Metrics for the Evaluation of the Southern Ocean in Coupled Climate Models and Earth System Models

Abstract

The Southern Ocean is central to the global climate and the global carbon cycle, and to the climate's response to increasing levels of atmospheric greenhouse gases, as it ventilates a large fraction of the global ocean volume. Global coupled climate models and earth system models, however, vary widely in their simulations of the Southern Ocean and its role in, and response to, the ongoing anthropogenic trend. Due to the region's complex water‐mass structure and dynamics, Southern Ocean carbon and heat uptake depend on a combination of winds, eddies, mixing, buoyancy fluxes, and topography. Observationally based metrics are critical for discerning processes and mechanisms, and for validating and comparing climate and earth system models. New observations and understanding have allowed for progress in the creation of observationally based data/model metrics for the Southern Ocean. Metrics presented here provide a means to assess multiple simulations relative to the best available observations and observational products. Climate models that perform better according to these metrics also better simulate the uptake of heat and carbon by the Southern Ocean. This report is not strictly an intercomparison, but rather a distillation of key metrics that can reliably quantify the “accuracy” of a simulation against observed, or at least observable, quantities. One overall goal is to recommend standardization of observationally based benchmarks that the modeling community should aspire to meet in order to reduce uncertainties in climate projections, and especially uncertainties related to oceanic heat and carbon uptake.

CMIP5 model selection for ISMIP6 ice sheet model forcing: Greenland and Antarctica

“…We have noted the unequal performance of coupled climate models over different realms, which we suggest highlights the importance of assessing model fidelity over a range of metrics combining the sub-surface ocean, surface ocean and atmosphere 385 conditions. It also explains why the present ranking of models differs from existing intercomparison studies specifically focused on the atmosphere (Agosta et al, 2015, e.g.,) or the ocean (e.g., Sallée et al, 2013;Meijers et al, 2012;Russell et al, 2018). As Agosta et al (2015) focuses purely on the model performance for ice-sheet surface mass balance, its results differ from this paper evaluating both the ocean and atmospheric metrics for the sake of providing the atmosphere-driven surface mass balance and the ocean-driven melt from the same coupled model as boundary conditions to ice-sheet models.…”

The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric

“…Initial conditions are also adjusted in the optimization to better fit the data. As such, utilizing a data‐assimilating model like B‐SOSE has advantages over a purely forward model and can even be useful for validating the large‐scale processes simulated in forward models (Russell et al, ). For additional information on the model configuration of the 2008–2012 solution and its extensive validation, the reader is encouraged to see Verdy and Mazloff ().…”

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…These OSSEs provided critical information about the number of floats required to reproduce observed patterns of oxygen, DIC, and air-sea CO 2 fluxes. The development of the Biogeochemical Southern Ocean State Estimate (B-SOSE; [151]), a data-assimilating numerical model which seeks to minimize model-observation differences using conserved model dynamics, now provides observationally informed output in a gridded format useful for prognostic model evaluation [152]. Looking ahead, the use of observations in conjunction with models, either through assimilating state estimate models or to validate prognostic models, is a powerful tool for synthesizing existing observations and extrapolating our understanding into the future.…”