Damian S. Grundle

Low oxygen eddies in the eastern tropical North Atlantic: Implications for N2O cycling

Nitrous oxide (N2O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N2O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N2O concentrations much higher (up to 115 nmol L−1) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N2O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N2O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N2O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N2O at the regional scale, and point out the possible need for a reevaluation of how we view N2O cycling in the ETNA.

Author Correction: Low oxygen eddies in the eastern tropical North Atlantic: Implications for NO cycling

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Transition to El Niño conditions in the eastern tropical Pacific in October 2015

A strong El Niño developed in early 2015. Measurements from a research cruise on the RV Sonne in October 2015 near the equator east of the Galapagos Islands and off the shelf of Peru, are used to investigate changes related to El Niño in the upper ocean in comparison with earlier cruises in this 10 region. At the equator at 85°30’W, a clear temperature increase leading to lower densities in the upper 350 m, despite a concurrent salinity increase from 40 to 350 m, developed in October 2015. Lower nutrient concentrations were also present in the upper 200 m, and higher oxygen concentrations were observed between 40 and 130 m. Except for the upper 60 m at 2°30’S, however, there was no obvious increase in oxygen concentrations at sampling stations just north (1°N) and south (2°30’S) of the 15 equator at 85°30’W. In the equatorial current field, the Equatorial Undercurrent (EUC) east of the Galapagos Islands almost disappeared in October 2015, with a transport of only 0.02 Sv in the equatorial channel between 1°S and 1°N, and a weak current band of 0.78 Sv located between 1°S and 2°30’S. Such near-disappearances of the EUC in the eastern Pacific seem to occur only during strong El Niño events. Off the Peruvian shelf at ~9°S, where the sea surface temperature (SST) was elevated, 20 upwelling was modified, and warm, saline and oxygen rich water was upwelled. Despite some weak El Niño related SST increase at ~12 to 16°S, the upwelling of cold, low salinity and oxygen-poor water was still active at the easternmost stations at three sections at ~12°S, ~14°S and ~16°S, while further west on these sections a transition to El Niño conditions appeared. Although in early 2015 the El Niño was strong and in October 2015 showed a clear El Niño influence on the EUC, in the eastern tropical 25 Pacific the measurements only showed developing El Niño water mass distributions. In particular the Ocean Sci. Discuss., doi:10.5194/os-2016-14, 2016 Manuscript under review for journal Ocean Sci. Published: 24 March 2016 c