Description: The mixed layer (ML) temperature and salinity changes in the central tropical Atlantic have been studied by a dedicated experiment (Cold Tongue Experiment (CTE)) carried out from May to July 2011. The CTE was based on two successive research cruises, a glider swarm, and moored observations. The acquired in situ data sets together with satellite, reanalysis, and assimilation model data were used to evaluate box-averaged ML heat and salinity budgets for two subregions: (1) the western equatorial Atlantic cold tongue (ACT) (23°-10°W) and (2) the region north of the ACT. The strong ML heat loss in the ACT region during the CTE was found to be the result of the balance of warming due to net surface heat flux and cooling due to zonal advection and diapycnal mixing. The northern region was characterized by weak cooling and the dominant balance of net surface heat flux and zonal advection. A strong salinity increase occurred at the equator, 10°W, just before the CTE. During the CTE, ML salinity in the ACT region slightly increased. Largest contributions to the ML salinity budget were zonal advection and the net surface freshwater flux. While essential for the ML heat budget in the ACT region, diapycnal mixing played only a minor role for the ML salinity budget. In the region north of the ACT, the ML freshened at the beginning of the CTE due to precipitation, followed by a weak salinity increase. Zonal advection changed sign contributing to ML freshening at the beginning of the CTE and salinity increase afterward.

Description: Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

Description: The coastal upwelling regime off Peru in December 2012 showed considerable vertical concentration gradients of dissolved nitrous oxide (N2O) across the top few meters of the ocean. The gradients were predominantly downward, i.e., concentrations decreased toward the surface. Ignoring these gradients causes a systematic error in regionally integrated gas exchange estimates, when using observed concentrations at several meters below the surface as input for bulk flux parameterizations – as is routinely practiced. Here we propose that multi-day near-surface stratification events are responsible for the observed near-surface N2O gradients, and that the gradients induce the strongest bias in gas exchange estimates at winds of about 3 to 6 m s−1. Glider hydrographic time series reveal that events of multi-day near-surface stratification are a common feature in the study region. In the same way as shorter events of near-surface stratification (e.g., the diurnal warm layer cycle), they preferentially exist under calm to moderate wind conditions, suppress turbulent mixing, and thus lead to isolation of the top layer from the waters below (surface trapping). Our observational data in combination with a simple gas-transfer model of the surface trapping mechanism show that multi-day near-surface stratification can produce near-surface N2O gradients comparable to observations. They further indicate that N2O gradients created by diurnal or shorter stratification cycles are weaker and do not substantially impact bulk emission estimates. Quantitatively, we estimate that the integrated bias for the entire Peruvian upwelling region in December 2012 represents an overestimation of the total N2O emission by about a third, if concentrations at 5 or 10 m depth are used as surrogate for bulk water N2O concentration. Locally, gradients exist which would lead to emission rates overestimated by a factor of two or more. As the Peruvian upwelling region is an N2O source of global importance, and other strong N2O source regions could tend to develop multi-day near-surface stratification as well, the bias resulting from multi-day near-surface stratification may also impact global oceanic N2O emission estimates.