Using the German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis framework,<br />four separate eddy tracer mixing coefficients are adjusted jointly with external forcing fields, such as to reduce<br />a global misfit between the model simulations and ocean observations over a single 10-yr period and weighted<br />by uncertainties. The suite of the adjusted eddy tracer mixing coefficients includes the vertical diffusivity kz,<br />the along-isopycnal surface diffusivity kredi, the isopycnal layer thickness diffusivity kgm, and the along-isothickness<br />advection coefficient kgmskew. Large and geographically varying adjustments are found in all four<br />parameters, which all together lead to an additional 10% reduction of the total cost function, as compared to<br />using only surface flux parameters. However, their relative contribution to the cost reduction varies from 1%<br />to 50% among the four coefficients, with the adjusted kgm contributing most. Regionally, the estimated kgm<br />ranges from less than 2800 to about 2500 m2 s21. Largest adjustments in kgm reside in the vicinity of large<br />isopycnal slopes and support a mixing length hypothesis; they also likewise support the hypothesis of a critical<br />layer enhancement and high potential density gradient suppression. In a few occasions, resulting negative net<br />kgm values can be found in the core of main currents, suggesting the potential for an inverse energy cascade<br />transfer there. Large adjustments of kredi and kgmskew are found in the vicinity of isopycnal slopes. The adjustments<br />of kz in the tropical thermoclines suggest deficiencies of the mixed layer parameterization.

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institute of oceanography, University of Hamburg