A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO

This study investigates how accurately the interannual variability over the Indian Ocean basin and the relationship between the Indian summer monsoon and the El Niño Southern Oscillation (ENSO) can be simulated by different modelling strategies. With a hierarchy of models, from an atmospherical general circulation model (AGCM) forced by observed SST, to a coupled model with the ocean component limited to the tropical Pacific and Indian Oceans, the role of heat fluxes and of interactive coupling is analyzed. Whenever sea surface temperature anomalies in the Indian basin are created by the coupled model, the inverse relationship between the ENSO index and the Indian summer monsoon rainfall is recovered, and it is preserved if the atmospherical model is forced by the SSTs created by the coupled model. If the ocean model domain is limited to the Indian Ocean, changes in the Walker circulation over the Pacific during El-Niño years induce a decrease of rainfall over the Indian subcontinent. However, the observed correlation between ENSO and the Indian Ocean zonal mode (IOZM) is not properly modelled and the two indices are not significantly correlated, independently on season. Whenever the ocean domain extends to the Pacific, and ENSO can impact both the atmospheric circulation and the ocean subsurface in the equatorial Eastern Indian Ocean, modelled precipitation patterns associated both to ENSO and to the IOZM closely resemble the observations.     

The significance of Mn‐rich ilmenite and the determination of P–T paths from zoned garnet in metasedimentary rocks from the western Cape Breton Highlands,Nova Scotia

The Jumping Brook Metamorphic Suite in the western Cape Breton Highlands of Nova Scotia is part of an inverted Barrovian sequence that formed during a Late Silurian–Early Devonian promontory–promontory collision in the Canadian Appalachians. In this study, systematic discrepancies between geochemical observations and thermodynamic model predictions led to the discovery of a systematic relationship linking the style of garnet core isopleth intersection (GCII) to the pyrophanite (MnTiO₃) component of co‐existing ilmenite. Samples that yielded tight GCIIs at or near the garnet‐in curve were found to contain ilmenite with negligible pyrophanite components, whereas samples yielding GCIIs far removed (up to 105°C) from the garnet‐in curve were found to contain ilmenite with significant pyrophanite and/or ecandrewsite (ZnTiO₃) components. Based on petrographic and geochemical observations, Mn(±Zn)‐rich ilmenite are interpreted to have sequestered Mn throughout prograde metamorphism due to sluggish intracrystalline diffusion. The amount of reactive Mn input into the thermodynamic models from whole‐rock analyses were, in some cases, overestimated, resulting in garnet‐in curve topologies that extend to erroneously low P–T conditions. Modifications to the whole‐rock chemistry that account for Mn sequestration into ilmenite, however, yielded robust model results. Our results show that, in addition to uncertainties in thermodynamic data sets and phenomenon related to reaction kinetics, Mn‐rich ilmenite may superimpose additional complexities related to the interpretation of predicted equilibria involving garnet. Numerical simulations of garnet crystallization were used to infer P–T paths of metamorphism for one sample from the garnet zone (Mn corrected) and two samples from the staurolite zone (Mn uncorrected) of the inverted sequence. Model results are remarkably similar among the three samples and indicate that garnet crystallization occurred along relatively steep (31–37°C/km) clockwise P–T paths. The peak conditions of garnet crystallization and metamorphism (560–590°C, 7.4–8.0 kbar) are interpreted to have been attained approximately simultaneously, such that the paths are characterized by tight prograde‐to‐retrograde transitions. The hairpin nature of the P–T paths is interpreted to represent the onset of thrust‐related exhumation and isograd inversion along ductile shear zones, consistent with available field and geochronological constraints.     

The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm⁻¹ spectral range with apodized resolutions of 2.54 or 0.53 cm⁻¹). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO₂. A firm detection of benzene (C₆H₆) is provided by CIRS at levels of about 3.5×¹⁰⁻⁹ around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν₄ band at 1304 cm⁻¹ and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C₂H₂, C₂H₆, C₃H₈, C₂H₄ and CO₂ to a significant enhancement at high northern (early winter) latitudes for HCN, HC₃N, C₄H₂, C₃H₄ and C₆H₆. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C₂H₂, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH₃D band at 1156 cm⁻¹ and found to be . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.     

Impacts of changing climate on the hydrology and hydropower production of the Tagus River basin

The Tagus River basin is an ultimately important water source for hydropower production, urban and agricultural water supply in Spain and Portugal. Growing electricity and water supply demands, over‐regulation of the river and construction of new dams, as well as large inter‐basin and intra‐basin water transfers aggravated by strong natural variability of climate in the catchment, have already imposed significant pressures on the river. The substantial reduction of discharge is observed already now, and projected climatic change is expected to alter the water budget of the catchment further.In this study, we address the effects of projected climate change on the water resources availability in the Tagus River basin and influence of potential changes on hydropower generation of the three important reservoirs in the basin. The catchment‐scale, process‐based eco‐hydrological model soil and water integrated model was set up, calibrated and validated for the entire Tagus River basin, taking into account 15 large reservoirs in the catchment. The future climate projections were selected from those generated within the Inter‐Sectoral Impact Model Intercomparison Project. They include five bias‐corrected climatic datasets for the region, obtained from global circulation model runs under two emissions scenario – moderate and extreme ones – and covered the whole century. The results show a strong agreement among model runs in projecting substantial decrease of discharge of the Tagus River discharge and, consequently, a strong decrease in hydropower production under both future climate scenarios. Copyright © 2016 John Wiley & Sons, Ltd.     

Guano core evidence of palaeoenvironmental change and Woodland Indian inhabitance in Fern Cave,Alabama, USA,from the mid‐Holocene to present

Bat guano cores have been used as a source of palaeoenvironmental information to aid in the reconstruction of past climates and vegetation. We collected a 104‐cm‐long (43 cm compacted) guano core from Fern Cave, Alabama, USA, that provided a c. 6000‐year record of guano accumulation. Pollen, nutrients (C, N, P) and stable isotopes (δ¹³C, δ¹⁵N) were measured on the guano core with the objective of reconstructing the environmental history of the area from the mid‐Holocene to present. Our data indicate that bats have utilized Fern Cave for at least 6000 years and that Woodland Indians also utilized the cave for a short period. A 3‐cm charcoal layer was dated to 2720±30 cal. a BP and inferred to be Woodland Indian in origin from microscopic inspection and thickness. Pollen and geochemical data showed that bat diets changed in the late Holocene possibly linked to food supply and climate changes. These results demonstrate that guano cores are a useful tool of palaeoenvironmental reconstruction when other forms of palaeorecords do not exist and can add to local archaeological information.     

A 50-year record of nitrate concentrations in the Slapton Ley Catchment,Devon, United Kingdom

Slapton Ley, a coastal lake, is the largest natural body of fresh water in south-west England. There was concern in the 1960s that the lake was becoming increasingly eutrophic. To quantify inputs of water, sediment and nutrients into the lake, Slapton Ley Field Centre initiated a programme of weekly water quality sampling in September 1970. Of all the chemical properties which have been measured over the decades, the nitrate record has been the subject of more research than any other. The weekly monitoring has been supplemented by research projects aimed at understanding aspects of processes and patterns of nitrate delivery to the stream network. Three aspects of the nitrate record are reviewed: short-term process dynamics; the annual cycle of influent streams and the lake itself; and long-term trends. In the first two decades of monitoring, there was increasing concern about a trend of rising nitrate concentrations, an issue in most lowland rivers in the United Kingdom at the time. In the 1990s, nitrate concentrations levelled off and then have fallen steadily in recent years. In relation to eutrophication, there are clear signs of improvement in the influent streams, but concerns remain about water quality in the lake itself.     

Conduit convection,magma mixing,and melt inclusion trends at persistently degassing volcanoes

Magmas progressively exsolve volatiles as they ascend towards the Earth's surface, such that their volatile content is a function of pressure. Water and carbon dioxide concentrations measured in melt inclusions from degassing volcanoes rarely coincide with modelled degassing trends. I show that observed melt inclusion trends can be reproduced through mixing of magmas, either during convection within the volcanic conduit, or within a subterranean magma reservoir. No fluxing gas phase or post-entrapment loss of water need be invoked. A permeable network allowing gas transport is still required to avoid fragmentation of magma at shallow depths.     

Human perturbations on the global biogeochemical cycles of coupled Si–C and responses of terrestrial processes and the coastal ocean

We present a model of the global biogeochemical cycle of silicon (Si) that emphasizes its linkages to the carbon cycle and temperature. The Si cycle is a crucial part of global nutrient biogeochemistry regulating long-term atmospheric CO₂ concentrations due to silicate mineral weathering reactions involving the uptake of atmospheric CO₂ and production of riverine dissolved silica, cations and bicarbonate. In addition and importantly, the Si cycle is strongly coupled to the other nutrient cycles of N, P, and Fe; hence siliceous organisms represent a significant fraction of global primary productivity and biomass. Human perturbations involving land-use changes, burning of fossil fuel, and inorganic N and P fertilization have greatly altered the terrestrial Si cycle, changing the river discharge of Si and consequently impacting marine primary productivity primarily in coastal ocean waters.     

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding

We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.