A Storm-driven Reversal of the Springtime North Atlantic CO₂ Sink

Abstract. The North Atlantic Ocean is a major sink for atmospheric carbon dioxide (CO₂), particularly in spring when phytoplankton blooms intensify CO₂ uptake. Its magnitude and variability affect how much anthropogenic CO₂ is sequestered. Yet short-term variability in this sink remains poorly constrained. Energetic events such as storms are rarely sampled, leaving their contribution to air-sea CO₂ exchange unresolved. Here we report a reversal of this sink during spring 2023, captured by validated measurements aboard a racing sailboat crossing a storm. Surface ocean CO₂ increased abruptly, reaching on average 34 µatm above atmospheric levels (magnitude comparable to the ~40 µatm undersaturation that typically sustains spring uptake) as strong winds drove vertical mixing and Ekman suction that ventilated an unusually shallow carbon-rich subsurface reservoir. The anomaly was further amplified by a concurrent marine heatwave. Our observations provide evidence that storms can ventilate subsurface carbon reservoirs and temporarily reverse CO₂ fluxes even during peak biological drawdown. They reveal a mode of short-lived but intense carbon outgassing that is unresolved in coarse resolution models. As storms and marine heatwaves intensify under climate change, such events will become an increasingly important component of the ocean carbon cycle and may weaken the North Atlantic CO₂ sink.