Evaluation of biogeochemical float pCO2 estimates and empirical approaches to estimate total alkalinity in the Southern Ocean using carbonate system observations from the Drake Passage

Biogeochemical profiling floats have provided new estimates of air-sea carbon dioxide (CO₂) flux in the Southern Ocean that differ significantly from approaches based on ship-board observations, particularly within the Antarctic Circumpolar Current (ACC). Estimates of the surface partial pressure of carbon dioxide (pCO₂) from biogeochemical floats are based on float observations of pH and total alkalinity (TA) estimated from empirical algorithms. However, the hydrographic data used to develop the TA algorithms in the Southern Ocean are mostly collected during austral spring and summer due to logistical constraints. The nearly two-decade Drake Passage Time-series (DPT) includes year-round carbonate system observations that span all ACC fronts including the Antarctic Polar Front (APF). Here, we utilize DPT carbonate system observations to evaluate current approaches used to estimate pCO₂ from biogeochemical float observations in the Southern Ocean by 1) examining comparisons of ship-board pCO₂ measurements and biogeochemical float pCO₂ estimates in the Drake Passage and 2) comparing TA estimates from empirical algorithms with TA values based on in situ carbonate system measurements. DPT TA values were calculated from underway pCO₂ and discrete measurements of total CO₂ and not directly measured TA, though the propagated uncertainty using this combination should be approximately four μmol kg^-1. We find large systematic discrepancies during winter south of the APF between TA based on DPT carbonate system measurements and TA estimated from empirical algorithms. These discrepancies are as large as 25 μmol kg^-1, which translates to systematic bias in biogeochemical float pCO₂ estimates on the order of 5 μatm. This bias has the effect of exaggerating the amplitude of the seasonal cycle of pCO₂ and leads to enhanced net flux of CO₂ into the atmosphere during the wintertime, since pCO₂ is higher than the atmosphere due to deep wintertime mixing with high CO₂ waters.