Search

A global coupled ocean-atmosphere modeling system is applied in a transient climate simulation to study the response to global warming of Dimethylsulfide (DMS) in the ocean, the DMS flux to the atmosphere, and the resulting DMS concentrations in the atmosphere. The DMS production and consumption processes in the ocean are linked to plankton dynamics simulated in the marine biogeochemistry model HAMOCC5.1, embedded in an ocean general circulation model (MPI-OM). The atmospheric model ECHAM5 is extended by the microphysical aerosol model HAM, treating the sulfur chemistry in the atmosphere and the evolution of microphysically interacting internally and externally mixed aerosol populations. For future conditions (2000-2100) we assume greenhouse gas concentrations, aerosol and aerosol precursor emissions according to the SRES A1B scenario. We analyzed the results in terms of simulated changes between the period 1861-1890 and 2061-2090. For the global annual mean DMS sea surface concentration and the DMS flux we found a reduction by 10%. The DMS burden in the atmosphere is reduced by only 3%, owing to a longer lifetime of DMS in the atmosphere in a warmer climate (+7%). Regionally the response and the underlying mechanisms are quite inhomogeneous. The largest reduction in the DMS sea surface concentration is simulated in the Southern Ocean (-40%) caused by an increase in the summer mixed layer depth, leading to less favorable light conditions for phytoplankton growth. In the mid and low latitudes DMS sea surface concentrations are predominantly reduced due to nutrient limitation of the phytoplankton growth through higher ocean stratification and less transport of nutrients into the surface layers. Copyright 2007 by the American Geophysical Union.

Shelves have been estimated to account for more than one-fifth of the global marine primary production. It has been also conjectured that shelves strongly influence the oceanic absorption of anthropogenic CO2 (carbon shelf pump). Owing to their coarse resolution, currently applied global climate models are inappropriate to investigate the impact of climate change on shelves and regional models do not account for the complex interaction with the adjacent open ocean. In this study, a global ocean general circulation model and biogeochemistry model were set up with a distorted grid providing a maximal resolution for the NW European shelf and the adjacent northeast Atlantic. Using model climate projections we found that already a~moderate warming of about 2.0 K of the sea surface is linked with a reduction by ~ 30% of the biological production on the NW European shelf. If we consider the decline of anthropogenic riverine eutrophication since the 1990s, the reduction of biological production amounts is even larger. The relative decline of NW European shelf productivity is twice as strong as the decline in the open ocean (~ 15%). The underlying mechanism is a spatially well confined stratification feedback along the continental shelf break. This feedback reduces the nutrient supply from the deep Atlantic to about 50%. In turn, the reduced productivity draws down CO2 absorption in the North Sea by ~ 34% at the end of the 21st century compared to the end of the 20th century implying a strong weakening of shelf carbon pumping. Sensitivity experiments with diagnostic tracers indicate that not more than 20% of the carbon absorbed in the North Sea contributes to the long-term carbon uptake of the world ocean. The rest remains within the ocean's mixed layer where it is exposed to the atmosphere. The predicted decline in biological productivity, and decrease of phytoplankton concentration (in the North Sea by averaged 25%) due to reduced nutrient imports from the deeper Atlantic will probably affect the local fish stock negatively and therefore fisheries in the North Sea.

This paper presents an estimate of the oceanic Lorenz energy cycle derived from a simulation forced by 6-hourly fluxes obtained from NCEP–NCAR reanalysis-1. The total rate of energy generation amounts to 6.6 TW, of which 1.9 TW is generated by the time-mean winds and 2.2 TW by the time-varying winds. The dissipation of kinetic energy amounts to 4.4 TW, of which 3 TW originate from the dissipation of eddy kinetic energy. The energy exchange between reservoirs is dominated by the baroclinic pathway and the pathway that distributes the energy generated by the time-mean winds. The former converts 0.7 to 0.8 TW mean available potential energy to eddy available potential energy and finally to eddy kinetic energy, whereas the latter converts 0.5 TW mean kinetic energy to mean available potential energy.This energy cycle differs from the atmospheric one in two aspects. First, the generation of the mean kinetic and mean available potential energy is each, to a first approximation, balanced by the dissipation. The interaction of the oceanic general circulation with mesoscale eddies is hence less crucial than the corresponding interaction in the atmosphere. Second, the baroclinic pathway in the ocean is facilitated not only by the surface buoyancy flux but also by the winds through a conversion of 0.5 TW mean kinetic energy to mean available potential energy. In the atmosphere, the respective conversion is almost absent and the baroclinic energy pathway is driven solely by the differential heating.

Continental shelf seas are known to support a large fraction of the global primary production. Yet, they are mostly ignored or neglected in global biogeochemical models. A number of processes that control the transfer of dissolved nutrients from rivers to the open ocean remain poorly understood. This applies in particular to dissolved silica which drives the growth of diatoms that form a large part of the phytoplankton biomass and are thus an important contributor to export production of carbon. Here, the representation of the biogeochemical cycling along continents is improved by coupling a high resolution database of riverine fluxes of nutrients to the global biogeochemical ocean general circulation model HAMOCC5-OM. Focusing on silicon (Si), but including the whole suite of nutrients – carbon (C), nitrogen (N) and phosphorus (P) in various forms – inputs are implemented in the model at coastal coupling points using the COSCAT global database of 156 mega-river-ensemble catchments from Meybeck et al. (2006). The catchments connect to the ocean through coastal segments according to three sets of criteria: natural limits, continental shelf topography, and geophysical dynamics. According to the model the largest effects on nutrient concentrations occur in hot spots such as the Amazon plume, the Arctic – with high nutrient inputs in relation to its total volume, and areas that encounter the largest increase in human activity, e.g., Southern Asia.

Ocean acidification is likely to impact calcification rates in many pelagic organisms, which may in turn cause significant changes in marine ecosystem structure. We examine effects of changes in marine CaCO3 production on total alkalinity (TA) in the ocean using the global biogeochemical ocean model HAMOCC. We test a variety of future calcification scenarios because experimental studies with different organisms have revealed a wide range of calcification sensitivities to CaCO3 saturation state. The model integrations start at a preindustrial steady state in the year 1800 and run until the year 2300 forced with anthropogenic CO2 emissions. Calculated trends in TA are evaluated taking into account the natural variability in ocean carbonate chemistry, as derived from repeat hydrographic transects. We conclude that the data currently available does not allow discerning significant trends in TA due to changes in pelagic calcification caused by ocean acidification. Given different calcification scenarios, our model calculations indicate that the TA increase over time will start being detectable by the year 2040, increasing by 5-30 mu mol kg(-1) compared to the present-day values. In a scenario of extreme reductions in calcification, large TA changes relative to preindustrial conditions would have occurred at present, which we consider very unlikely. However, the time interval of reliable TA observations is too short to disregard this scenario. The largest increase in surface ocean TA is predicted for the tropical and subtropical regions. In order to monitor and quantify possible early signs of acidification effects, we suggest to specifically target those regions during future ocean chemistry surveys.

HOPE is a primitive-equation model of the global ocean circulation, but may also be used for regional studies. Prognostic variables are the three-dimensional horizontal velocity fields, the sea-surface elevation, and the thermohaline variables. The equa- tions are discretized on prescribed horizontal surfaces using Arakawa-E-type grids. Two time levels are used for the integration. A Hibler-type dynamic sea-ice model allows a prognostic calculation of sea-ice thick- ness, compactness, and velocities. The thermodynamic growth of sea ice is calculated with heat balance equations using simple bulk formulae. A snow layer is included. HOPE is in particular useful for altimetry data assimilation purposes because of the prognostic calculation of the sea-surface elevation.

The Hamburg Oceanic Carbon Cycle Circulation Model (Cycle 1)

Several multi-century and multi-millennia simulations have been performed with a complex Earth System Model (ESM) for different anthropogenic climate change scenarios in order to study the long-term evolution of sea level and the impact of ice sheet changes on the climate system. The core of the ESM is a coupled coarse-resolution Atmosphere–Ocean General Circulation Model (AOGCM). Ocean biogeochemistry, land vegetation and ice sheets are included as components of the ESM. The Greenland Ice Sheet (GrIS) decays in all simulations, while the Antarctic ice sheet contributes negatively to sea level rise, due to enhanced storage of water caused by larger snowfall rates. Freshwater flux increases from Greenland are one order of magnitude smaller than total freshwater flux increases into the North Atlantic basin (the sum of the contribution from changes in precipitation, evaporation, run-off and Greenland meltwater) and do not play an important role in changes in the strength of the North Atlantic Meridional Overturning Circulation (NAMOC). The regional climate change associated with weakening/collapse of the NAMOC drastically reduces the decay rate of the GrIS. The dynamical changes due to GrIS topography modification driven by mass balance changes act first as a negative feedback for the decay of the ice sheet, but accelerate the decay at a later stage. The increase of surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long term. Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced to 3/4 of the original size. From then, the reduction in the albedo of Greenland strongly accelerates the decay of the ice sheet.

Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (+/- 1 sigma) downward flux of organic matter across 75 m depth is 17 +/- 6 Pg C yr(-1). Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics. [References: 112]

We implemented a process-based DMS module into the global carbon cycle ocean model (HAMOCC5) which includes a simple module for plankton dynamics and investigated the regional and seasonal variations of the marine sulfur cycle. The turnover rates within the DMS cycle are only poorly known. Therefore we developed, on the basis of a global DMS data set, an optimization routine for the free parameters controlling DMS production and removal. The resulting seasonal and regional distributions of DMS concentration are fully consistent with the underlying hydrodynamical and biogeochemical processes. We investigated a series of DMS model approaches with various complexities. The distinction between different DMS producing phytoplankton species and the consideration of the regionally and seasonally varying bacterial activity on converting dDMSP to DMS and on DMS consumption appears to have a crucial effect on the quality of the results in the given model conception.

Trends and variability in the ocean-atmosphere CO2 flux and the uptake of anthropogenic CO2 are simulated for the period 1948–2003, using a biogeochemical carbon cycle model (HAMOCC5) coupled online to a global ocean general circulation model (MPI-OM). The model is forced by daily National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1948 to 2003. We find a global interannual variability of ±0.50 PgC yr−1 (2σ) which is largely dominated by ocean dynamics in the equatorial Pacific. On decadal scale, two patterns are of global importance. First, the regime shift that occurs in 1975–1977 when the modeled interannual variability of the equatorial Pacific changes from ±0.32 PgC yr−1 to ±0.23 PgC yr−1 and the mean outgasing of CO2 decreases from 0.70 PgC yr−1 to 0.58 PgC yr−1. Second, the trend that occurs in the Southern Ocean, where we find an increase of the CO2 fluxes over the full 56-year simulation period. The flux results from stronger upwelling and deeper mixed layers which are caused by increasing wind velocities. We estimate an average CO2 flux into the ocean of 1.49 PgC yr−1 for 1980–1989 and 1.74 PgC yr−1 for 1990–1999, with extremes of 1.20 PgC yr−1 at the La Niña event in 1996 and 2.10 PgC yr−1 during the El Niño events in 1993 and 1998. Because of the rising buffer factor, the uptake of anthropogenic CO2 is slowing down in regions of shallow mixing toward the end of the simulation. Overall, about 124 Pg of anthropogenic carbon has accumulated in the model ocean until the end of 2003

With a volume transport of similar to134 x 10(6) m(3)/s at the Drake Passage, the Antarctic Circumpolar Current (ACC) is the strongest ocean current. In the interest of estimating the secondary magnetic fields generated by the magnetohydrodynamic interaction of this flow with Earth's main field, we compare numerical results for the magnetic fields obtained using flow from three different ocean general circulation models. These simulations all expect detectable ocean signals in the magnetic records at ground and satellite altitude ( 400 km). The variability of this contribution is highly correlated with the ACC transport, a very important variable for climate studies. Observed magnetic fields could then be used, in principle, to derive an index of variability of the ACC. However given its small amplitude compared with other magnetic contributions, extracting the ocean's signal from observations remains a challenge at this time

The Earth's Dole effect describes the isotopic O-18/O-16-enrichment of atmospheric oxygen with respect to ocean water, amounting under today's conditions to 23.5parts per thousand. We have developed a model of the Earth's Dole effect by combining the results of three-dimensional models of the oceanic and terrestrial carbon and oxygen cycles with results of atmospheric general circulation models (AGCMs) with built-in water isotope diagnostics. We obtain a range from 22.4parts per thousand to 23.3parts per thousand for the isotopic enrichment of atmospheric oxygen. We estimate a stronger contribution to the global Dole effect by the terrestrial relative to the marine biosphere in contrast to previous studies. This is primarily caused by a modeled high leaf water enrichment of 5-6parts per thousand. Leaf water enrichment rises by similar to1parts per thousand to 6-7parts per thousand when we use it to fit the observed 23.5parts per thousand of the global Dole effect. The present model is designed to be utilized in forthcoming paleo studies allowing a quantitative analysis of long-term observations from polar ice cores. [References: 73]