Evolution of biogeochemical Properties Inside Poleward Undercurrent Eddies in the Southeast Pacific Ocean

Abstract. Oceanic eddies are ubiquitous features of the circulation through to be involved in transporting water mass properties over long distances from their source region. Among these is a particular type with a core within the thermocline with little signature visible from space. Despite their significance, their role in the ocean circulation remains largely undocumented from observations. This study characterizes the variations in internal biogeochemistry, disparities with external properties, and processes influencing the dissolved oxygen budget of Poleward undercurrent eddies (PUDDIES) during their transit to oceanic waters. Employing a high-resolution coupled simulation of the Southeast Pacific, we scrutinize eddy dynamics and biogeochemical processes associated with the nitrogen cycle, including characteristic mechanisms of Eastern Boundary Upwelling Systems (EBUS) such as denitrification. Our findings reveal that Puddies capture a biogeochemical signal contingent upon their formation location, particularly associated with the core of the Peru-Chile Undercurrent at the core of the Oxygen Minimum Zone (OMZ). While permeability at the periphery facilitates exchange with external waters, thereby modulating the original properties, the core signal retains negative oxygen (O₂) anomalies and positive anomalies of other biogeochemical tracers. These disturbances likely contribute to average properties that exceed the 90^th percentile threshold in the open ocean, contrasting with the formation zone where they surpass 50^th percentile levels. Suboxic cores are prevalent near the coast but decrease in abundance with distance from shore, giving way to a predominance of hypoxic cores, indicative of core ventilation during transit. The principal mechanism governing O₂ input into, or output from the eddy core entails lateral and vertical advection, with vertical mixing supplying O₂ to a lesser extent. Biological activity consumes O₂ for approximately 6 to 12 months more intensely the first 100 days, thereby facilitating the persistence of low O₂ conditions and extending the lifetime of biogeochemical anomalies within the core. The ammonium and nitrite depleted out of time in the eddy core with a decay rate greater than the nitrate and nitrous oxide, while these are accumulating in open sea. Our observations suggest that southern regions of the southeast Pacific OMZ undergo greater deoxygenation and nutrient enrichment due to Puddies compared to northern regions. This underscores the significant role of Puddies in modifying biogeochemical conditions in the open ocean and in extending the boundaries of the Southern tip of the OMZ.