Tim Fischer

Mixed layer heat and salinity budgets during the onset of the 2011 Atlantic cold tongue

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.

Eddy diffusivities estimated from observations in the Labrador Sea

Eddy diffusivities in the Labrador Sea (LS) are estimated from deep eddy resolving float trajectories, moored current meter records, and satellite altimetry. A mean residence time of 248 days in the central LS is observed with several floats staying for more than 2 years. By applying a simple random walk diffusion model, the observed distribution of float residence times in the central LS could be explained by a mean eddy diffusivity of about 300 m2 s−1. Estimates from float trajectories themselves and from moored current meter records yield significantly higher eddy diffusivities in the central LS of 950–1100 m2 s−1. This discrepancy can be explained by an inhomogeneity of the eddy diffusivity at middepth with high/low values in the central LS/region between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE. between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE.

Nitrous oxide during the onset of the Atlantic cold tongue

The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18±2.59 µmol m−2 d−1) suggests that in May-July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O. This article is protected by copyright. All rights reserved.

Gas exchange estimates in the Peruvian upwelling regime biased by multi-day near-surface stratification

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 nearsurface 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 nearsurface 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 multiday near-surface stratification as well, the bias resulting from multi-day near-surface stratification may also impact global oceanic N2O emission estimates.

Circulation, eddies, oxygen and nutrient changes in the eastern tropical South Pacific Ocean

Reviewer #1: 12 Main comment: This paper presents a set of observations collected during an oceanographic 13 cruise off Peru in November-December 2012. The zonal circulation in the south 14 eastern Pacific has been measured, as well as the alongshore flow off the Peru coast. 15 The trajectories and properties measured by drifting floats launched in the area are 16 also described in detail. In the end of the paper, circulation trends are estimated by 17 comparing the data with those of previous cruises (Feb 1993). Nutrient and oxygen 18 trends are also inferred from the present and previous measurements. This paper 19 is following a suite of several papers presenting invaluable observations recently collected 20 off Peru during a series of oceanographic cruises. The data certainly deserves 21 to be published as there has not been many papers presenting recent observations in 22 this upwelling region which has peculiarities because of its very intense oxygen minimum 23 zone. However, the present paper is mainly a repetition of a previous paper by 24 Czeschel et al. (JGR, 2011, doi:10.1029/2010JC006565.) which presented data from 25 a previous cruise with quasi similar transects. It was structured similarly, described current 26 sections, floats trajectories, etc.. Despite the similarities between the two papers, 27 there is almost no discussion of the new results in the light of the previous findings of 28 Czeschel et al. (2011). Moreover, while the quality of the figures is good, the text is 29 extremely difficult to read. It lacks structure, there are lots of repetitions, there are no 30 transitions between the paragraphs, parts of the manuscript are really unclear. A lot 31 of it deserves rewriting (see my detailed comments below). Therefore I believe that 32 the paper could be significantly improved by (i) introducing a more developed section 33 with a detailed discussion of the previous findings of Czeschel, (ii) careful rewriting of 34 several paragraphs. Thus, I am in favour of a major revision. 35

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

Halocarbons (water) measured during Maria S. Merian cruise MSM18/3

Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane (CH2I2). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr3 of up to 44.7 and up to 9.2 pmol/L for CH2Br2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water, CH2Br2 was often more elevated in greater depth than CHBr3, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH2Br2 indicates an additional source in comparison to CHBr3 or a slower degradation of CH2Br2. Concentrations of CH3I of up to 12.8 pmol/L in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a biological source. CH2I2 was very low in the near surface water with maximum concentrations of only 3.7 pmol/L. CH2I2 showed distinct maxima in deeper waters similar to CH2Br2. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and one of the main driving factors of their emissions into the atmosphere in the ACT-region. The calculated production rates of the compounds in the mixed layer are 34 ± 65 pmol/m**3/h for CHBr3, 10 ± 12 pmol/m**3/h for CH2Br2, 21 ± 24 pmol/m**3/h for CH3I and 384 ± 318 pmol/m**3/h for CH2I2 determined from 13 depth profiles.