Carbon dioxide and ocean acidification based on temperature
From Sabine, et al., 2004:


  The capacity for ocean waters to take up anthropogenic CO2 from the
  atmosphere is inversely proportional to the value of the Revelle
  Factor, hence the lower the Revelle Factor, the higher the oceanic
  equilibrium  concentration of anthropogenic CO2 for a given
  atmospheric CO2 perturbation.


The Revelle Factor is, "the ratio of instantaneous change in carbon dioxide to the change in total dissolved inorganic carbon (DIC)"

Therefore, the controlling factor for CO$_2$ absorption in seawater is presence of DIC and buffering. If you add CO$_2$ and a relatively small portion of it becomes other DIC species such as HCO$_3^{-}$ or CO$_3^{2-}$, then you have a low Revelle factor and high uptake of atmospheric CO$_2$. Alkaline waters (lower pH) can absorb more CO$_2$ because they have less of the acidic CO$_3^{2-}$ and neutral buffering HCO$_3^{-}$.