDUMThe cover shows a model output surface temperature sequence from a Gulf of Mexico run. The run is identical to Case B3 of Sections 3.1 and 5.3 except the lateral momentum diffusivity is ten times larger (200 m 2 /sec). The panels are 30 days apart. The run (with Prandtl number equal to 10) thus approximately emulates layered models, which generally have small or zero thermal diffusivity, while having finite momentum diffusivity. The increased lateral momentum diffusivity reduces the amplitude of paired cool core cyclonic recirculation eddies that are observed in the western Gulf of Mexico. Increasing lateral heat diffusivity to 200 m 2 /sec eliminates them. In such higher diffusivity cases, the old warm core western GOM eddies elongate along the shelfbreak as they dissipate. The substantial effects of Reynolds number on GOM dynamics is discussed further in this report. Color VHS animation of the Case B3 model results and model output data are available on request.All GOM runs in this report were done on a Silicon Graphics workstation using approximately 20 km lateral resolution with 20 layers (83 x 60 x 20 grid) and depths truncated at 3.5 km. In the South China Sea run (Section 6.1) with 1 10-element vectors, the model runs at over 205 megaflops on a single Cray YMP pipe. This is equivalent to less than 2 minutes per layer per model year with our GOM resolution (comparable to the fastest ocean models). The entire model vectorizes and over 99 percent of it compiles to parallel vectors. No modifications of the original SOMS model were required to achieve this full vectorization and parallelization by the Cray YMP compiler.The version of the model used here assumes constant horizontal grid increments while using the actual latitudinal Coriolis parameter distribution. The latitudinal increment is about 22 km, while the longitudinal is about 18 km. The latter is slightly less than the minimum (about 19 km) that would occur on a true sphere at the northern GOM boundary (based on 1/5 degree spacing of the bathymetry data used). This reduces the port separation between the Yucatan and Florida Straits by about 10 percent. A fully spherical version of our model is also available. 7 c 93-15471IllllhI!E~lii'y ABSTRACT To improve coastal and semi-enclosed seas modeling, a new ocean model called DieCAST (.Drtrich-Center for Air Sea Technology) has been developed. In application to the Gulf of Mexico (GOM), this model gives remarkably similar results to that obtained from the Sandia Ocean Model System (SOMS) and at a computational speed much faster than SOMS or other ocean models presently applicable using high coastal resolution.In this report, the effects of lateral diffusivity, resolution, and other parameters on SOMS and DieCAST modeled Gulf of Mexico eddies are explored. Results show: (1) that diffusivity on the order of 100 m 2 /sec or larger significantly decreases maximum velocity and frontal intensity, and greatly decreases the amplitudes of modeled paired eddies near the western GOM shelfbreak similar to th...

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