Abstract

Oceanic eddies are ubiquitous features of the circulation thought to be involved in transporting water mass properties over long distances from their source region. Among these is a particular type that has a core within the thermocline with little surface expression. Despite their significance, their role in ocean circulation remains largely undocumented by 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 document biogeochemical properties and processes associated with the nitrogen cycle inside PUDDIES and contrast them with those of the surrounding environment. Our findings reveal that PUDDIES capture a biogeochemical signal contingent upon their formation location along the coast, 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 (O-2) anomalies and positive anomalies of other biogeochemical tracers. These anomalous conditions result in tracer values exceeding the 90th percentile of their distribution in the open ocean, in contrast to the formation zones, where anomalies only surpass the 50th percentile. This indicates that PUDDIES may play a role in modulating the average properties of the open ocean. Suboxic (O-2<20 M) cores are prevalent near the coast but decrease in abundance with distance from shore, giving way to a predominance of hypoxic (20 mu M < O-2<45 M) cores (predominating at 60 % in the open sea), suggesting core ventilation during transit. The principal mechanism governing O-2 input to or output from the eddy core entails lateral and vertical advection, with vertical mixing supplying O-2 to a lesser extent. Biological activity consumes O-2 inside PUDDIES for around 6 to 12 months, especially intensely for the first 100 d, thus facilitating the persistence of low O-2 conditions and extending the lifetime of biogeochemical anomalies within the core (up to 800 km offshore). Ammonium and nitrite deplete earlier in the eddy core with a decay rate greater than that of nitrate and nitrous oxide, while these accumulate in the open sea (up to 16 % and 100 % higher than the mean state, respectively). Our results suggest that southern regions of the southeast Pacific OMZ undergo greater deoxygenation and nutrient enrichment due to PUDDIES compared to northern regions. However, the combination of various physical conditions can generate zones with more pronounced changes in the nitrite (subsurface water masses due to interactions with the PUDDIES, such as at 30 degrees S). The maximum contribution of NO takes place in particular along this latitude, with a 460 % increase compared to the mean state, near the coastal zone. In summary, PUDDIES formed along the Chilean coast capture distinct biogeochemical signatures depending on where they form. In the north, minimal ventilation fosters suboxic conditions and denitrification - leading to deficits of nitrate (NO) and nitrous oxide (N2O) but high NO and ammonium (NH) - whereas central and southern subregions show increased NO and higher N2O. Moreover, cross-shore exchange between Equatorial Subsurface Water and Subantarctic Water further amplifies this variability, giving rise to eddies with diverse nutrient and oxygen properties as they move offshore.