ALBERT

Description: Mooring observations and model simulations point to an instability of the Labrador Current (LC) during winter, with enhanced eddy kinetic energy (EKE) at periods between 2 and 5 days and much less EKE during other seasons. Linear stability analysis using vertical shear and stratification from the model reveals three dominant modes of instability in the LC: 1) a balanced interior mode with along-flow wavelengths of about 30–45 km, phase velocities of 0.3 m s−1, maximal growth rates of 1 day−1, and surface-intensified but deep-reaching amplitudes; 2) a balanced shallow mode with along-flow wavelengths of about 0.3–1.5 km, phase velocities of 0.55 m s−1, about 3 times larger growth rates, but amplitudes confined to the mixed layer (ML); and 3) an unbalanced symmetric mode with the largest growth rates, vanishing phase speeds, and along-flow structure, and very small cross-flow wavelengths, also confined to the ML. Both balanced modes are akin to baroclinic instability but operate at moderate-to-small Richardson numbers Ri with much larger growth rates as for the quasigeostrophic limit of Ri ≫ 1. The interior mode is found to be responsible for the instability of the LC during winter. Weak stratification and enhanced vertical shear due to local buoyancy loss and the advection of convective water masses from the interior result in small Ri within the LC and up to 3 times larger growth rates of the interior mode in March compared to summer and fall conditions. Both the shallow and the symmetric modes are not resolved by the model, but it is suggested that they might also play an important role for the instability in the LC and for lateral mixing.

Description: The formation of an anticyclonic mode water eddy in Jan/Feb 2013 within the Peru-Chile Undercurrent is presented based on a multi-platform observational study. Two consecutive research cruises, a glider swarm experiment and moored measurements were conducted as part of the interdisciplinary "SFB 754 Climate-Biogeochemistry Interactions in the Tropical Ocean" project within the Peruvian upwelling regime at 12°S. The dataset allows a detailed investigation of the eddy generation process and its impacts on the near-coastal hydrography and biogeochemistry in space and time. The near-coastal horizontal circulation off Peru at 12°S changes significantly over the two months of observation. In early January, we observe a strong but clear Peru-Chile Undercurrent with maximal pole-ward velocities of ~25 cm/s in 100 - 200 m depth. A week later the vertical shear starts to increases and finally a mode water eddy forms. The eddy has a velocity maximum of ~0.3 m/s in 100 - 200 m depth and a radius of ~45 km. The eddy induced circulation strongly influences the near-coastal hydrography: Across-shore velocities result in an exchange of water masses between the shelf-break and the offshore ocean. At the eddy edge small scale salinity anomalies are found, which seem to be formed by mesoscale stirring. Energetic near-inertial oscillations are observed in the deeper water column during eddy generation that appear to be associated with this feature. After its generation close to the shelf break the eddy propagates westwards.

Description: In meso- and submesoscale regimes a strong physical-biogeochemical coupling is thought to exist. A swarm experiment with seven gliders equipped with sensors measuring pressure, temperature, salinity, oxygen and chlorophyll fluorescence was conducted in early 2013 within the upwelling region off Peru. The goal was to study near-coastal pathways for the supply of oxygen from the mixed layer to the oxygen minimum zone. Each glider carried out about one dive per hour measuring two multi-parameter profiles with a lateral resolution of about 500 m. More than 15.000 profiles were recorded during the two-months deployment within a small spatial area to capture both the temporal and spatial variability of the physical and biochemical parameters. The glider-based data show small-scale structures of different tracers such as salinity and oxygen at different depths. There are pronounced density compensated salinity anomalies at about 150 m depth, where lateral eddy-stirring of the lateral background salinity gradient is suggested as the main formation mechanism. Near the surface, non-density compensated salinity and oxygen intrusions are observed, which reached well below the mixed layer. These structures are found in areas of strong lateral density fronts. This suggests that submesoscale frontal processes are responsible for the observed structures. The role of these meso- and submesoscale processes for the near coastal vertical oxygen supply is discussed.

Description: A swarm experiment with seven gliders equipped with sensors measuring pressure, temperature, salinity, oxygen and chlorophyll fluorescence was conducted in early 2013 within the upwelling region off Peru. The goal was to study the role of meso- and submesoscale proccesses for the near-coastal oxygen ventilation of the Peruvian oxygen minimum zone. Each glider carried out about one dive per hour measuring two multi-parameter profiles with a lateral resolution less than 300 m. About 15.000 profiles were recorded during the two-months deployment within a small spatial area to capture both the temporal and spatial variability of the physical and biochemical parameters. Two main results are presented in the talk: 1) The formation of a low oxygen mode water eddy within the Peru Chile Undercurrent is observed. The near-coastal horizontal circulation off Peru at 12°S changes significantly over the two months of observation. In early January, we observe a pronounced Peru-Chile Undercurrent with maximal poleward velocities of 25 cm/s in 100 - 200 m depth. A week later the circulation start to change and a mode water eddy forms within the glider field. The physical and biogeochemical eddy properties and impacts on the near-coastal salinity and oxygen distribution are described in detail. 2) During an upwelling event the formation and decay of a submesoscale cold water filament is present in the sea surface temperature and glider data. Near the surface, non-density compensated salinity and oxygen intrusions are observed which seem to be associated with this feature. These anomalies reach well below the mixed layer into the thermocline and frontal subduction is suggested as their formation mechanism.

Description: Mooring observations and model simulations point to an instability of the Labrador Current (LC) during winter, with enhanced eddy kinetic energy (EKE) at periods between 2 to 5 days, and much less EKE during other seasons. Linear stability analysis using vertical shear and stratification from the model reveals three dominant modes of instability in the LC: - a balanced interior mode with along-flow wavelengths of about 30–45 km, phase velocities of 0.3 m/s, maximal growth rates of 1 d−1 and surface intensified, but deep reaching amplitudes, - a balanced shallow mode with along-flow wavelengths of about 0.3–1.5 km, about three times larger phase speeds and growth rates, but amplitudes confined to the mixed layer (ML), - and an unbalanced symmetric mode with largest growth rates, vanishing phase speeds and along-flow structure, and very small cross-flow wavelengths, also confined to the ML. Both balanced modes are akin to baroclinic instability, but operate at moderate to small Richardson numbers Ri with much larger growth rates as for the quasi-geostrophic limit of Ri ≫ 1. The interior mode is found to be responsible for the instability of the LC during winter. Weak stratification and enhanced vertical shear due to local buoyancy loss and the advection of convective water masses from the interior result in small Ri within the LC, and to three times larger growth rates of the interior mode in March compared to summer and fall conditions. Both the shallow and the symmetric mode are not resolved by the model, but it is suggested that they might also play an important role for the instability in the LC and for lateral mixing.

Description: The water mass transformation process in the Labrador Sea during winter plays an important role for the Atlantic meridional overturning circulation and the global climate system. The Labrador Sea Water (LSW) is exported within the deep Labrador Current (LC) after the convection process. LSW takes up large amounts of atmospheric tracer gases as CO2 and oxygen, and is thus one of the major agent for ventilation of the abyssal ocean. It is shown that enhanced eddy kinetic energy (EKE) along the LC shows up in a 1/12 ° ocean model simulation during the transformation process. Moored in-situ measurements within the LC also show enhanced EKE levels during winter. This instability processes within the LC is important as it might alter the water mass properties of the (LSW) by frontal mixing processes during the water mass transformation and export within the LC. The frontal instability process, which lead to enhanced EKE along the LC during winter is investigated using ageostrophic linear stability analysis. Dense and weakly stratified water masses produced during the wintertime transformation process lead to weaker stratification and a strengthening of the lateral density gradients within the LC. Weak stratification and enhanced vertical shear result in low Richardson numbers and the growth rate of baroclinic waves increases significantly within the shelf break LC during winter. Rapid frontogenesis along the whole LC sets in resulting in enhance EKE. During the rest of the year strong stratification and weak vertical shear leads to larger Richardson numbers and smaller growth rates. Ageostrophic linear stability analysis shows that a geostrophic interior mode has similar wavelengths as the first wavelike disturbances in the model simulations. A shallow mode with lateral scales O (1 km) is also predicted, which can be associated with mixed layer instabilities and submesoscale variability but remains unresolved by the model simulation.

Description: At the submesocale a strong physical-biogeochemical coupling exists due to similar temporal and spatial scales of the physical and biogeochemical processes involved. A swarm experiment with seven gliders equiped with biophysical sensors measuring pressure, temperature, salinity, oxygen, chlorophyll fluorescence and turbidity was conducted in early 2013 in the upwelling region off Peru. The goal of the experiment was to sample a relatively small spatial area as synoptically as possible, in order to allow seperation between temporal and spatial variability. In particular, the role of submesoscale processes for the near coastal vertical oxygen distribution was assessed. Each glider carried out about one dive per hour measuring two multi-parameter profiles with a lateral resolution of about 500 m. Alltogether more than 15.000 profiles were recorded during the two-months deployment. The glider data shows small scale filamentary structures of different tracers such as salinity and oxygen at different depths. There are density compensated salinity filaments at about 150 m depth were lateral mesoscale eddy stirring of the lateral background salinity gradient is suggested as the main formation mechasim. Closer to the surface, however, non-density compensated salinity and oxygen intrusions reaching well below the mixed layer can be seen. These structures are found at strong lateral mixed layer density fronts, suggesting that submesoscale frontal processes are responsible for the observed structures. This findings suggest that beside diapycnal mixing vertical velocities due to submesoscale frontal processes lead to an additional oxygen supply from the oxygenated mixed layer into the oxygen minimum zone.

Description: Important for the carbon export to the Deep Sea, Transparent Exopolymer Particles (TEP) also serve as nutritional substrate and attachment surfaces for bacteria, supporting biological oxygen consumption. Spatial distribution of TEP in the Eastern South Pacific Ocean (ESP), an area influenced by a highly productive upwelling system and underlying extensive Oxygen Minimum Zone (OMZ), is largely unknown. In attempt to recognize how TEP stocks are affected by complex biogeochemistry of ESP and vice versa, we determined TEP distribution in the ESP (12°-14°S, 76°-79°W) during the Meteor 93 cruise (February - March 2013). Highest TEP concentrations (〉1900µgXeq/l) were observed close to the coast, coinciding with upwelling of nutrient-rich waters. Generally TEP accumulated in the oxygenated photic layer and correlated significantly with chlorophyll-a-fluorescence (r=0.7, n=323, P〈0.05). At the upper boundary the OMZ (70 m), TEP concentrations were moderate (〈80µgXeq/l), slightly attenuating toward its lower border (〈60µgXeq/l; 500m depth). The role of TEP for organic matter cycling in the ESP and their potential influence on oxygen and carbon fluxes in the OMZ are discussed.