Southern Ocean concentrations

Profiles in the Japanese Sea are similar for model and observational data. Concentration decreases down to 1000 m and remains constant below. Surface concentrations are lower for modeled profiles, most likely due to the emission scenario, that assumes identical temporal behaviour for all source points and does not capture all emitted mass. Due to the limited horizontal resolution of models, the topography of the ocean differs from the real one. In the Southern ocean concentrations were low throughout all depths, and for the measurements often below the detection limit of 6 pg/L. 

The model results were compared with the HOx concentrations measured by the FAGE (Fluorescence Assay by Gas Expansion) technique during four days of clean Southern Ocean marine boundary layer (MBL) air. The models overestimated OH concentrations by about 10% on two days and about 20% on the other two days. HO2 concentrations were measured during two of these days and the models overestimated the measured concentrations by about 40%. Better agreement with measured HO2 was observed by using data from several MBL aerosol measurements to estimate the aerosol surface area and by increasing the HO2 uptake coefficient to unity. This reduced the modelled HO2 overestimate by 40%, with little effect on OH, because of the poor HO2 to OH conversion at the low ambient NOx concentrations. 

Southern Ocean controi on thermociine nutrient concentrations. APF = Antarctic poiar front,... 

Surface concentrations of nitrate and DSi (iJuM) (a) in the upweiiing area iocated offshore of West Africa (18°N 16°W) during March-Aprii 1971 and (b) in the Southern Ocean (45°S to 65°S, along 65°W) during March 1977. Source-. After Dugdaie, R. C., et al. (1995). Deep-Sea Research I 42, 697-719. 

The DSi stripped out of the Southern Ocean s surfece waters at the site of SAMW formation is converted to BSi. This BSi eventually sinks into the deep waters (CDW), where it is remineralized to DSi and driven back south to be upwelled again into the surface waters. This trapping effect is a large part of why half of the global marine sedimentary sink of BSi is located in the Southern Ocean. South of 55°S, iron limitation is so severe, as compared to the rate of upweiiing supply of DSi, that the diatoms are not able to reduce silicic acid concentrations to zero. 

Penkett, S. A., P. S. Monks, L. J. Carpenter, K. C. Clemitshaw, G. P. Ayers, R. W. Gillett, I. E. Galbally, and C. P. Meyer, Relationships between Ozone Photolysis Rates and Peroxy Radical Concentrations in Clean Marine Air over the Southern Ocean, J. Geophys. Res., 102, 12805-12817(1997). 

Average DMS concentrations in surface waters and in the atmosphere over the Southern Ocean were similar to values reported for other world ocean areas. 

Winds, currents and tides all contribute to horizontal mixing, which can also affect DMS concentrations. As an example, during a Lagrangian iron-addition experiment in the Southern Ocean a sulphur-hexafluoride-labelled patch of water spread from about 70-1,000 km2 in 18 days which led to rapid dilution of the bloom with water containing low biomass and DMS (Turner et al., in prep). Vertical and horizontal mixing processes can thus be overruling loss processes after a local built-up of the DMS concentration, which warrants inclusion of these processes in models. 

Daly KL, DiTullio GR (1996) Particulate dimethylsulfoniopropionate removal and dimethylsulfide production by zooplankton in the Southern Ocean. In Kiene RP, Visscher PT, Keller MD, Kirst GO (eds) Biological and environmental chemistry of DMSP and related sulfonium compounds. Plenum Press, New York, pp 223-238 Davidson AT, Marchant HJ (1987) Binding of manganese by Antarctic Phaeocystis pouchetii and the role of bacteria in its release. Marine Biol 95 481-487 DiTullio GR, Smith WO (1995) Relationship Between Dimethylsulfide And Phytoplankton Pigment Concentrations In The Ross Sea, Antarctica. Deep-Sea Res Part 142 873-892... 

Annual patterns of P. antarctica in the Southern Ocean may thus be determined by the interplay between variable micronutrient concentrations and irradiance availability. In contrast, patterns in high latitudes in the northern hemisphere, where iron is not typically limiting, are primarily determined by nutrient conditions and large-scale hydrographical phenomena. 

Case Study II — Photochemical control of ozone in the remote marine boundary layer (MBL) - An elegant piece of experimental evidence for the photochemical destruction of ozone comes from studies in the remote MBL over the southern ocean at Cape Grim, Tasmania (41 In the MBL, the photochemical processes are coupled to physical processes that affect the observed ozone concentrations, namely deposition to the available surfaces and entrainment from the free troposphere. The sum of these processes can be represented in the form of an ozone continuity equation (a simplified version of Equation 2.6), viz... 

Figure 1.5c shows that preformed NO represents a very substantial fraction of the total NO in the thermocHne and deep ocean, and that it is responsible for about half of the surface to deep gradient in NO3. It also shows that much of the NOJ increase in the deep Atlantic along the path of NADW is actually caused by mixing of a high preformed NOj end-member from the Southern Ocean while the NOj increase from remineralization is relatively small (Fig. 1.5b). The highest concentrations of remineralized NO are found in the thermocline, as expected from the fact that about 90% of the organic nitrogen exported from the upper ocean is reminer-alized above 1000 m (e.g., Martin et al. (1987)).

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