Dependence of Near-Inertial Variance in Sea Surface Height on Different Representations of Ekman Layer Dynamics

Abstract:

Balanced and unbalanced contributions to sea surface height are analysed in numerical simulations for which high-frequency forcing adds near-inertial motion to geostrophic turbulence. In this study, we include in our definition of balance not only geostrophic modes but also slowly varying Ekman and nonlinear Ekman flows. Similarly, imbalance includes both inertia-gravity modes and high-frequency vortical modes. To examine how this picture impacts the interpretation of sea surface height (SSH), we first employ (without offering compensation or a benefit package) a two-layer model with a slab ocean embedded in the upper layer. The slab layer dynamics include strong near-inertial oscillations and nonlinear corrections to the Ekman flow. We find that how near-inertial variance in SSH is partitioned between different dynamical modes is strongly dependent on details of the slab layer dynamics. This motivates testing how different representations of the upper ocean frictional layer influences sea surface height in high-resolution simulations using the MITgcm. For example, we vary vertical resolution and whether or not ocean surface velocity is included in the wind stress formulation, which is done for a range of flow regimes and model topographies.