A study of the atmospheric circulation between equator and 60oN during the winter and summer seasons

Summary The mean zonal and meridional wind components and the mean mass circulation in different latitudes are discussed in relation to previous studies. Divergence and vertical motion are calculated for various latitude belts. There is evidence for a strong tropical Hadley cell with a temperate latitude indirect circulation during the winter season. During summer, the northern Hadley cell is weaker and displaced poleward; a circulation in the opposite sense appears in equatorial latitudes.The regional patterns of divergence and vertical motion appear related particularly to the position of the subtropical high pressure cells. Subsidence and lower-layer divergence are characteristic of the eastern flanks of the subtropical anticyclones, while the opposite pattern prevails on its western flanks. These longitudinal contrasts appear particularly pronounced during the summer season. The three-dimensional flow pattern in the tropics is illustrated by selected trajectories for the winter and summer seasons.     

Role of melt during deformation in the deep crust

Deformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr).
Partial melting of deep crustal rocks can lead to melt-enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture.
Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt-enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt-enhanced embrittlement occurs. The occurrence of melt-enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt-enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.     

Changing predictability of Indian monsoon rainfall anomalies?

The predictability of Indian summer monsoon rainfall from pre-season circulation indices is explored from observations during 1939–91. The predictand is the all-India average of June–September precipitation NIR, and the precursors examined are the latitude position of the 500 mb ridge along 75°E in April (L), the pressure tendency April minus January at Darwin (DPT), March-April-May temperature at six stations in west central India (T6), the sea surface temperature (SST) anomaly in the northeastern Arabian Sea in May (ASM), SST anomaly in the Arabian Sea in January (ANJ), northern hemisphere temperature anomaly in January–February (NHT), and Eurasian snow cover in January (SNOW). Monsoon rainfall tends to be enhanced with a more northerly ridge position, small Darwin pressure tendency, warmer pre-season conditions, and reduced winter snow cover. However, relationships have varied considerably over the past half-century, with the strongest associations during 1950–80, and a drastic weakening in the 1980s. Four prediction models were constructed based on stepwise multiple regression, using as predictors combinations of L, DPT, T6, ASM, and NHT, with 1939–68 as “dependent” dataset, or training period, and 1969–91 as “independent” dataset or verification period. For the 1969–80 portion of the verification period calculated and observed NIR values agreed closely, with the models explaining 74–79% of the variance. By contrast, after 1980 predictions deteriorated drastically, with the explained variance for the 1969–89 time span dropping to 25–31%. The monsoon rainfall of 1990 and 1991 turned out to be again highly predictable from models based on stepwise multiple regression and linear discriminant analysis and using as input L + DPT or L + DPT + NHT, and with this encouragement an experimental real-time forecast was issued of the 1992 monsoon rainfall. These results underline the need for investigations into decadal-scale changes in the general circulation setting and raise concern for the continued success of seasonal forecasting.     

Correlations and sequence stratigraphic model for Messinian carbonate platforms of the western and central Mediterranean

Distance correlations of Late Tortonian–Messinian littoral carbonate complexes are proposed from the study of eight platforms in the western and central Mediterranean. Correlations are based on the identification of two major biological sedimentary cycles and of two index surfaces. Surface A is a maximum flooding surface during cycle 1 at around 6.7 Ma. Surface B is a regional marine planation surface at around 5.95 Ma, at the base of cycle 2 (Terminal Carbonate Complex). A general sedimentary model is proposed for the 7–5.6-Ma time-span. The boundary between cycles 1 and 2 is coincident with the onset of the Messinian Salinity Crisis, and appears to be related to major environmental–paleo-oceanographic changes in the Mediterranean, rather than to a major sea-level drop or to climatic change.     

Screening and ranking of sedimentary basins for sequestration of CO<Subscript>2</Subscript> in geological media in response to climate change

Sedimentary basins are suitable to different degrees for CO ₂ geological sequestration as a result of various intrinsic and extrinsic characteristics, of which the geothermal regime is one of the most important. Warm basins are less favorable for CO ₂ sequestration than cold basins because of reduced capacity in terms of CO ₂ mass, and because of higher CO ₂ buoyancy, which drives the upward CO ₂ migration. A set of 15 criteria, with several classes each, has been developed for the assessment and ranking of sedimentary basins in terms of their suitability for CO ₂ sequestration. Using a parametric normalization procedure, a basin's individual scores are summed to a total score using weights that express the relative importance of different criteria. The total score is ranked to determine the most suitable basin or region thereof for the geological sequestration of CO ₂. The method is extremely flexible in that it allows changes in the functions that express the importance of various classes for any given criterion, and in the weights that express the relative importance of various criteria. Examples of application are given for Canada's case and for the Alberta basin in Canada.     

Couplex Benchmark Computations Obtained with the Software Toolbox UG

This paper describes the numerical results for the COUPLEX benchmark obtained with the simulation software UG using vertex centered finite volume and higher order discontinuous Galerkin schemes. Multigrid solvers on unstructured grids, local mesh refinement and parallel computation are employed to yield very accurate solutions. Since the full range of results required in the benchmarks is too large to be displayed in this paper we focus on the comparison of discretization schemes, assessment of numerical errors and the presentation of parallel computations.     

Reef development on a deepening platform: two Early Cretaceous coralgal patch reefs (Catí, Llàcova Formation, eastern Spain) compared

Two coralgal patch reefs of the Hauterivian Llàcova Formation (Maestrat Basin, eastern Spain), exposed at two consecutive stratigraphic levels within a single section, have been studied to document taxonomic implications of a changing environment. These two reefal palaeocommunities differ substantially in coral taxonomic composition, microbialite formation pattern and in abundance and composition of encrusters and bioeroders. Of a total of 14 coral species, just one (Stylina parvistella) occurs in both reefs, yet is abundant in the (lower) reef A and rare, occurring near the reef base, in the reef B assemblage. Reef A is dominated by a phototrophic fauna and coral species with small corallites and imperforate septa (a stylinid-thamnasteriid-heterocoeniid-actinastreid association), along with an encruster association dominated by Bacinella and Lithocodium. Reef B is characterised by a balanced phototrophic-heterotrophic fauna that gradually passes into a heterotrophic-dominated assemblage. During this latest growth stage, microsolenid corals dominated the assemblage. The encruster fauna is characterised by sponges, polychaetes and bryozoans. Moderate deepening during a transgressive systems tract (TST) depositional sequence and elevated nutrient supply are interpreted to represent the driving environmental parameters that caused faunal compositions to differ between these two reefal palaeocommunities. Nine coral taxa, previously known only from younger (Barremian–Cenomanian) strata, have been identified, namely Dimorphocoenia? rudis, Eocomoseris raueni, Eocomoseris sp., Holocoenia jaccardi, Latusastrea irregularis, Mesomorpha sp., Microsolena kugleri, Polyphylloseris mammillata and Polyphylloseris sp. This observation emphasises the importance of the Hauterivian Stage as a period of evolutionary transition in Late Jurassic–Cretaceous coral faunas.     

Anaerobic ammonium oxidation by nitrite (anammox): Implications for N2 production in coastal marine sediments

The respiratory reduction of nitrate (denitrification) is acknowledged as the most important process that converts biologically available nitrogen to gaseous dinitrogen (N₂) in marine ecosystems. Recent findings, however, indicate that anaerobic ammonium oxidation by nitrite (anammox) may be an important pathway for N₂ formation and N removal in coastal marine sediments and in anoxic water columns of the oceans. In the present study, we explored this novel mechanism during N mineralization by ¹⁵N amendments (single and coupled additions of ¹⁵NH₄⁺, ¹⁴NO₃⁻ and ¹⁵NO₃⁻) to surface sediments with a wide range of characteristics and overall reactivity. Patterns of ^29/30N₂ production in the pore water during closed sediment incubations demonstrated anammox at all 7 of the investigated sites. Stoichiometric calculations revealed that 4% to 79% of total N₂ production was due to this novel route. The relative importance of anammox for N₂ release was inversely correlated with remineralized solute production, benthic O₂ consumption, and surface sediment Chl a. The observed correlations indicate competition between reductants for pore water nitrite during early diagenesis and that additional factors (e.g. availability of Mn-oxides), superimposed on overall patterns of diagenetic activity, are important for determining absolute and relative rates of anammox in coastal marine sediments.