Constraining Southern Ocean CO₂ Flux Uncertainty Using Uncrewed Surface Vehicle Observations

Abstract

Remote, harsh conditions of the Southern Ocean challenge our ability to observe the region’s influence on the climate system. Southern Ocean air‐sea CO2 flux estimates have significant uncertainty due to the reliance on limited ship‐dependent observations in combination with satellite‐based and interpolated data products. We utilize a new approach, making direct measurements of air‐sea CO2, wind speed, and surface ocean properties on an Uncrewed Surface Vehicle (USV). In 2019, the USV completed the first autonomous circumnavigation of Antarctica providing hourly CO2 flux estimates. Using this unique data set to constrain potential error in different measurements and propagate those through the CO2 flux calculation, we find that different wind speed products and sampling frequencies have the largest impact on CO2 flux estimates with biases that range from −4% to +20%. These biases and poorly constrained interannual variability could account for discrepancies between different approaches to estimating Southern Ocean CO2 uptake.

Observational Evidence of Ventilation Hotspots in the Southern Ocean

“…These float-based estimates show that the regions around the Polar Front exhibit significant outgassing during autumn and winter, likely associated with the large-scale upwelling of old, carbon-rich waters (Gray et al, 2018). The development and deployment of unmanned surface vehicles has further expanded the collection of ΔpCO 2 concentrations across the atmosphere and surface ocean (Sutton et al, 2021). However, the role of localized regions of enhanced EKE has not yet been diagnosed using such methods, in terms of the effects on air-sea exchange of CO 2 .…”

The sensitivity of pCO<sub>2</sub> reconstructions in the Southern Ocean to sampling scales: a semi-idealized model sampling and reconstruction approach

“…Bushinsky et al, 2019). Over the last decade, the advent of a range of new autonomous ocean observing platforms has opened doors towards closing the seasonal and intra-seasonal sampling biases created by the high cost of ship operations in the Southern Ocean outside the summer window (Majkut et al, 2014;Monteiro et al, 2015;Williams et al, 2017;Gray et al, 2018;Bushinsky et al, 2019;Sutton et al, 2021).…”

“…Thus resolving the mean seasonal cycle through in situ observations not only is a challenging exercise that has followed several avenues from extrapolating findings from the Drake Passage Time-series (DPT) like in Fay et al (2018), to utilizing measurements from extended deployments of autonomous ocean observing platforms such as Wavegliders (Monteiro et al, 2015), biogeochemical Argo floats (Williams et al, 2017;Gray et al, 2018;Bushinsky et al, 2019), and more recently Saildrones (Sutton et al, 2021). These advances have allowed the density of the Southern Ocean surface CO2 observing networks to increase.…”

“…These advances have allowed the density of the Southern Ocean surface CO2 observing networks to increase. Consequently, the problem of general sparseness in observations and particularly the sampling biases (Monteiro et al, 2015;Gloege et al, 2021) was partially addressed but not resolved by the ocean CO2 in-situ observations community (Bushinsky et al, 2019;Sutton et al, 2021). For example, under-sampling in winter by ships has been addressed by the 10-day resolution SOCCOM profiling floats and/or Lagrangian platforms that are carried zonally by water currents (Majkut et al, 2014;Williams et al, 2017;Gray et al, 2018;Bushinsky et al, 2019).…”

“…Notwithstanding these new platforms, sparse and scale sensitive observations in the Southern Ocean continue to be a barrier in constraining the seasonal cycle and inter-annual variability of surface ocean 𝑝CO ! (Monteiro et al, 2015;Rödenbeck et al, 2015;Sutton et al, 2021).…”