Dissolved oxygen is a major element for marine biogeochemical processes and ecosystems. Some areas of the ocean show low oxygen concentrations. They are found all around the world, but the most extensive are located in the tropical and subtropical Pacific Ocean. They occur due to a combination of factors including high surface productivity, weak ventilation, and stratification - but not only. Understanding such a phenomenon and its effects involves studying the chemical, biological processes, and also the physical ones, at the surface and in-depth, and at meso- and submesoscale. This knowledge is critical for predicting how these low-oxygen areas and their ecosystems may respond to climate change.

The largest oxygen deficient zone is located in the eastern tropical North Pacific. A study focuses on the area of it west of central America, combining physical and biogeochemical in situ data with satellite information for sea level anomaly, finite-size Lyapunov exponents (FSLE), chlorophyll a, and wind patterns, with particle tracking tools, to elucidate the mechanisms driving the distribution of oxygen within this oxygen deficient zone.

The at-sea campaign done during this study traversed two anticyclonic eddies, detected in altimetry sea level anomaly maps and in the Mesoscale Eddy Tracking Atlas - born respectively in the Tehuantepec and in the Papagayo gulfs, both generated by the winds crossing the mountains of central America (see previous images of the month on Tehuantepec eddies). Chlorophyll filaments from the two gulfs coincide with regions of high FSLE, indicating intense stirring and mixing processes. Otherwise, these two mesoscale eddies exhibited different biogeochemical signatures. The Papagayo eddy showcases elevated oxygen levels, likely advected from the coastal region, while the Tehuantepec eddy although crossing an oxygen-depleted zone still manifested localized oxygen maxima at depth. This is probably due to the effect of the winds in driving upwelling but also enhancing vertical mixing, facilitating the oxygenation of these deeper waters before they are subducted and transported offshore as part of the regional circulation.

Observation of small-scale processes and filamentation is thus important to better understand the distribution of biogeochemical properties and the stirring. Swot may help in this respect, especially close to the coasts.