Storm‐Driven pCO2 Feedback Weakens the Response of Air‐Sea CO2 Fluxes in the Sub‐Antarctic Southern Ocean.

The sub‐seasonal CO2 flux (FCO2) variability across the Southern Ocean is poorly understood due to sparse observations at the required temporal and spatial scales. Twinned surface and profiling gliders experiments were used to investigate how storms influence FCO2 through the air‐sea gradient in partial pressure of CO2 (ΔpCO2) in the sub‐Antarctic zone. Winter‐spring storms caused ΔpCO2 to weaken (by 22–37 μatm) due to mixing/entrainment and weaker stratification. This weakening in ΔpCO2 was in phase with the increase in wind stress resulting in a reduction of the storm‐driven CO2 uptake by 6%–27%. During summer, stronger stratification explained the weaker sensitivity of ΔpCO2 to storms, instead temperature changes dominated the ΔpCO2 variability. These results highlight the importance of observing synoptic‐scale variability in ΔpCO2, the absence of which may propagate significant biases to the mean annual FCO2 estimates from large‐scale observing programmes and reconstructions. Plain Language Summary: The sub‐Antarctic zone of the Southern Ocean is a region that mostly experiences carbon dioxide (CO2) uptake because of its low temperature, strong winds and lower CO2 content. The wind can influence the CO2 uptake through two pathways: the speed of CO2 transfer between the air‐sea interface (kw) and the difference in CO2 concentration in the surface ocean and overlying atmosphere (ΔpCO2). Using autonomous robots that can measure hourly air and water conditions simultaneously, we show that not resolving ΔpCO2 during a storm event can lead to overestimating the CO2 uptake. This is particularly important during winter and spring when the ocean's surface layers are less stratified. The warmer temperatures during summer meant a more stratified surface layer resulting in a weaker and delayed impact of storms on the ΔpCO2. This study shows that the various annual CO2 uptake estimation methods used by the research community should not neglect ΔpCO2 responses during storms. Key Points: Hourly glider observations show that the impact of storms on both kw and ΔpCO2 simultaneously modulates the ocean CO2 uptake variabilityWinter‐spring storms weaken ΔpCO2 through enhanced entrainment and mixing, partially counteracting the increase in CO2 uptake due to kw aloneBy not accounting for the storm‐linked positive feedback in ΔpCO2, the cumulative seasonal CO2 uptake was found to be overestimated by ∼6% [ABSTRACT FROM AUTHOR]