Analysis of water mass transformations and the spurious thermohaline overturning circulation in numerical ocean models

Abstract

Numerical models have become an important tool for investigating the oceans heat and salt contents. The large-scale thermohaline overturning circulation in the world ocean is directly linked to the transformation of water masses caused by small-scale diapycnal mixing, which is parameterized in models. However, in addition to this physically justified "physical mixing", numerical transport schemes rely on additional "numerical mixing" for stability reasons. Thus, the simulated overturning circulation in ocean models is strongly affected by this spurious mixing.Diagnostics of spurious mixing in terms of local tracer variance decay offer a detailed analysis of water mass transformations (WMT). Vice versa, analysis methods for WMT can be used to deduce information about the effects of mixing. In contrast to direct mixing diagnostics based on discrete variance decay (DVD) in geographical space, the WMT analysis framework is based on a mapping to tracer space, where diatracer fluxes that quantify the WMT can directly be diagnosed. Recently, a new local framework was derived, which combines the classical WMT framework with the local DVD analysis (Klingbeil & Henell, 2023). The derived analytical relations between dia-surface fluxes and mixing were demonstrated in an isohaline framework by Henell et al. (2023) [see corresponding submission to this session].We will present how this methodology can be transferred to the world ocean in order to diagnose local diapycnal mixing and to quantify the spurious contribution to the simulated thermohaline overturning circulation in ocean models. In particular, the extension to density space requires the consistent quantification of density DVD, which is challenging in numerical models with prognostic equations for salinity and temperature and a non-linear equation of state.   Henell, E., H. Burchard, U. Gräwe, K. Klingbeil (2023) Spatial composition of the diahaline overturning circulation in a fjord-type, non-tidal estuarine system. Journal of Geophysical Research (Oceans). https://dx.doi.org/10.1029/2023JC019862. Klingbeil, K. and E. Henell (2023) A Rigorous Derivation of the Water Mass Transformation Framework, the Relation between Mixing and Diasurface Exchange Flow, and Links to Recent Theories in Estuarine Research. Journal of Physical Oceanography. https://doi.org/10.1175/JPO-D-23-0130.1.