CANDIE: A new version of the DieCAST ocean circulation model

The development and verification of a new version of the DieCAST ocean circ ulation model to be referred to as CANDIE (Canadian Diecast) are considered . Both CANDIE and DieCAST have many features in common with the well-known Modular Ocean Model (MOM) of the Geophysical Fluid Dynamics Laboratory. Of particular relevance to the present study are the rigid-lid approximation a nd the use of standard Cartesian coordinates. The DieCAST formulation in te rms of the surface pressure, rather than the volume transport streamfunctio n, is also used in CANDIE to reduce numerical sensitivity to ocean depth va riations. The major difference between MOM and DieCAST is the use of a mixe d C and A grid formulation in DieCAST rather than the B grid formulation us ed in MOM. CANDIE differs from DieCAST in the use of a standard C grid form ulation and a reduction in the magnitude of the time truncation error assoc iated with the implicit treatment of the Coriolis force. The implementation of the rigid-lid approximation is retained. To assess the reliability of both DieCAST and CANDIE, the authors have appl ied these models to a problem used by Haidvogel and Beckmann in a compariso n of several different model formulations. The tests include the influence of a steep-sided coastal canyon that represents a significant challenge for the step topography of Cartesian coordinate models. Haidvogel and Beckmann 's tests show general agreement between models based on topography followin g coordinates, but significantly different results were obtained with MOM. The results of DieCAST for the homogeneous test case also differ substantia lly from those of the sigma-coordinate models, largely due to dissipation a ssociated with low-order interpolations used adjacent to solid boundaries i n DieCAST. However. the results of CANDIE are in good agreement with those of the sigma-coordinate models for both homogeneous and stratified coastal canon experiments, These results clearly demonstrate that the differences f ound for both MOM and DieCAST are not due to intrinsic limitations associat ed with the use of Cartesian coordinates.