Reiner Schlitzer

Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms

Todays surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

Impacts of Atmospheric Anthropogenic Nitrogen on the Open Ocean

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the oceans external (nonrecycled) nitrogen supply and up to ∼3% of the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide (N2O) per year. Although ∼10% of the oceans drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

Evaluation of ocean carbon cycle models with data-based metrics

New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models

We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model, failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.

Evaluating global ocean carbon models: The importance of realistic physics

A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

Interactive analysis and visualization of geoscience data with ocean data view

Ocean Data View (ODV) is a freeware package for the interactive exploration and graphical display of multi-parameter profile or sequence data. Although originally developed for oceanographic observations only, the underlying concept is more general, and data or model output from other areas of geosciences, like for instance geology, geophysics, geography and atmospheric research can be maintained and explored with ODV as well. The data format of ODV is designed for dense storage and direct data access, and allows the construction of very large datasets, even on affordable and portable hardware. ODV supports display of original data by colored dots or actual data values at the measurement locations. In addition, two fast and reliable variable-resolution gridding algorithms allow color shading and contouring of gridded fields along sections and on general 3D surfaces. A large number of derived quantities can be selected and calculated online. These variables are displayed and analyzed in the same way as the basic variables stored in disk files. ODV runs on PCs under Windows and on UNIX workstations under SUN Solaris. The software and extensive sets of coastline, topography, river-, lake- and border outlines as well as various gazetteers of topographic features are available at no cost over the Internet. In addition, the electronic atlas eWOCE that consists of oceanographic data from the World Ocean Circulation Experiment (WOCE) is also available free of charge over the Internet. A gallery of prepared plots of property distributions along WOCE sections provides a quick overview over hydrographic, nutrient, oxygen and transient tracer fields in the ocean and, apart from the scientific use for oceanographic research, can serve as tutorial material for introductory or advanced courses on oceanography.

Particle fluxes in the ocean: comparison of sediment trap data with results from inverse modeling

Biological production lowers the CO2 concentrations in the surface layer of the ocean, and sinking detritus ‘‘pumps’’ nutrients and CO2 into the deep ocean. Quantifying the efficiency of the biological pump is a prerequisite for global CO2 budgets. Sediment traps are commonly used to directly measure the vertical particle flux; however, for logistical and financial reasons, traps cannot provide area-wide data sets. Moreover, it has been shown that sediment traps can under- or overestimate particle fluxes considerably. In this paper, we present a new technique to estimate the downward flux of particulate matter with an adjoint model. Hydrographic and nutrient data are used to calculate the mean ocean circulation together with parameters for particle fluxes using the AWI Adjoint Model for Oceanic Carbon Cycling (AAMOCC). The model is fitted to the property concentrations by systematically varying circulation, air–sea fluxes, export production and remineralization rates of particulate biogenic matter simultaneously. The deviations of model fluxes based on nutrient budgets from direct measurements with sediment traps yield an independent estimate of apparent trapping efficiencies. While consistent with hydrographic and nutrient data, model particle fluxes rarely agree with sediment trap data: (1) At shallow water depth (V1000 m), sediment trap fluxes are at the average 50% lower than model fluxes, which confirms flux calibrations using radionuclides; (2) in the very deep traps, model fluxes tend to be lower compared to data, which might be explained by lateral inputs into the traps. According to these model results, particle fluxes from the euphotic zone (EP) into mid water depth are considerably higher and the shallow loop of nutrient is more vigorous than would be derived from sediment trap data. Our results imply that fluxes as collected with sediment traps are inconsistent with model derived long-term mean particle fluxes based on nutrient budgets in the water column. In agreement with recent radionuclide studies, we conclude that reliable export flux estimates can only be obtained from sediment trap data if appropriate corrections are applied. D 2003 Elsevier Science B.V. All rights reserved.

A chlorofluoromethane and hydrographic section across Drake Passage: Deep water ventilation and meridional property transport

New hydrographic and nutrient data obtained on a section across Drake Passage (F/S Meteor January 1990, World Ocean Circulation Experiment Hydrographic Program section S1) are in close agreement with property sections reported previously. The chlorofluoromethanes CFM 11 and CFM 12 were measured in Drake Passage for the first time. CFM concentrations are found to decrease from the surface down into the Upper Circumpolar Deep Water, for which they confirm water renewal from the south. For the Lower Circumpolar Deep Water, in which CFM concentrations were above detection limit only south of the Polar Front, very little water renewal on the CFM time scale is implied. Nonvanishing CFM is again found in the Weddell Sea Deep Water and the Southeast Pacific Deep Water toward the bottom in the south, but recent ventilation for the latter water mass is rejected. CFM 11 and CFM 12 concentrations vary essentially in constant proportion down to very low concentrations, questioning the possibility of using CFM ratios as “age” markers. The observed ratios are shown to be a natural feature of the upwelling regime of the southern ocean. Property concentrations on isopycnal surfaces display large undulations, reaching down into the Upper Circumpolar Deep Water. Their extrema, due to varying contribution of young water of southern origin, are situated at the boundaries of the current bands of the Antarctic Circumpolar Current The feature is ascribed to property advection by rings and is taken to support previous claims that rings are an important transport mechanism across the Antarctic Circumpolar Current and that they might assist in maintaining its fronts.

High-resolution modeling of sediment erosion and particle transport across the northwest African shelf

[1] The region off Cape Blanc along the northwest African coast is dominated by persistent upwelling and strong activity of small-scale eddies, filaments, and jets. Vertical particle camera profiles obtained during recent cruises in this region show that there exist two well-marked maxima of particle abundance in the water column, one at the surface and the other in subsurface layers between 200 m and 400 m depths. Using a highresolution (2.7 km) terrain-following coordinate ocean model with built-in ecosystem and sediment transport modules, we show that the surface particle maximum can be explained by local productivity, while the deeper, subsurface particle cloud most likely originates from particulate material eroded from the shallow shelf and transported offshore by vigorous filament activity and dynamic features of the flow. In the numerical experiments, particles are produced either by primary production in the surface layer or from prescribed sediment sources to mimic suspension and erosion along the shelf areas. Good agreement of modeled particle distributions with the data is achieved with a typical settling velocity of 5 m day � 1 . Time-averaged effective transport patterns of particles reveal distinct maxima between 20.5N and 23.5N off Cape Blanc. In the south of Cape Bojador and off Cape Timiris, on the other hand, the effective transport distance patterns suggest energetic offshore activity.

On the importance of intermediate water flows for the global ocean overturning

A steady state inverse global ocean model is used together with the available original, historical hydrographic database to study and quantify the large-scale global ocean circulation. The model has a variable resolution grid with grid sizes as small as 2.5° longitude by 2° latitude along boundaries, straits or over steep topography and a default resolution of 5° by 4° in “quiet” open ocean regions. The model has 26 vertical levels with 60 m resolution near the surface. The adjoint method is applied to drive the model to the hydrographic data and to optimize horizontal flows, air-sea heat fluxes, and mixing coefficients in an iterative way. Mass, heat, and salt budgets are satisfied exactly by the model. After assimilation, both simulated temperature and salinity fields are in good agreement with observations. Sensitivity experiments show that different circulation patterns with varying relative importance of intermediate water versus warm water transports and varying warm water inflow from the Indian Ocean into the Atlantic are consistent with the hydrographic data. However, for all solutions we find that the water mass that dominates the compensation of North Atlantic Deep Water export is Antarctic Intermediate Water. The northward transport rates of intermediate water in the South Atlantic and South Pacific in our model solutions range between 10 and 15 Sv in each ocean and are considerably larger than previously published values.