The nonlinear association between ENSO and the Euro-Atlantic winter sea level pressure

A nonlinear projection of the tropical Pacific sea surface temperature anomalies (SSTA) onto the Northern Hemisphere winter sea level pressure (SLP) anomalies by neural networks (NN) was performed to investigate the nonlinear association between El Niño-Southern Oscillation (ENSO) and the Euro-Atlantic winter climate. While the linear impact of ENSO on the Euro-Atlantic winter SLP is weak, the NN projection reveals statistically significant SLP anomalies over the Euro-Atlantic sector during both extreme cold and warm ENSO episodes, suggesting that the Euro-Atlantic climate mainly responds to ENSO nonlinearly. The nonlinear response, mainly a quadratic response to the SSTA, reveals that regardless of the sign of the SSTA, positive SLP anomalies are found over the North Atlantic, stretching from eastern Canada to Europe (with anomaly center located just northwestward of Portugal), and negative anomalies centered over Scandinavia and Norwegian Sea, consistent with the excitation of the positive North Atlantic Oscillation pattern.     

Model studies of the impact of chemical inhomogeneity on SO2 oxidation in warm stratiform clouds

A one-dimensional, time-dependent model of the physics and chemistry of a warm stratiform cloud is used to study the possible impact of chemical inhomogeneity among cloud and raindrops on the oxidation of SO₂ in clouds. The effects of chemical inhomogeneity are examined using two contrasting models: In Model 1 a bulk-solution parameterization is adopted which effectively treats all cloud and raindrops as if they are chemically homogeneous; in Model 2 we allow the cloud and raindrops to have a dichotomous distribution. The dichotomous distribution in Model 2 is simulated by assuming that the two groups of cloud droplets nucleate from two chemically distinct populations of condensation nuclei; one being acidic and the other being alkaline. While the two models yield essentially identical results when the ambient levels of H₂O₂ are greater than the ambient levels of SO₂, the rate of conversion of SO₂ to sulfuric acid and the amount of sulfate removed in the precipitation can be significantly enhanced in Model 2 over that of Model 1 under conditions of oxidant limitation (i.e., H₂O₂ < SO₂). This enhancement is critically dependent upon the fraction of alkaline nuclei assumed to be present in Model 2 and arises from the rapid increase in the aqueous-phase reaction between O₃+S_IV at high pH. Our results suggest that cloud models which adopt a bulk-solution parameterization for cloud droplet chemistry, may underestimate the amount of in-cloud SO₂ oxidation under oxidant-limited conditions.     

Transition from wavelets to ripples in a laboratory flume with a diverging channel

An experimental investigation on the initiation and development of bed forms on a bed of fine silica sand was conducted under alluvial flow conditions in a laboratory flume with a diverging channel. The main aims of the study were to assess： i） the steepness of bed forms in the transition stage of development; and ii） the threshold height of wavelets （ηt） that triggered the start of ripple development. Detailed bed profile measurements were carried out using an acoustic Doppler probe, traversed longitudinally over the sediment bed at various experimentation times. The bed form dimensions were extracted from such bed profile records and analysed for the wavelet, transition and equilibrium stages. It was found that the steepness of ripples in the transition and equilibrium stages were similar, confirming predictions of previous mathematical model simulations. A lognormal distribution fitted the wavelet length data. The wavelet threshold height was estimated as ηt ≈ 7 mm, or ηt≈ 80 in wall units. Such a height magnitude suggested that ripple development could be triggered by the wavelets reaching the outer flow zone of a turbulent boundary layer. The ηt value obtained corresponded generally to the intersection point between two predictive equations for bed form dimensions. A formulation was developed to predict ηt as a function of the sediment grain size, which was confirmed for the fine sand used in this study.     

Deglacial to postglacial palaeoenvironments of the Celtic Sea: lacustrine conditions versus a continuous marine sequence

Recent work on the last glaciation of the British Isles has led to an improved understanding of the nature and timing of the retreat of the British?Irish Ice Sheet (BIIS) from its southern maximum (Isles of Scilly), northwards into the Celtic and Irish seas. However, the nature of the deglacial environments across the Celtic Sea shelf, the extent of subaerial exposure and the existence (or otherwise) of a contiguous terrestrial linkage between Britain and Ireland following ice retreat remains ambiguous. Multiproxy research, based on analysis of 12 BGS vibrocores from the Celtic Deep Basin (CDB), seeks to address these issues. CDB cores exhibit a shell‐rich upward fining sequence of Holocene marine sand above an erosional contact cut in laminated muds with infrequent lonestones. Molluscs, in situ Foraminifera and marine diatoms are absent from the basal muds, but rare damaged freshwater diatoms and foraminiferal linings occur. Dinoflagellate cysts and other non‐pollen palynomorphs evidence diverse, environmentally incompatible floras with temperate, boreal and Arctic glaciomarine taxa co‐occurring. Such multiproxy records can be interpreted as representing a retreating ice margin, with reworking of marine sediments into a lacustrine basin. Equally, the same record may be interpreted as recording similar conditions within a semi‐enclosed marine embayment dominated by meltwater export and deposition of reworked microfossils. As assemblages from these cores contrast markedly with proven glaciomarine sequences from outside the CDB, a glaciolacustrine interpretation is favoured for the laminated sequence, truncated by a Late Weichselian transgressive sequence fining upwards into fully marine conditions. Reworked rare intertidal molluscs from immediately above the regional unconformity provide a minimum date c. 13.9 cal. ka BP for commencement of widespread marine erosion. Although suggestive of glaciolacustrine conditions, the exact nature and timing of laminated sediment deposition within the CDB, and the implications this has on (pen)insularity of Ireland following deglaciation, remain elusive.     

The Požarevac loess–paleosol sequence: a record of increased aridity in the south-eastern margin of the Carpathian Basin during the last 350 ka

The loess sequence preserved in the Požarevac brickyard in north-eastern Serbia comprises eight loess units separated by seven paleosols. Geochronological investigation using amino acid racemization and luminescence dating support stratigraphic correlations of loess units L3, S2LL1 and L1 at the Požarevac section with loess of glacial cycles E [Marine Isotope Stage (MIS) 10], D (MIS 9–8), C (MIS 7–6) and B (MIS 5–2) across central Europe. Correlation with the marine oxygen-isotope stratigraphy and associated paleoclimatic inferences are further supported by magnetic susceptibility, particle size and carbonate content measured in Požarevac sediments. Malacological investigations at the Požarevac section reveal the continuous presence of the Chondrula tridens and Helicopsis striata faunal assemblages throughout the last 350 ka. The loess malacological fauna, which is characterized by the complete absence of cold-resistant and cold-preferring species, suggests a stable, dry and relatively warm glacial and interglacial climate, compared with other central European loess localities. Together these data suggest that the south-eastern part of the Carpathian (Pannonian, Middle Danube) Basin was a refugium for warm-preferring and xerophilous land-snails during the generally unfavorable glacial climates of the late Middle and Late Pleistocene.     

Linearization of in situ stress change formulae for gauges at arbitrary down-hole orientation

The vibrating wire stress gauge is a borehole device for measuring stress changes in situ. Three gauge arrays are required for determining changes in the secondary principal stresses. Standard data reduction formulae that use the principal axes for reference are in error because of possible changes in the principal directions. Data reduction formulae that allow for such changes and for arbitrary gauge orientations within an array are presented here. This is of considerable practical importance when installing gauges because of scheduling restrictions and hole irregularities that may preclude installation at the planned angles for which data reduction formulae exist. Linearization and thus direct inversion of the resulting system of equations, is achieved in a fixed but otherwise arbitrarily oriented Cartesian system of reference.     

Geomagnetic reversal history

The history of geomagnetic polarity reversals in the Cenozoic and Late Mesozoic is well known since the Late Jurassic (Oxfordian). A continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies. Most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies. This has led to the generation of progressively refined and improved geomagnetic reversal time-scales that provide a framework for absolute dating of palaeontological zonations. By serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the Earth's core. The rate of reversals since the Late Cretaceous shows a steady increase on which a cyclical variation appears to be superposed. A stochastic model for reversals predicts a Poisson distribution of polarity interval lengths. The polarity time scales contain many fewer short (± 50 kyr) polarity chrons than a Poisson distribution, and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model. The statistical arguments give no convincing reason for abandoning the model and other, physical reasons must be sought to explain the incompleteness of the reversal record. The discovery and verification of short chrons in the oceanic record may best be investigated by deep-tow magnetometer surveys. The reversal history before the Late Jurassic is not well known. Magnetostratigraphy in coeval Early Jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence. Evaluation of geomagnetic polarity history for the Early Mesozoic and the Palaeozoic will require unambiguous magnetostratigraphy in well-dated sections where verification of the polarity pattern is possible at the fossil zone or stage level.