Tracer Evolution in the Oceanic Mixed Layer

Abstract

Ocean tracers such as carbon dioxide, nutrients, and plankton evolve primarily in the oceanic mixed layer where air-sea gas exchange occurs and light is plentiful for photosynthesis. It is well known from prior observational and computational studies that there can be substantial heterogeneity, or patchiness, in the spatial distribution of ocean tracers due to both vertical and horizontal turbulent mixing across a wide range of scales. The contribution of submesoscale turbulent processes to these distributions is not completely understood, however, particularly in the sub-kilometer range. Within this range, both large-scale, quasi-geostrophic eddies and small-scale, three-dimensional turbulence are active, resulting in substantial dynamical complexity from which tracer heterogeneity can arise. In this talk, results from large eddy simulations of the evolution of a large scale temperature front are used to examine the role of multi-scale turbulent mixing on distributions of idealized ocean tracers from scales of 20km down to 5m. The simulations include the effects of wave-driven Langmuir turbulence by solving the wave-averaged Boussinesq equations with an imposed Stokes drift velocity. Tracers with different source, initial, and boundary conditions are examined in order to understand the respective roles of both small-scale, near-surface vertical mixing and larger-scale upwelling motions typically associated with submesoscale eddies. Tracer evolution is characterized using spatial concentration distributions, multi-scale fluxes, and spectra, and the implications of the results for computational modeling of air-sea gas exchange and upper-ocean tracer transport are outlined.

KATHERINE M. SMITH

Department of Mechanical Engineering, University of Colorado Boulder