Interaction between near-inertial and geostrophic flow in idealized ocean circulation models

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Ocean circulation at large to intermediate scales can be thought of as a superposition of nearly-geostrophic and near-inertial components of motion. The slowly evolving geostrophic fields contains about 90% of the kinetic energy while the faster near-inertial field (frequencies near the Coriolis frequency) accounts for most of the remainder. This difference is attributable in large part to a fundamental difference in the way energy in these two classes of flow behave in geophysical turbulence. Near-inertial energy is readily transferred to dissipation scales, whereas geostrophic energy is not. Modelling the geostrophic circulation thus involves making assumptions regarding energy dissipation mechanism. A common assumption is that geostrophic energy is removed through interaction with the bottom boundary layer. Because the sea floor is inaccessible, however, bottom drag coefficients are in large part determined via a tuning exercise. This has led to the search for alternative routes to dissipation for geostrophic energy. In particular, geostrophic-to-near-inertial energy transfers coupled with a forward cascade of near-inertial energy has been proposed. Much of the earlier work along these lines has focused on spontaneous generation of unbalanced motion by an otherwise (geostrophically) balanced flow. This loss of balance, however, is weak and more recent work emphasizes the role of external forcing of the near-inertial modes. Much of the work on this stimulated loss of balance i) focusses on relatively small scales and ii) uses different diagnostics to define or model balance. The work proposed here focuses primarily on the ocean mesoscale (10-100 km) and uses complementary diagnostics to examine the effect of forced near-inertial motion on the balanced flow. The approach allows for a range of projects using theory and models of different complexity. It will provide opportunities for HQP at the undergraduate, MSc and, PhD levels to gain a solid foundation in the theory and modelling of ocean circulation. Because some of the problems treated also have atmospheric counter-parts, this research framework also allows me to attract students whose a priori interest may be more centred on atmospheric rather than oceanic applications. Students will exit my program with a firm rooting in geophysical fluid dynamics and an appreciation of how their work fits into the broader contexts of physical oceanongraphy, meteorology, and climate dynamics.