Inconsistency between ship- and Argo float-based pCO2 at the intense upwelling region of the Drake Passage, Southern Ocean

Abstract

The Southern Ocean absorbs a quarter of anthropogenic carbon dioxide (CO2) from the atmosphere to modulate the climate system. However, less attention has been paid to the CO2 outgassing phenomenon at the Antarctic Circumpolar Current (ACC) region of the Southern Ocean due to strong upwelling. Recent studies using autonomous biogeochemical-Argo float revealed a greater winter CO2 outgassing than previously estimated at ACC zone of the Southern Ocean, which, however, remains controversial and urgently needs to be validated. Here we take the Drake Passage as a case study to present new insights into the Southern Ocean carbon cycle and examine the validity of float-based CO2 outgassing. Upon integrating the ship-based data over the past two decades, we investigate the spatiotemporal variability of sea surface CO2 partial pressure (pCO2) in Drake Passage. We show that Drake Passage is acting as a year-round weak CO2 sink, although some CO2 uptake is counteracted by winter CO2 outgassing. The float-based pCO2 values are overall higher than ship-based values in winter, by 6 to 20 µatm (averaged 14 µatm) at the most intensive upwelling region. We then develop a surface carbon balance calculation (considering mixing between surface, subsurface, and upwelled waters) to estimate the potential of surface pCO2 increase due to upwelling, and we find that upwelling of CO2-rich subsurface waters in Drake Passage cannot support an excess ΔpCO2 of 14 µatm as suggested by float detections. We further compare our results to previous study and find that, although we used same datasets and obtained comparable results, the way to conclude the bias in float-based pCO2 would cause significant difference: an uncertainty of ±2.7% (i.e., ± 11 µatm) in float-based pCO2 estimated by other study seems acceptable, however, it is five times larger than the typical ship-based pCO2 uncertainty ( ± 2 µatm), and would cause ~180% bias in CO2 flux estimates. Going forward, there is special need for caution when interpreting the float-based CO2 flux; meanwhile, further comparisons and corrections between float- and ship-based pCO2 are clearly warranted.

Subantarctic pCO2 estimated from a biogeochemical float: comparison with moored observations reinforces the importance of spatial and temporal variability

“…Extending the comparison to the first 10 profiles, which were within 1˚latitude from the mooring, pCO 2 PF estimates were about 6.0 μtam (± 11.82) higher than mooring observations within 6h of each profile. This agrees with previous reports (using various methods) of a bias in float pCO 2 estimates of about 4 -8 μatm higher (Gray et al, 2018;Williams et al, 2018;Bushinsky et al, 2019;Long et al, 2021;Wu and Qi, 2022). It is also important to note that Maurer et al (2021) stated that float pH measurements adjusted according to the method described in their paper (and applied in this study) are accurate to approximately 0.007 (based on validation efforts which compared adjusted float pH to shipboard data taken alongside deployment).…”

Carbon Outgassing in the Antarctic Circumpolar Current Is Supported by Ekman Transport From the Sea Ice Zone in an Observation‐Based Seasonal Mixed‐Layer Budget

“…However, the uncertainty of the ocean carbon sink is highest in the Southern Ocean (Fay & McKinley, 2021; Friedlingstein et al., 2022; Gloege et al., 2021) and the discrepancy between models and observations is also most significant there compared to other ocean basins (Hauck et al., 2020). There is also discrepancy between observational estimates of the Southern Ocean carbon sink, or the carbonate parameters necessary to determine it, based on the method or data set used (Bushinsky & Cerovečki, 2023; Coggins et al., 2023; Long et al., 2021; Mackay & Watson, 2021; Wu et al., 2022; Wu & Qi, 2022). Further, sampling alias linked to the spatial distribution of available observations has been shown to be significant (Hauck et al., 2023).…”

Carbon outgassing in the Antarctic Circumpolar Current is supported by Ekman transport from the sea ice zone in an observation-based seasonal mixed-layer budget

“…However, the uncertainty of the ocean carbon sink is highest in the Southern Ocean Friedlingstein et al, 2022;Gloege et al, 2021) and the discrepancy between models and observations is also most significant there compared to other ocean basins (Hauck et al, 2020). There is also discrepancy between observational estimates of the Southern Ocean carbon sink, or the carbonate parameters necessary to determine it, based on the method or data set used (Bushinsky & Cerovečki, 2023;Coggins et al, 2023;Long et al, 2021;Mackay & Watson, 2021;Wu & Qi, 2022). Further, sampling alias linked to the spatial distribution of available observations has been shown to be significant (Hauck et al, 2023).…”