High resolution climate simulations over the Arctic

The regional atmospheric climate model HIRHAM has been applied to the Arctic. Simulations for the whole year 1990 and for an ensemble of winter months (January of 1985-1995) have been performed. The comparison of the simulations with observational data analyses shows that the general spatial patterns are in good agreement with the data, in both the vertical structure and the annual cycle. For an additional validation of the model results, a multivariate classification of large-scale circulation patterns has been applied to the January ensemble model simulations.     

Cenomanian oysters from the Sergipe Basin, Brazil

Cenomanian (mid Cretaceous) oysters from the Sergipe Basin in northeastern Brazil are described, with revisions of previously described forms. Nine genera and subgenera, including eleven species, are distinguished: Rastellum diluvianum (Linné, 1767), Amphidonte (Ceratostreon) reticulata (Reuss, 1846), A. (Ceratostreon) flabellata (Goldfuss, 1833), Exogyra (Costagyra) olisiponensis Sharpe, 1850, Ilymatogyra (Afrogyra) africana (Lamarck, 1801), Rhynchostreon (Rhynchostreon)mermeti (Coquand, 1862), R. (Laevigyra) obliquatum (Pulteney, 1813), R. (Laevigyra) sp., Pycnodonte (Phygraea) vesiculosa (J. Sowerby, 1823), Curvostrea rouvillei (Coquand, 1862) and Ambigostrea sp. No undoubted Turonian oysters are known from Sergipe, althoughR. (R.) mermeti possibly straddles the Cenomanian–Turonian boundary. This is in sharp contrast with the situation in the more northerly Brazilian basins, from where several Turonian but no Cenomanian forms have been described. Well-preserved material from Sergipe confirms the close relationship between Exogyra (Costagyra) Vialov andVultogryphaea Vialov. The palaeobiogeographical affinity of the oyster fauna is typically Tethyan with many taxa that are known particularly from the southern Tethys. The inferred palaeoenvironment as implied by the oysters is that of a shallow shelf.     

Local stability and estimation of uncertainty for solutions to inverse problems

 We present a method for studying local stability of a solution to an inverse problem and evaluate the uncertainty in determining true values of particular observables. The investigation is done under the assumption that only the Gaussian part of fluctuations about the local minimum of the cost (likelihood) function is essential. Our approach is based on the spectral analysis of the Hessian operator associated with the cost function at its extremal point, and we put forward an effective iterative algorithm suitable for numerical implementation in the case of a computationally large problem. Received: 16 May 2001 / Accepted: 22 October 2001     

Crustal structure and local seismicity in Colombia

Using P-wave travel time data from local seismicity, the crustal structure ofthe central and southern part of Colombia was determined. A very stableand narrow range of possible velocity models for the region was obtainedusing travel time inversion. This range of models was tested with earthquakelocations to select the best velocity model. The 1D velocity modelproposed has five layers over a halfspace, with interfaces at depths of 4,25, 32, 40 and 100 km and P-wave velocities of 4.8, 6.6, 7.0, 8.0, 8.1and 8.2 km/sec, respectively. According to this model the Moho lies at32 km depth on average. For P-waves, the station corrections range from–0.62 to 0.44 sec and for S-wave they range from –1.17 to 0.62 sec.These low variations in station residuals indicate small lateral velocitychanges and therefore the velocity model found should be well suited forearthquake locations and future starting model for 3D tomography studies.Using this new velocity model, the local earthquakes were relocated. Theshallow seismicity, < 30 km, clearly shows the borders betweentectonic plates and also the main fault systems in the region. The deepseismicity, > 80 km, shows two subduction zones in the country: theCauca subduction zone with a strike of N120^°E, dip of 35^°and thickness of 35 km, and the Bucaramanga subduction zone which has,for the northern part, a strike of N103^°E, dip of 27^° andthickness undetermined and, for the southern part, a strike ofN115^°E, dip of 40^° and thickness of 20 km. Based ondifferences of thickness of brittle crust in the subducted slab and spatialdistribution of the seismicity, the Cauca and Bucaramanga subduction zonesseem to represent independent processes. The Cauca subduction seems tobe connected to the process of the Nazca plate being subducted under theNorth Andes Block. In the Bucaramanga subduction zone, the transitionbetween southern and northern parts and changes in geometry of the slabseem to be gradual and there is no evidence of a tear in the slab, howeverthe local seismicity does not allow us to determine which plate or plates arebeing subducted. The Bucaramanga nest appears to be included into thesubducted slab.     

Variability of the mixing zones and estuarine turbidity maxima in the Elbe and Weser estuaries

The neighboring coastal plain estuaries of the Elbe and Weser Rivers in Northern Germany exhibit pronounced estuarine turbidity maxima (ETM). Common features and differences between the longitudinal distributions of salinity and suspended particulate matter (SPM) in both estuaries are compared as well as the mechanisms effecting them. Monthly transects of the near surface SPM indicate that the long-term variability of salinity and the ETM is mainly influenced by the freshwater runoff. The variability is reduced to certain characteristic patterns by application of Empirical Orthogonal Functions analysis. The coefficients of these patterns are then correlated to runoff and the resulting functional regressions are used for the construction of a statistical model for the distribution of salinity and SPM along the estuaries; for SPM this has not been successful.     

Uncertainty and sensitivity analyses of OH-initiated dimethyl sulphide (DMS) oxidation kinetics

A numerical experiment has been conducted on the OH-initiated tropospheric oxidation of DMS. This involved the selection of a set of reactions describing the OH-initiated oxidation kinetics and the conversion of the present level of uncertainty of the system into uncertainty ranges and distributions for the relevant system parameters (kinetic constants and initial concentrations). Uncertainties have been propagated through the model onto the output variables of interest. This has allowed (a) the uncertainty in model prediction to be quantified and compared with observations (uncertainty analysis) and (b) the relative importance of each input parameter in determining the output uncertainty to be quantified (sensitivity analysis). Output considered were the ratio of MSA/(SO₂ + H₂SO₄) concentration at a given time, the ratio SO₂/H₂SO₄, the total peroxynitrate species concentrations and the relative fraction of SO₂ and H₂SO₄ formed through the various pathways. Conditional upon the model and data assumptions underlying the experiment, the following main conclusions were drawn:

1. The possibility of direct formation of SO₃ without SO₂ as intermediate as suggested by Bandyet al. (1992) and Yinet al. (1990), involving direct thermal decomposition of CH₃SO₃ · does not seem to play a major role in the overall generation of sulphate. This is relevant to the issue of gas to particle conversion over remote areas.
2. Reaction of CH₃SOO · intermediate may be the most important pathway to the formation of SO₂.
3. The dominating peroxynitrate is CH₃S(O)₂O₂NO₂.

Through sensitivity analysis the kinetic constants have been identified which — because of their uncertainty and of their impact on the output — mostly contribute to the output uncertainty.     

Climate variability and physical forcing of the food webs and the carbon budget on panarctic shelves

Brief overviews of the Arctic’s atmosphere, ice cover, circulation, primary production and sediment regime are given to provide a conceptual framework for considering panarctic shelves under scenarios of climate variability. We draw on past ‘regional’ studies to scale-up to the panarctic perspective. Within each discipline a synthesis of salient distributions and processes is given, and then functions are noted that are critically poised and/or near transition and thereby sensitive to climate variability and change. The various shelf regions are described and distinguished among three types: inflow shelves, interior shelves and outflow shelves. Emphasis is on projected climate changes that will likely have the greatest impact on shelf-basin exchange, productivity and sediment processes including (a) changes in wind fields (e.g. currents, ice drift, upwelling and downwelling); (b) changes in sea ice distribution (e.g. radiation and wind regimes, enhanced upwelling and mixing, ice transport and scour resuspension, primary production); and (c) changes in hydrology (e.g. sediment and organic carbon delivery, nutrient supplies). A discussion is given of the key rate-controlling processes, which differ for different properties and shelf types, as do the likely responses; that is, the distributions of nutrients, organic carbon, freshwater, sediments, and trace minerals will all respond differently to climate forcing.A fundamental conclusion is that the changes associated with light, nutrients, productivity and ice cover likely will be greatest at the shelf-break and margins, and that this forms a natural focus for a coordinated international effort. Recognizing that the real value of climate research is to prepare society for possible futures, and that such research must be based both on an understanding of the past (e.g. the palaeo-record) as well as an ability to reliably predict future scenarios (e.g. validated models), two recommendations emerge: firstly, a comprehensive survey of circumpolar shelf-break and slope sediments would provide long-term synchronous records of shelf-interior ocean exchange and primary production at the shelf edge; secondly, a synoptic panarctic ice and ocean survey using heavy icebreakers, aircraft, moorings and satellites would provide the validation data and knowledge required to properly model key forcing processes at the margins.     

Current estimates of freshwater flux through Arctic and subarctic seas

As the world warms, the expectation is that the freshwater outflows from the Arctic Ocean to the North Atlantic will strengthen and may act to suppress the rate of the climatically-important Atlantic meridional overturning circulation. Hitherto, however, we have lacked the system of measurements required to estimate the totality of the freshwater flux through subarctic seas. Though observations remain patchy and rudimentary in places, we piece-together the results from recent large-scale observational programmes together with associated modelling, to establish preliminary maps of the rates and pathways of freshwater flux through subarctic seas. These fluxes are calculated according to two reference salinities, S = 34.8 to conform with the majority of estimates reported in the literature, and S = 35.2, the salinity of the inflowing Atlantic water, to calculate the freshwater balance of the ‘Arctic Mediterranean’. We find that 148 mSv of freshwater enters the Nordic Seas across its northern boundary. There it is supplemented by around 54 mSv of freshwater from Baltic runoff, Norwegian runoff, P − E and Greenland ice melt, so that the total freshwater contribution to the Nordic Seas from all sources is 202 mSv. Of this, around 51 mSv of freshwater is estimated to pass south to the deep Atlantic in the dense water overflows leaving an assumed balance of 151 mSv to leave the Nordic Seas in the upper water export through Denmark Strait. The corresponding estimate for the freshwater outflow west of Greenland is 103 mSv relative to 35.2 so that the total freshwater flux reaching the North Atlantic through subarctic seas is around 300 mSv.