Air-sea CO2 flux measurements on the first autonomous circumnavigation of Antarctica

In August 2019, an autonomous surface vehicle making hourly seawater and air CO₂ measurements completed a 196-day, 11,879 nm circumnavigation of Antarctica. The 7 m long, wind-powered saildrone carried an Autonomous Surface Vehicle CO₂ (ASVCO₂), a system utilizing an air-sea equilibration method with infrared CO₂ gas analyzer calibrated in situ with certified CO₂ reference gas, which is based on proven technology used for nearly two decades on moored surface buoys. The mission started in New Zealand moving east, capturing nearly all of the Austral summer and spring seasons as well as the first month of winter conditions. The range of observed ΔpCO₂ (sea-air) was -40 to 60 µatm with a mean of -3 ± 13 µatm. Similar to previous results based on calculated seawater pCO₂ from Biogeochemical Argo float observations, this mission observed less of a CO₂ sink during winter (in this case only July) in the Antarctic Southern Zone compared to previous ship-based estimates. In addition to direct sea-air CO₂ observations, the saildrone also directly measured wind speeds up to 36 m/s, providing a valuable dataset for determining the uncertainty of CO₂ flux calculated using satellite-based wind products. Saildrones have the potential to play an important role filling the more persistent gaps in the ocean observing system given they are not reliant on ships for deployment and recovery, are navigable, and can survive some of the most extreme open ocean conditions.