Net Global Enhancement of Surface Chlorophyll by Oceanic Fine-scale Eddies

Abstract Fine-scale oceanic eddies (FEs), with horizontal scale smaller than the Rossby deformation radius, cover the variability at submesoscale and small-mesoscale wavelengths. They serve as critical drivers for vertical transport that helps close the nutrient budget and sustain observed levels of primary production. However, the fine-scale eddies have never been directly observed on a global scale prior to the launch of the Surface Water and Ocean Topography (SWOT) satellite mission. Consequently, their global contribution to surface chlorophyll variability has remained unknown. By combining SWOT observations with multi-platform datasets, we show that FEs are ubiquitous throughout the global ocean and are roughly an order of magnitude more abundant than large mesoscale eddies observed by traditional altimetry missions. In contrast to mesoscale eddies, fine-scale cyclones are systematically stronger than anticyclones by approximately 10% on average. Globally composited results reveal that the magnitude of positive surface chlorophyll anomalies driven by cyclonic fine-scale eddies exceeds the magnitude of negative chlorophyll anomalies of anticyclones by more than twofold, producing a consistent net positive effect on surface chlorophyll concentrations across the global oceans, which is driven by the combined effect of horizontal and vertical motions of fine-scale eddies. Our results provide the first observational benchmark for characterizing FEs and their biogeochemical influence, offering critical constraints for future coupled physical-biogeochemical models and helping to reduce longstanding uncertainties.