Adjoint-Based Estimation of Eddy-Induced Tracer Mixing Parameters in the Global Ocean

Abstract

Abstract Using the German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis framework, four separate eddy tracer mixing coefficients are adjusted jointly with external forcing fields, such as to reduce a global misfit between the model simulations and ocean observations over a single 10-yr period and weighted by uncertainties. The suite of the adjusted eddy tracer mixing coefficients includes the vertical diffusivity kz, the along-isopycnal surface diffusivity kredi, the isopycnal layer thickness diffusivity kgm, and the along-iso-thickness advection coefficient kgmskew. Large and geographically varying adjustments are found in all four parameters, which all together lead to an additional 10% reduction of the total cost function, as compared to using only surface flux parameters. However, their relative contribution to the cost reduction varies from 1% to 50% among the four coefficients, with the adjusted kgm contributing most. Regionally, the estimated kgm ranges from less than −800 to about 2500 m2 s−1. Largest adjustments in kgm reside in the vicinity of large isopycnal slopes and support a mixing length hypothesis; they also likewise support the hypothesis of a critical layer enhancement and high potential density gradient suppression. In a few occasions, resulting negative net kgm values can be found in the core of main currents, suggesting the potential for an inverse energy cascade transfer there. Large adjustments of kredi and kgmskew are found in the vicinity of isopycnal slopes. The adjustments of kz in the tropical thermoclines suggest deficiencies of the mixed layer parameterization.