Open boundary conditions for tidally and subtidally forced circulation in a limited-area coastal model using the Regional Ocean Modeling System (ROMS)

摘要

In limited-area ocean models, open boundary conditions (OBCs) often create dynamic inconsistencies and perform poorly in resolving tidal or subtidal flow when both forces exist. Orlanski-type radiation OBCs are reasonably efficient at treating the subtidally forced flow, and Flather-type OBCs are commonly adapted for the tidally forced flow. However, neither of them performs well when tidal and subtidal forces simultaneously drive the flows. We have developed a novel OBC that integrates the active OBC in Gan and Allen (2005) and a Flather-type OBC. This new OBC accommodates the concurrent Tidal and Subtidal (TST) forcing, and the respective tidal or subtidal forcing, at the open boundary of a limited-area model. This new TST-OBC treats the tidal component with a Flather-type OBC, and it separates subtidal barotropic and baroclinic components into local (forced) and global (unforced) components. Then an unforced Orlanski-type OBC can be applied to the global part. We applied the TST-OBC to all model variables to reduce dynamic inconsistence. Using the Regional Ocean Modeling System, we applied the TST-OBC to the shallow East China Sea shelf where strong tidal and subtidal forces over complex topography govern the circulation. Our numerical experiments and analyses suggest that the TST-OBC was robust for both concurrent tidal-subtidal forcing and solely tidal or subtidal forcing at the open boundary. It reduced spurious energy reflection, and, overall, it performed better than an Orlanski-type or Flather-type OBC in reproducing realistic tidal and subtidal shelf circulation.