Modeled Impacts of Sea Ice Exchange Processes on Arctic Ocean Carbon Uptake and Acidification (1980-2015)

A regional Arctic ice ocean model incorporating biogeochemical processes occurring inside the sea ice and water column is used to assess changes to the Arctic Ocean's carbon system, including oceanic carbon uptake and ocean acidification, over the recent period of Arctic sea ice decline (1980-2015). Two novel modifications are the following: (1) incorporation of carbon uptake by sea ice algae and (2) modification of the sea ice carbon pump to allow for vertical transport by brine plumes with high concentrations of dissolved inorganic carbon (DIC) and total alkalinity (TA) to the bottom of the mixed layer. The simulated carbon uptake north of 66.5 degrees N increases by 20%, from 110 to 135 Tg C yr(-1)from 1980-2015, and the mean pan-Arctic sea surface pH decreases from 8.1 to 8.0. There is substantial regional and seasonal variability, highlighting potential problems with interpolating sparse measurements. Two sensitivity studies assess the effects of modifications to the near-surface carbonate system. First, excluding the sea ice carbon pump results in a marked decrease in seasonal variability of pan-Arctic-mean sea surface DIC (similar to 25% less than the standard run) and TA (similar to 10% less), suggesting that neglecting the ice carbon pump results in models overestimating the saturation state in summer. Second, neglecting the sea ice algae results in an underestimation of the Arctic Ocean's annual carbon uptake (similar to 3% per year), indicating that the accumulating year-on-year underestimation of annual carbon uptake, as a result of neglecting sea ice algae, would lead to increasing error over multidecadal runs of polar ocean models.