Background
The authors indicate that over the last two centuries the world's oceans have absorbed 25-30% of the total amount of CO2 emitted to the atmosphere by fossil-fuel burning, cement production and land-use changes, citing Sabine et al. (2004) and Le Quere et al. (2009); and they say that this oceanic carbon sink has partially mitigated the CO2-induced warming of the globe by slowing the rate-of-rise in the air's CO2 content. Hence, they state that "mechanistic representation of oceanic carbon uptake and storage is essential to robust climate prediction."

What was done
Working towards this end, Long et al. compared ocean carbon uptake and storage - as simulated by the Community Earth System Model, version 1-Biogeochemistry [CESM1(BGC)] - where ocean and ice component models were forced by atmospheric observations and reanalyses, and where biogeochemical fields were initialized using data-based climatologies, after which the fully coupled model was integrated for a period of 1000 years, in order to allow the deep ocean to approach equilibrium. In this paper, therefore, Long et al. examined two 20th-century simulations branched off this steady-state run after 150 years of integration.

What was learned
The five researchers report that (1) "modeled ΔpCO2 [= pCO2^seawater - pCO2^atmosphere] is larger than observed in the eastern equatorial Pacific and over much of the Southern Ocean north of about 60°S," and that (2) "the term ΔpCO2 is under-estimated, by contrast, in the polar Southern Ocean." In this region, in fact, they say that (3) "the model predicts ΔpCO2 values of the opposite sign" than what is actually observed there, as per Takahashi et al. (2009). They also report that (4) "modeled salinity-normalized surface dissolved inorganic carbon and alkalinity concentrations tend to be too low over much of the ocean." In fact, they say that (5) "salinity-normalized surface alkalinity is underestimated virtually everywhere," although (6) alkalinity is "over-estimated at depth."

Long et al. additionally note that (7) "in the polar Southern Ocean, annual-mean pCO2^seawater is substantially lower in the model than in observations," and that (8) "the model predicts stronger seasonality, with much lower austral summer December-February pCO2^seawater values than in the Takahashi et al. (2009) climatology." And they also indicate that (9) "summertime mixed layer depths along the Antarctic Circumpolar Current are too shallow in the model by 20-50 meters."

In the North Atlantic (49-80°N), on the other hand, the five U.S. scientists state that the CESM1 predicts an annual-mean pCO2^seawater that is comparable to observations. However, they report that (10) "the amplitude of the seasonal cycle in the model is ~5-fold larger." In addition, they note that (11) "high chlorophyll biases in this region indicate that the magnitude of the simulated Arctic phytoplankton bloom is too strong," citing Moore et al. (2013). And they note that "while the amplitude of the high-latitude seasonal cycle in pCO2^seawater is generally much larger than inter-annual variability, (12) "the opposite is true in the tropics."

The authors also write that (13) "contemporary CO2 uptake is weaker than ΔpCO2-based flux estimates between about 40 and 55°S," whereas [14] "south of 60°S the models show stronger uptake." And they state that (15) "Southern Hemisphere sea ice coverage is far too extensive," as currently modeled, and that (16) "the total sea ice area is consistently about 62% greater than satellite observations over the seasonal cycle," citing Landrum et al. (2012), which they say (17) "is likely due to westerly winds in the coupled model that are stronger than observed," citing Danabasoglu et al. (2012).

What it means
Long et al.'s final conclusion is that "substantial improvements in the physical parameterizations controlling mixing and overturning in the model are necessary to improve the representation of ventilation," and that presently lacking such, "the current CESM configuration can be expected to continue to underestimate Cant [anthropogenic CO2] uptake under twenty-first-century scenarios."

References
Danabasoglu, G., Bates, S., Briegleb, B.P., Jayne, S.R., Jochum, M., Large, W.G., Peacock, S. and Yeager, S.G. 2012. The CCSM4 ocean component. Journal of Climate 25: 1361-1389.

Landrum, L., Holland, M.M., Schneider, D.P. and Hunke, E. 2012. Antarctic sea ice climatology, variability, and late twentieth-century change in CCSM4. Journal of Climate 25: 4817-4838.

Le Quere, C., Raupach, M.R., Canadell, J.G., Marland, G., Bopp, L., Ciais, P., Conway, T.J., Doney, S.C., Feely, R.A., Foster, P., Friedlingstein, P., Gurney, K., Houghton, R.A., House, J.I., Huntingford, C., Levy, P.E., Lomas, M.R., Majkut, J., Metzl, N., Ometto, J.P., Peters. G.P., Prentice, I.C., Randerson, J.T., Running, S.W., Sarmiento, J.L., Schuster, U., Sitch, S., Takahashi, T., Viovy, N., van der Werf, G.R. and Woodward, F.I. 2009. Trends in the sources and sinks of carbon dioxide. Nature Geoscience 2: 831-836.

Moore, J.K., Lindsay, K., Doney, S.C., Long, M.C. and Misumi, K. 2013. Marine ecosystem dynamics and biogeochemical cycling in the Community Earth System Model [CESM1(BGC)]: Comparison of the 1990s with the 2090s under the RCP 4.5 and RCP 8.5 scenarios. Journal of Climate 26: 6775-6800.

Sabine, C.A., Feely, R.A., Gruber, N., Key, R.M., Lee, K., Bullister, J.L., Wanninkhof, R., Wong, C.S., Wallace, D.W.R., Tilbrook, R., Millero, F.J., Peng, T.-H., Kozvr, A., Ono, T. and Rios, A.F. 2004. The oceanic sink for anthropogenic CO2. Science 305: 367-371.

Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C, Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, A., Bakker, D.C.E, Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R. and de Baar, H.J.W. 2009. Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans. Deep-Sea Research II 56: 554-577.