Hidden vortices: near-equatorial low-oxygen extremes driven by high-baroclinic-mode vortices

Abstract. Long-term time series of dissolved oxygen (DO) measurements from the upper 500 m of the eastern tropical North Atlantic (ETNA), collected over a period of up to 15 years at three different mooring sites, reveal recurring extreme low-oxygen events lasting for several weeks. Similarly, observations from 15 individual meridional ship sections between 6 and 12° N along 23° W show DO concentrations far below 60 µmol kg−1 in the upper 200 m – significantly lower than the climatological values in this depth range (>80 µmol kg−1). Two-third of these low-oxygen events could be related with high-baroclinic-mode vorticies (HBVs) with their cores located well below the mixed layer. Despite the energetic equatorial circulation and the expected dominance of wave-like structures in the near-equatorial region, these HBVs persist as relatively long-lived and coherent features. Based on moored and shipboard observations from the ETNA, and supported by an eddy-resolving ocean-biogeochemistry model, we characterize their dynamics and DO distribution. Observed water mass properties and model analyses suggest that most HBVs originate from the eastern boundary and can persist for more than six months. As they propagate westward into regions of higher potential vorticity (PV), anticyclonic HBVs with low-PV cores remain more effectively isolated and have longer lifespans compared to cyclonic HBVs with high-PV core. The vertical structure of the dominant anticyclonic HBVs corresponds to baroclinic modes 4–10, with associated Rossby radii ranging from 34 to 13 km, respectively. This is consistent with observed eddy sizes and is well below the corresponding 1st baroclinic Rossby radius of deformation (>100 km). Since none of the observed HBVs exhibit a surface signature, a substantial portion of the near-equatorial eddy field may remain undetected by satellites, yet still exert significant influence on local ocean ecosystems and biogeochemical cycles.