Impact of symmetric instability parametrization scheme on the upper ocean layer in a high-resolution global ocean model

Abstract

The primary effect of symmetric instability (SI) is the shoaling of the mixed layer. This has been shown to improve the vertical mixing of a passive tracer between the surface mixed layer and the ocean interior in several ideal models. In this study, we apply an SI parameterization scheme on a resolved front in a realistic, high-resolution global ocean model, and evaluate global-scale SI effects. We analyze the model outputs for two typical front regions: the Kuroshio Extension (KE) shows conditions favorable for SI generation whereas in the Subantarctic Front (SAF), east of Australia, SI is weak. We demonstrate that the SI parameterization has a detectable effect on the global-scale sea surface temperature (SST), mixed layer depth (MLD), especially in the strong front and current-topography interactions regions, in winter in the northern and southern hemispheres. The variation of MLD changes by 30% in a local region when SI parameterization scheme is considered. These spatial and temporal distribution characteristics are consistent with those expected for the SI mechanism. However, the SI parameterization only results in submesoscale positive and negative staggered patch structures in the MLD and SST, analogous to turbulence structures. Therefore, its global-scale direct impacts on the MLD and SST are not systematic. The submesoscale disturbance induced by the SI parameterization is of the same order of magnitude as the errors in the current SST observation data; as such, it is assumed to be statistically insignificant in the scope of the current study in a global model. The submesoscale MLD and SST disturbance induced by SI is probably important for local air-sea exchange, but this requires further investigation with the use of long-time-integral coupled models in the future.