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


Russell, Joellen L.
Kamenkovich, Igor
Bitz, Cecilia
Ferrari, Raffaele
Gille, Sarah T.; Goodman, Paul J.; Hallberg, Robert; Johnson, Kenneth; Khazmutdinova, Karina; Marinov, Irina; Mazloff, Matthew; Riser, Stephen; Sarmiento, Jorge L.; Speer, Kevin; Talley, Lynne D. and Wanninkhof; Rik

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.

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Figure 7: Annual Mean CO2 flux (sea to air, gC/m2/yr, positive (red) is out of the ocean). The observed fluxes are taken from Takahashi et al. (2009) climatology for reference year 1995, available through CDIAC, with a fairly coarse 4°-by-5° resolution. Model output for years 1986-2005 from the HISTORICAL forcing scenario. Panel A) Takahashi climatology (from NCEI); B) B-SOSE; C) CanESM2; D) GFDL-ESM2M; E) HadGEM2-ES; F) MIROC-ESM; and G) MRI-ESM1. Panel H) is the zonal mean of each of the datasets (in gC/m2/yr); and panel I) is the cumulative integral of the net CO2 flux from 90°S to 30°S (in PgC/yr). The CanESM2 simulation has an order-of-magnitude more uptake and outgassing than the other simulations. The location of the Subantarctic Front (as seen in Figure 3) from the WOA13 is shown in magenta in Panel A.

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J. Geophys. Res. Oceans. Accepted Author Manuscript. doi:10.1002/2017JC013461