Revisiting Winter Southern Ocean CO2 Uptake Based on CALIPSO Observations.

The absorption of atmospheric carbon dioxide (CO2) in the Southern Ocean represents a critical component of the global oceanic carbon budget. Previous assessments of air‐sea carbon flux variations and long‐term trends in polar regions during winter have faced limitations due to scarce field data and the lack of ocean color satellite imagery, causing uncertainties in estimating CO2 flux estimation. This study utilized the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite to construct a continuous 16‐year (2006–2021) time series of sea surface partial pressure of CO2 (pCO2) in the Southern Ocean. Our findings revealed that the polar region in South Ocean acts as a carbon sink in winter, with CO2 flux of ∼30 TgC in high‐latitude areas (South of 50°S). This work highlights the efficacy of active remote sensing for monitoring sea surface pCO2 and contributes insights into the dynamic carbonate systems of the Southern Ocean. Plain Language Summary: Climate change data from recent decades have consistently shown an increase in atmospheric CO2 concentration. The Southern Ocean, a major carbon sink, is critical in this regard. However, limitations in ocean color remote sensing and infrequent sampling hinder a complete understanding of carbon uptake in high‐latitude regions during winter. Previous reconstructions inadequately considered the biological effect on the air‐sea CO2 exchange process in winter. This study used observations from an active remote sensing satellite to represent the biological effects of CO2 and construct a long‐term time series of sea surface CO2 level for the Southern Ocean. Additionally, the study reassessed the CO2 uptake capacity of the Southern Ocean in winter. These findings suggest that previous estimates may underestimate the CO2 uptake capacity in the high‐latitude regions during winter, potentially due to underestimations of biological effects. This research underscores the value of active remote sensing for obtaining critical biogeochemical parameters in high‐latitude oceans, providing an essential tool for monitoring carbonate systems. Key Points: A new method was proposed to reconstruct pressure of CO2 (pCO2) using CALIPSO‐derived bbp dataA 16‐year Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation observation‐based pCO2 product was constructedThe CO2 uptake capacity in the Southern Ocean during winter was estimated [ABSTRACT FROM AUTHOR]