Modeling Oceanic Carbon Uptake and Storage
Long, M.C., Lindsay, K., Peacock, S., Moore, J.K. and Doney, S.C. 2013. Twentieth-century oceanic carbon uptake and storage in CESM1(BGC). Journal of Climate 26: 6775-6800.
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 1,000 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.
In describing their findings the five researchers report that (1) "modeled ΔpCO2 [= pCO2seawater - pCO2atmosphere] 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 pCO2seawater is substantially lower in the model than in observations," and that (8) "the model predicts stronger seasonality, with much lower austral summer December-February pCO2seawater 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 pCO2seawater 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 pCO2seawater 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  "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).
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."
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