| The property ALK-DIC, the concentration difference of alkalinity and total dissolved
inorganic carbon in seawater has recently been used as a metric for the data-based evaluation
of ocean climate and carbon cycle models. The rational behind this is that ALK-DIC is
thought to serve as a good approximation of the carbonate ion concentration, [CO32-]. Since
[CO32-] is inversely related to the Revelle Factor, it is considered a good proxy of the buffer
capacity of seawater and hence the chemical capacity of the ocean to take up atmospheric
CO2. ALK-DIC has hence been suggested as an easy to calculate metric to judge a models
chemical potential to take up atmospheric CO2 in comparison with the real ocean. Since
[CO32-] is also an important property controlling the dissolution of CaCO3, computing
ALK-DIC in the deep ocean might also help to evaluate CaCO3 cycle modules in ocean
biogeochemical models.
In this study, the distribution of ALK-DIC and [CO32-] in observations of the ocean
(GLODAP-data) and ocean model output available from six models contributing to the
Climate Model Intercomparison Project (CMIP5) is compared and the potential of ALK-DIC
to serve as a suitable proxy of [CO32-] in the context of data-based model evaluation is
explored.
In most of the surface ocean (GLODAP observational dataset) ALK-DIC is by 35-45%
larger than [CO32-] with the important exception of the Southern Ocean where the
difference is smaller. This difference decreases further in the interior ocean and can even turn
sign, for example in oxygen minimum zones where [CO32-] becomes slightly
large than ALK-DIC. While overall patterns of the evaluated CMIP5 models are
similar to observations, distinct differences are observed. For example, minimum
ALK-DIC in observations is usually close to zero while extremely negative values are
observed in some of the CMIP5 models (e.g. -156 umol/kg in IPSL-CM5A-LR,
and up to -418, -426, and -450 umol/kg in GFDL-ESM2M, MPI-ESM-LR and
NorESM1-ME, respectively). Possible causes are discussed and the incomplete
implementation of processes affecting ocean alkalinity is suggested as the most likely
cause. |