ALBERT

Beschreibung: A series of high resolution multibeam echo sounding surveys near San Francisco, CA suggests the occasional migration of marine sandwave crests in the direction opposite to that indicated by the shape asymmetry of the individual sandwaves, in contrast to all previous observations of the relationship between bedform shape asymmetry and migration. The anti-asymmetry migration occurs over approximately the same time period that a new sand wave crest is formed within the large and relatively stable field. The new sandwave crest appears approximately midway between two larger crests, in the vicinity of structural defects in a nearly two-dimensional portion of the field. The sandwaves in the vicinity of the new crest are found to have migrated away from the new crest regardless of their shape asymmetry. Later a large section of the new crest disappears, and the neighboring sandwaves migrated back toward the vacated crest location. This migration occurs for sandwaves up to at least ten wavelengths away from the new crest.

Beschreibung: The Irish landmass, now at the western extremity of the Eurasian Plate, was formed in the Caledonian Orogeny during the Palaeozoic assembly of Pangea. The associated closure of the Iapetus Ocean is recorded in the NE–SW structural trends that dominate the tectonic set-up of Ireland today. The deep-crustal dynamics of the orogeny and the effect on the crust of the subsequent extension and magmatism in the North Atlantic are debated. Fabrics within deep crustal rocks preserve a record of deformation during and after the continental collisions. Here, we measured Rayleigh-wave phase velocities using seismograms recorded by permanent and temporary intermediate-band stations in Ireland and inverted the data for phase-velocity maps, including azimuthal anisotropy. The observed isotropic phase-velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across Ireland. Anisotropy of Rayleigh waves at 10–20 s periods shows a NE–SW fast-propagation direction and is largest (up to 2%) at a 15 s period, at which Rayleigh waves sample primarily the middle and lower crust. The NE–SW trend of the deep-crustal anisotropic fabric is parallel to tectonic trends, in particular the Iapetus Suture Zone, which indicates that suture-parallel flow in the middle and lower crust accommodated the continental collision. The apparent preservation of the Caledonian-age fabric also shows that the deep crust of the Eurasian margin in Ireland was neither stretched by the NW–SE extension associated with the opening of the North Atlantic, nor modified significantly by the Cenozoic magmatism in the region.

Beschreibung: Results from the first assessment of air quality over the Canadian oil sands–one of the largest industrial undertakings in human history–using satellite remote sensing observations of two pollutants, nitrogen dioxide (NO2) and sulfur dioxide (SO2), are presented. High-resolution maps were created that revealed distinct enhancements in both species over an area (roughly 30 km × 50 km) of intensive surface mining at scales of a few kilometers. The magnitude of these enhancements, quantified in terms of total mass, are comparable to the largest seen in Canada from individual sources. The rate of increase in NO2 between 2005 and 2010 was assessed at 10.4 ± 3.5%/year and resulted from increases both in local values as well as the spatial extent of the enhancement. This is broadly consistent with both surface-measurement trends and increases in annual bitumen production. An increase in SO2 was also found, but given larger uncertainties, it is not statistically significant.

Beschreibung: The diurnal cycle in the oceanic surface winds in the tropical eastern Pacific is shown, through numerical experiments with a regional atmospheric model, to be associated with the migrating diurnal atmospheric thermal tide, forced by absorption of solar near-IR radiation by tropospheric water vapor, and a topographically-modified extended sea-breeze, forced by diurnal land heating. Idealized experiments prove capable of discriminating the effects of both processes, showing that beyond 2000 km from the coast, the thermal tide is dominant, while closer to the coast both processes are of the same order. The shortwave forcing due to water vapor is also found to produce a diurnal cycle in precipitation, but the process appears to be independent from the thermal tide and it is proposed that this effect is mediated by the radiatively-forced changes in the column stability.

Beschreibung: In the summer 2010 Western Russia was hit by an extraordinary heat wave, with the region experiencing by far the warmest July since records began. Whether and to what extent this event is attributable to anthropogenic climate change is controversial. Dole et al. (2011) report the 2010 Russian heat wave was “mainly natural in origin” whereas Rahmstorf and Coumou (2011) write that with a probability of 80% “the 2010 July heat record would not have occurred” without the large-scale climate warming since 1980, most of which has been attributed to the anthropogenic increase in greenhouse gas concentrations. The latter explicitly state that their results “contradict those of Dole et al. (2011).” Here we use the results from a large ensemble simulation experiment with an atmospheric general circulation model to show that there is no substantive contradiction between these two papers, in that the same event can be both mostly internally-generated in terms of magnitude and mostly externally-driven in terms of occurrence-probability. The difference in conclusion between these two papers illustrates the importance of specifying precisely what question is being asked in addressing the issue of attribution of individual weather events to external drivers of climate.

Beschreibung: Current consensus on global climate change predicts warming trends driven by anthropogenic forcing, with maximum temperature changes projected in the Northern Hemisphere (NH) high latitudes during winter. Yet, global temperature trends show little warming over the most recent decade or so. For longer time periods appropriate to the assessment of trends, however, global temperatures have experienced significant warming trends for all seasons except winter, when cooling trends exist instead across large stretches of eastern North America and northern Eurasia. Hence, the most recent lapse in global warming is a seasonal phenomenon, prevalent only in boreal winter. Additionally, we show that the largest regional contributor to global temperature trends over the past two decades is land surface temperatures in the NH extratropics. Therefore, proposed mechanisms explaining the fluctuations in global annual temperatures should address this apparent seasonal asymmetry.

Beschreibung: The behaviour of hydraulically ‘tight’ barrier rocks is a key determinant of the long-term integrity of potential underground storage sites for the waste products from low-carbon emission energy production technologies (including nuclear waste and CO2 captured from fossil fuels). Here we isolate the relationship between crack-induced permeability and porosity using an initially crack-free natural crystalline material. We vary secondary porosity from an initial value of zero, and demonstrate that the bulk permeability K varies with total connected porosity Φ above the percolation threshold Φc as K = K0(Φ − Φc)n, where n = 3.8 ± 0.4, i.e., similar to results obtained for higher porosity rocks, indicating universality of this scaling law. Close to the percolation threshold a modest change in total porosity from 1% to 5% or so results in a massive change in permeability of 7 orders of magnitude or more. The results are consistent with a continuum percolation model that reflects the microstructure of the pore/induced microcrack network in the natural material.

Beschreibung: Unusually cold conditions in Arctic winter 2010/11 led to large stratospheric ozone loss. We investigate this with UV-visible measurements made at Eureka, Canada (80.05°N, 86.42°W) from 1999–2011. For 8–22 March 2011, OClO was enhanced, indicating chlorine activation above Eureka. Ozone columns were lower than in any other year in the record, reaching minima of 237 DU and 247 DU in two datasets. The average NO2 column inside the vortex, measured at visible and UV wavelengths, was 46 ± 30% and 45 ± 27% lower in 2011 than the average NO2 column from previous years. Ozone column loss was estimated from two ozone datasets, using a modeled passive ozone tracer. For 12–20 March 2011, the average ozone loss was 27% and 29% (99 DU and 108 DU). The largest percent ozone loss in the 11-year record of 47% (250 DU and 251 DU) was observed on 5 April 2011.

Beschreibung: The turbulent structure of airflow over a barchan sand dune is determined using quadrant analysis of wind velocity data derived from sonic anemometers. Results indicate an increased frequency of ejection and sweep events in the toe region of the dune, characteristic of the turbulent bursting process. In contrast, at the crest there was a significant increase in the occurrence of outward interactions. Combined with high frequency saltation data our analyses show that turbulent structures characterised by a positive streamwise fluctuating velocity (+u′; sweeps at the toe and outward interactions at the crest) have a dominant influence on sand transport on the dune, together accounting for up to 83% and 95% of transporting events at the toe and crest respectively.

Beschreibung: Arctic sea ice cover has decreased dramatically over the last three decades. Global climate models under-predicted this decline, most likely a result of the misrepresentation of one or more processes that influence sea ice. The cloud feedback is the primary source of uncertainty in model simulations, especially in the polar regions. A better understanding of the interaction between sea ice and clouds, and specifically the impact of decreased sea ice on cloud cover, will provide valuable insight into the Arctic climate system and may ultimately help in improving climate model parameterizations. In this study, an equilibrium feedback assessment is employed to quantify the relationship between changes in sea ice and clouds, using satellite-derived sea ice concentration and cloud cover over the period 2000–2010. Results show that a 1% decrease in sea ice concentration leads to a 0.36–0.47% increase in cloud cover, suggesting that a further decline in sea ice cover will result in an even cloudier Arctic.

Beschreibung: Over historic time Hawai‘i's dryland forests have been largely replaced by grasslands for grazing livestock. On-going efforts have been undertaken to restore dryland forests to bring back native species and reduce erosion. The reestablishment of native ecosystems on land severely degraded by long-term alternative use requires reversal of the impacts of erosion, organic-matter loss, and soil structural damage on soil hydraulic properties. This issue is perhaps especially critical in dryland forests where the soil must facilitate native plants' optimal use of limited water. These reforestation efforts depend on restoring soil ecological function, including soil hydraulic properties. We hypothesized that reforestation can measurably change soil hydraulic properties over restoration timescales. At a site on the island of Maui (Hawai‘i, USA), we measured infiltration capacity, hydrophobicity, and abundance of preferential flow channels in a deforested grassland and in an adjacent area where active reforestation has been going on for fourteen years. Compared to the nearby deforested rangeland, mean field-saturated hydraulic conductivity in the newly restored forest measured by 55 infiltrometer tests was greater by a factor of 2.0. Hydrophobicity on an 8-point scale increased from average category 6.0 to 6.9. A 4-point empirical categorization of preferentiality in subsurface wetting patterns increased from an average 1.3 in grasslands to 2.6 in the restored forest. All of these changes act to distribute infiltrated water faster and deeper, as appropriate for native plant needs. This study indicates that vegetation restoration can lead to ecohydrologically important changes in soil hydraulic properties over decadal time scales.

Beschreibung: This bi-polar analysis resolves ice edge changes on space/time scales relevant for investigating seasonal ice-ocean feedbacks and focuses on spatio-temporal changes in the timing of annual sea ice retreat and advance over 1979/80 to 2010/11. Where Arctic sea ice decrease is fastest, the sea ice retreat is now nearly 2 months earlier and subsequent advance more than 1 month later (compared to 1979/80), resulting in a 3-month longer summer ice-free season. In the Antarctic Peninsula and Bellingshausen Sea region, sea ice retreat is more than 1 month earlier and advance 2 months later, resulting in a more than 3-month longer summer ice-free season. In contrast, in the western Ross Sea (Antarctica) region, sea ice retreat and advance are more than 1 month later and earlier respectively, resulting in a more than 2 month shorter summer ice-free season. Regardless of trend magnitude or direction, and at latitudes mostly poleward of 70° (N/S), there is strong correspondence between anomalies in the timings of sea ice retreat and subsequent advance, but little correspondence between advance and subsequent retreat. These results support a strong ocean thermal feedback in autumn in response to changes in spring sea ice retreat. Further, model calculations suggest different net ocean heat changes in the Arctic versus Antarctic where autumn sea ice advance is 1 versus 2 months later. Ocean-atmosphere changes, particularly in boreal spring and austral autumn (i.e., during ∼March-May), are discussed and compared, as well as possible inter-hemispheric climate connections.

Beschreibung: Monsoons, the most energetic tropical climate system, exert a great social and economic impact upon billions of people around the world. The global monsoon precipitation had an increasing trend over the past three decades. Whether or not this increasing trend will continue in the 21st century is investigated, based on simulations of three high-resolution atmospheric general circulation models that were forced by different future sea surface temperature (SST) warming patterns. The results show that the global monsoon area, precipitation and intensity all increase consistently among the model projections. This indicates that the strengthened global monsoon is a robust signal across the models and SST patterns explored here. The increase of the global monsoon precipitation is attributed to the increases of moisture convergence and surface evaporation. The former is caused by the increase of atmospheric water vapor and the latter is due to the increase of SST. The effect of the moisture and evaporation increase is offset to a certain extent by the weakening of the monsoon circulation.

Beschreibung: The Big Dry, a recent drought over southeast Australia, began around 1997 and continued until 2011. We show that between 2002–2010, instead of a localized drought, there was a continent-wide reduction in water storage, vegetation and rainfall, spanning the northwest to the southeast of Australia. Trends in water storage and vegetation were assessed using Gravity Recovery and Climate Experiment (GRACE) and Normalized Difference Vegetation Index (NDVI) data. Water storage and NDVI are shown to be significantly correlated across the continent and the greatest losses of water storage occurred over northwest Australia. The frequency of tropical cyclones over northwest Australia peaked just prior to the launch of the GRACE mission in 2002. Indeed, since 1981, decade-scale fluctuations in tropical cyclone numbers coincide with similar variation in rainfall and vegetation over northwest Australia. Rainfall and vegetation in southeast Australia trended oppositely to the northwest prior to 2001. Despite differences between the northwest and southeast droughts, there is reason to believe that continental droughts may occur when the respective climate drivers align.

Beschreibung: To investigate the effect of photochemical aging on the stable carbon isotopic ratio (δ13C) of oxalic acid (OxA), a dominant organic species in atmospheric aerosols, we conducted a laboratory photolysis of OxA under H2O2-Fe3+(Fe2+)-UV system in aqueous phase and measured δ13C of remaining OxA. Our results showed that a significant photolysis of OxA occurred with OH radical but the isotopic fractionation of OxA was insignificant. In contrast, in the presence of Fe3+ (Fe2+), we found a significant enrichment of 13C in remaining OxA. We also found that kinetic isotope effect (KIE) of OxA largely depends on photochemical age (irradiation time) and concentration ratios of OxA to iron; 3.20 ± 0.49‰ (2.18 ± 1.18‰) and 21.62 ± 5.41‰ in 90 min and 180 min irradiation, in which OxA and Fe3+ (Fe2+) ratios were 50:1 and 200:1, respectively. The enrichment of 13C in remaining OxA was more significant during the photolysis catalyzed by Fe3+ (7‰) than by Fe2+ (3‰) in 90 min irradiation when OxA and iron ratios are the same (50:1). This study provides a laboratory evidence for the isotopic enrichment of 13C in OxA with photochemical aging. This approach is useful for better interpretation of atmospheric isotopic measurements in terms of the extent of atmospheric processing of aerosols.

Beschreibung: Lake Superior, the northern-most of the Laurentian Great Lakes, is the largest (by surface area) freshwater lake on the planet. Due in part to its high water surface to land area ratio, over one-third of the Lake Superior basin water budget is derived from precipitation falling directly on the lake surface. For most of the Great Lakes (including Lake Superior), historical precipitation estimates extend back to the early 1880s, and are based primarily on land-based gauge measurements. While alternatives to gauge-based estimates have been explored, there is no clear history of applying regional climate models (RCMs) to improve historical over-lake precipitation estimates. To address this gap in regional research, and to advance the state-of-the-art in Great Lakes regional hydrological modeling, we compare 21 years of output (1980–2000) from an RCM to conventional gauge-based precipitation estimates for the same time period over the Lake Superior basin. We find that the RCM, unlike the gauge-based method, simulates realistic variations in over-lake atmospheric stability, which propagate into basin-wide precipitation estimates with a relatively low over-lake to over-land precipitation ratio in warm months (roughly 0.7 to 0.8 in June, July, and August) and a relatively high over-lake to over-land precipitation ratio in cold months (roughly 1.3 to 1.4 in December and January), compared to gauge-based estimates. Our findings underscore a need to potentially update historical gauge-based precipitation estimates for large lake systems, including Lake Superior, and that RCMs appear to provide a robust and defensible basis for making those updates.

Beschreibung: The failure of frictional interfaces and the spatiotemporal structures that accompany it are central to a wide range of geophysical, physical and engineering systems. Recent geophysical and laboratory observations indicated that interfacial failure can be mediated by slow slip rupture phenomena which are distinct from ordinary, earthquake-like, fast rupture. These discoveries have influenced the way we think about frictional motion, yet the nature and properties of slow rupture are not completely understood. We show that slow rupture is an intrinsic and robust property of simple non-monotonic rate-and-state friction laws. It is associated with a new velocity scale cmin, determined by the friction law, below which steady state rupture cannot propagate. We further show that rupture can occur in a continuum of states, spanning a wide range of velocities from cmin to elastic wave-speeds, and predict different properties for slow rupture and ordinary fast rupture. Our results are qualitatively consistent with recent high-resolution laboratory experiments and may provide a theoretical framework for understanding slow rupture phenomena along frictional interfaces.

Beschreibung: Total mercury (HgT) and monomethyl mercury (MMHg) concentrations in fog collected from 4 locations in and around Monterey Bay, California during June-August of 2011 were 10.7 ± 6.8 and 3.4 ± 3.8 ng L−1 respectively. In contrast, mean HgT and MMHg concentrations in rain water from March-June, 2011 were 1.8 ± 0.9 and 0.1 ± 0.04 ng L−1 respectively. Using estimates of fog water deposition from 6 sites in the region using a standard fog water collector (SFC), depositions of HgT and MMHg via fog were found to range from 42–4600 and 14–1500 ng m−2 y−1, which accounted for 7–42% of HgT and 61–99% of MMHg in total atmospheric deposition (fog, rain, and dry deposition), estimated for the coastal area. These initial measurements suggest that fog precipitation may constitute an important but previously overlooked input of MMHg to coastal environments. Preliminary comparisons of these data with associated chemical, meteorological and oceanic data suggest that biotically formed MMHg from coastal upwelling may contribute to the MMHg in fog water.

Beschreibung: Model simulations performed with a three-dimensional, high-resolution, process study ocean model of eastern boundary upwelling systems are used to describe a mechanism that efficiently transports sediment-derived dissolved iron offshore in the subsurface through the bottom boundary layer (BBL) during downwelling-favorable wind events. In the model, sediment-derived iron accumulates in the BBL on the outer shelf when the winds are upwelling-favorable. When the wind reverses, the iron-laden BBL is mixed into the water column and transported offshore along isopycnals that intersect the bottom. Depending on the frequency of wind reversal, between 10–50% of the shelf sediment-derived iron flux is exported offshore through this previously unidentified subsurface pathway. If this mechanism operates on all coastal upwelling regimes, the global export of sediment-derived iron to the open ocean would be equivalent to ten times larger than the estimated source of dissolved iron from aerosols.

Beschreibung: Spanning a latitudinal range typical for deserts, the Indian peninsula is fertile instead and sustains over a billion people through monsoonal rains. Despite the strong link between climate and society, our knowledge of the long-term monsoon variability is incomplete over the Indian subcontinent. Here we reconstruct the Holocene paleoclimate in the core monsoon zone (CMZ) of the Indian peninsula using a sediment core recovered offshore from the mouth of Godavari River. Carbon isotopes of sedimentary leaf waxes provide an integrated and regionally extensive record of the flora in the CMZ and document a gradual increase in aridity-adapted vegetation from ∼4,000 until 1,700 years ago followed by the persistence of aridity-adapted plants after that. The oxygen isotopic composition of planktonic foraminifer Globigerinoides ruber detects unprecedented high salinity events in the Bay of Bengal over the last 3,000 years, and especially after 1,700 years ago, which suggest that the CMZ aridification intensified in the late Holocene through a series of sub-millennial dry episodes. Cultural changes occurred across the Indian subcontinent as the climate became more arid after ∼4,000 years. Sedentary agriculture took hold in the drying central and south India, while the urban Harappan civilization collapsed in the already arid Indus basin. The establishment of a more variable hydroclimate over the last ca. 1,700 years may have led to the rapid proliferation of water-conservation technology in south India.

Beschreibung: Hot temperatures in combination with high humidity cause human discomfort and may increase morbidity and mortality. A global climate model with an embedded urban model is used to explore the urban-rural contrast in the wet-bulb globe temperature, a heat stress index accounting for temperature and humidity. Wet-bulb globe temperatures are calculated at each model time step to resolve the heat stress diurnal cycle. The model simulates substantially higher heat stress in urban areas compared to neighbouring rural areas. Urban humidity deficit only weakly offsets the enhanced heat stress due to the large night-time urban heat island. The urban-rural contrast in heat stress is most pronounced at night and over mid-latitudes and subtropics. During heatwaves, the urban heat stress amplification is particularly pronounced. Heat stress strongly increases with doubled CO2 concentrations over both urban and rural surfaces. The tropics experience the greatest increase in number of high-heat-stress nights, despite a relatively weak ∼2°C warming. Given the lack of a distinct annual cycle and high relative humidity, the modest tropical warming leads to exceedance of the present-day record levels during more than half of the year in tropical regions, where adaptive capacity is often low. While the absolute urban and rural heat stress response to 2 × CO2 is similar, the occurrence of nights with extremely high heat stress increases more in cities than surrounding rural areas.

Beschreibung: Large-scale die-off of tidal marsh vegetation, caused by global change, is expected to change flow patterns over tidal wetlands, and hence to affect valuable wetland functions such as reduction of shoreline erosion, attenuation of storm surges, and sedimentation in response to sea level rise. This study quantified for the first time the effects of large-scale (4 ha) artificial vegetation removal, as proxy of die-off, on the spatial flow patterns through a tidal marsh channel and over the surrounding marsh platform. After vegetation removal, the flow velocities measured on the platform increased by a factor of 2 to 4, while the channel flow velocities decreased by almost a factor of 3. This was associated with a change in flow directions on the platform, from perpendicular to the channel edges when vegetation was present, to a tendency of more parallel flow to the channel edges when vegetation was absent. Comparison with hydrodynamic model simulations explains that the vegetation-induced friction causes both flow reduction on the vegetated platform and flow acceleration towards the non-vegetated channels. Our findings imply that large-scale vegetation die-off would not only result in decreased platform sedimentation rates, but also in sediment infilling of the channels, which together would lead to further worsening of plant growth conditions and a potentially runaway feedback to permanent vegetation loss.

Beschreibung: In this study we examine the hydrological processes that underpin non-stationarity in hydrological prediction. This is achieved by analysis of linkages between rainfall, groundwater storage, and runoff in Southwest Western Australia (SWWA), a region experiencing stream flow decline since the mid-1970s. We find a close connection between rainfall and changes in catchment groundwater storage, with increases in storage in years with annual rainfall above a threshold (1050–1400 mm), and declines during low rainfall years. Where groundwater is in contact with the stream bed, runoff, as a proportion of rainfall, is highly correlated with groundwater storage. Recent drought years have reduced groundwater storage and runoff ratio. In the absence of replenishing wetter years, lower runoff ratios are subsequently maintained. Runoff from a given depth of annual rainfall is now far lower than that produced 15 years ago. In this way groundwater storage acts as the catchment's “memory”. This study highlights the importance of catchment groundwater storage that may be used to improve runoff prediction in a drying climate.

Beschreibung: Recent warming and freshening of the Canada Basin has led to the year-round storage of solar radiation as the near-surface temperature maximum (NSTM). Using year-round ocean (from ice tethered profilers and autonomous ocean flux buoys), sea-ice (from ice mass balance buoys), and atmosphere (from NCEP/NCAR reanalysis) data from 2005–2010, we find that heat from the NSTM is entrained into the surface mixed layer (SML) during winter. Entrainment can only occur when the base of the SML reaches the top of the NSTM. If this condition is met, the surface forcing and stratification together determine whether the SML deepens into the NSTM. Heat transfer occurs by diffusion or by the erosion of the summer halocline. The average temperature of the SML warmed by as much as 0.06°C during storm events. Solar radiation began warming the SML about 1 month early during the winter of 2007–2008 and this can be explained by thin sea ice.

Beschreibung: Arctic column ozone reached record low values (∼310 DU) during March of 2011, exposing Arctic ecosystems to enhanced UV-B. We identify the cause of this anomaly using the Oslo CTM2 atmospheric chemistry model driven by ECMWF meteorology to simulate Arctic ozone from 1998 through 2011. CTM2 successfully reproduces the variability in column ozone, from week to week, and from year to year, correctly identifying 2011 as an extreme anomaly over the period. By comparing parallel model simulations, one with all Arctic ozone chemistry turned off on January 1, we find that chemical ozone loss in 2011 is enhanced relative to previous years, but it accounted for only 23% of the anomaly. Weakened transport of ozone from middle latitudes, concurrent with an anomalously strong polar vortex, was the primary cause of the low ozone When the zonal winds relaxed in mid-March 2011, Arctic column ozone quickly recovered.

Beschreibung: This study integrates available surface-based and satellite observations of solar radiation at the surface and the top of the atmosphere (TOA) with a comprehensive set of satellite observations of atmospheric and surface optical properties and a Monte Carlo Aerosol-Cloud-Radiation (MACR) model to estimate the three fundamental components of the planetary solar radiation budget: Albedo at the TOA; atmospheric solar absorption; and surface solar absorption. The MACR incorporates most if not all of our current understanding of the theory of solar radiation physics including modern spectroscopic water vapor data, minor trace gases, absorbing aerosols including its effects inside cloud drops, 3-D cloud scattering effects. The model is subject to a severe test by comparing the simulated solar radiation budget with data from 34 globally distributed state-of-the art BSRN (Baseline Surface Radiation Network) land stations which began data collection in the mid 1990s. The TOA over these sites were obtained from the CERES (Cloud and Earth's Radiant Energy System) satellites. The simulated radiation budget was within 2 Wm−2 for all three components over the BSRN sites. On the other hand, over these same sites, the IPCC-2007 simulation of atmospheric absorption is smaller by 7–8 Wm−2. MACR was then used with a comprehensive set of model input from satellites to simulate global solar radiation budget. The simulated planetary albedo of 29.0% confirms the value (28.6%) observed by CERES. We estimate the atmospheric absorption to be 82 ± 8 Wm−2 to be compared with the 67 Wm−2 by IPCC models as of 2001 and updated to 76 Wm−2 by IPCC-2007. The primary reasons for the 6 Wm−2 larger solar absorption in our estimates are: updated water vapor spectroscopic database (∼1 Wm−2), inclusion of minor gases (∼0.5 Wm−2), black and brown carbon aerosols (∼4 Wm−2), the inclusion of black carbon in clouds (∼1 Wm−2) and 3-D effect of clouds (∼1 Wm−2). The fundamental deduction from our study is the remarkable consistency between satellite measurements of the radiation budget and the parameters (aerosols, clouds and surface reflectivity) which determine the radiation budget. Because of this consistency we can account for and explain the global solar radiation budget of the planet within few Wm−2.

Beschreibung: The study of contrails and their impact on global climate change requires a cloud model that statistically represents contrail radiative properties. In this study, the microphysical properties of global contrails are statistically analyzed using collocated Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. The MODIS contrail pixels are detected using an automated contrail detection algorithm and a manual technique using the brightness temperature differences between the MODIS 11 and 12 μm channels. The scattering and absorption properties of typical contrail ice crystals are used to determine an appropriate contrail model to minimize the uncertainties arising from the assumptions in a particular cloud model. The depolarization ratio is simulated with a variety of ice crystal habit fractions and matched to the collocated MODIS and CALIOP observations. The contrail habit fractions are determined and used to compute the bulk-scattering properties of contrails. A parameterization of shortwave and longwave contrail optical properties is developed for the spectral bands of the Rapid Radiative Transfer Model (RRTM). The contrail forcing at the top of the atmosphere is investigated using the RRTM and compared with spherical and hexagonal ice cloud models. Contrail forcing is overestimated when spherical ice crystals are used to represent contrails, but if a hexagonal ice cloud model is used, the forcing is underestimated for small particles and overestimated for large particles in comparison to the contrail model developed in this study.

Beschreibung: Compressional (Vp) and shear (Vs) wave velocities of mid-ocean-ridge basalt (MORB) was investigated at in situ high pressure and high temperature conditions of the mantle transition region by using a combination of ultrasonic and in situ x-ray measurements. Both Vp and Vs of MORB are lower than the previously predicted velocities of the major mantle mineral phases. We found that the Vp and Vs of MORB along a typical geotherm are lower by about 2 and 5%, respectively than those of seismological models, and do not match any global and regional seismological models in the deeper parts of the mantle transition region. Thus, the existence of a basaltic layer in this region is unlikely, suggesting that the oceanic crust materials are transported into the lower mantle.

Beschreibung: We report on changes in P and S-wave velocities and rock microstructure induced by devolatilization reactions using gypsum as a reference analog material. Cylindrical samples of natural alabaster were dehydrated in air, at ambient pressure, and temperatures between 378 and 423 K. Dehydration did not proceed homogeneously but via a reaction front moving sample inwards separating an outer highly porous rim from the remaining gypsum which, above approximately 393 (±5) K, concurrently decomposed into hemihydrate. Overall porosity was observed to continuously increase with reaction progress from approximately 2% for fully hydrated samples to 30% for completely dehydrated ones. Concurrently, P and S-wave velocities linearly decreased with porosity from 5.2 and 2.7 km/s to 1.0 and 0.7 km/s, respectively. It is concluded that a linearized empirical Raymer-type model extended by a critical porosity term and based on the respective time dependent mineral and pore volumes reasonably replicates the P and S-wave data in relation to reaction progress and porosity.

Beschreibung: A unique microphysical structure of rainfall is observed by the surface laser optical Particle Size and Velocity (Parsivel) disdrometers on 25 April 2011 during Midlatitude Continental Convective Clouds Experiment (MC3E). According to the systematic differences in rainfall rate and bulk effective droplet radius, the sampling data can be divided into two groups; the rainfall mostly from the deep convective clouds has relatively high rainfall rate and large bulk effective droplet radius, whereas the reverse is true for the rainfall from the shallow warm clouds. The Weather Research and Forecasting model coupled with spectral bin microphysics (WRF-SBM) successfully reproduces the two distinct modes in the observed rainfall microphysical structure. The results show that the up-to-date model can demonstrate how the cloud physics and the weather condition on the day are involved in forming the unique rainfall characteristic.

Beschreibung: We investigate the relationship between the Last Glacial Maximum (LGM) and climate sensitivity across the PMIP2 multi-model ensemble of GCMs, and find a correlation between tropical temperature and climate sensitivity which is statistically significant and physically plausible. We use this relationship, together with the LGM temperature reconstruction of Annan and Hargreaves (2012), to generate estimates for the equilibrium climate sensitivity. We estimate the equilibrium climate sensitivity to be about 2.5°C with a high probability of being under 4°C, though these results are subject to several important caveats. The forthcoming PMIP3/CMIP5 models were not considered in this analysis, as very few LGM simulations are currently available from these models. We propose that these models will provide a useful validation of the correlation presented here.

Beschreibung: Eddies and vortices associated with breaking waves rapidly disperse pollution, nutrients, and terrestrial material along the coast. Although theory and numerical models suggest that vorticity is generated near the ends of a breaking wave crest, this hypothesis has not been tested in the field. Here we report the first observations of wave-generated vertical vorticity (e.g., horizontal eddies), and find that individual short-crested breaking waves generate significant vorticity [O(0.01 s−1)] in the surfzone. Left- and right-handed wave ends generate vorticity of opposite sign, consistent with theory. In contrast to theory, the observed vorticity also increases inside the breaking crest, possibly owing to onshore advection of vorticity generated at previous stages of breaking or from the shape of the breaking region. Short-crested breaking transferred energy from incident waves to lower frequency rotational motions that are a primary mechanism for dispersion near the shoreline.

Beschreibung: In order to probe the subsurface dynamics associated with geyser eruptions, we measured ground deformation at Old Faithful Geyser of Calistoga, CA. We present a physical model in which recharge during the period preceding an eruption is driven by pressure differences relative to the aquifer supplying the geyser. The model predicts that pressure and ground deformation are characterized by an exponential function of time, consistent with our observations. The geyser's conduit is connected to a reservoir at a depth of at least 42 m, and pressure changes in the reservoir can produce the observed ground deformations through either a poroelastic or elastic mechanical model.

Beschreibung: Extratropical cyclones are often associated with heavy precipitation events and can have major socio-economic impacts. This study investigates how much of the total precipitation in the Northern Hemisphere is associated with extratropical cyclones. An objective feature tracking algorithm is used to locate cyclones and the precipitation associated with these cyclones is quantified to establish their contribution to total precipitation. Climatologies are produced from the Global Precipitation Climatology Project (GPCP) daily dataset and the ERA-Interim reanalysis. The magnitude and spatial distribution of cyclone associated precipitation and their percentage contribution to total precipitation is closely comparable in both datasets. In some regions, the contribution of extratropical cyclones exceeds 90/85% of the total DJF/JJA precipitation climatology. The relative contribution of the most intensely precipitating storms to total precipitation is greater in DJF than JJA. The most intensely precipitating 10% of storms contribute over 20% of total storm associated precipitation in DJF, whereas they provide less than 15% of this total in JJA.

Beschreibung: Shatsky Rise is a Large Igneous Province (LIP) currently located in the northwestern Pacific. New downhole magnetic logging data from Integrated Ocean Drilling Program (IODP) Hole U1347A at Tamu Massif of Shatsky Rise captured the magnetic architecture in the uppermost lava sequence, providing a rare opportunity to investigate a time series of the intra-plate volcanism in conjunction with the Pacific plate construction history centered at the triple junction. Logging data results indicate that Tamu Massif was formed during normal polarity periods south of the paleoequator and crossed the equator at some point in the M19–M17 period. Combining these new observations with previous interpretations of the massif's tectonic history, a time series of the latitudinal tectonic motion of a LIP and the underlying Pacific plate during the plateau formation is postulated.

Beschreibung: It is shown that a global atmospheric model with horizontal resolution typical of that used in operational numerical weather prediction is able to simulate non-gaussian probability distributions associated with the climatology of quasi-persistent Euro-Atlantic weather regimes. The spatial patterns of these simulated regimes are remarkably accurate. By contrast, the same model, integrated at a resolution more typical of current climate models, shows no statistically significant evidence of such non-gaussian regime structures, and the spatial structure of the corresponding clusters are not accurate. Hence, whilst studies typically show incremental improvements in first and second moments of climatological distributions of the large-scale flow with increasing model resolution, here a real step change in the higher-order moments is found. It is argued that these results have profound implications for the ability of high resolution limited-area models, forced by low resolution global models, to simulate reliably, regional climate change signals.

Beschreibung: Various seismicity patterns before large earthquakes have been reported in the literature. They include foreshocks (medium-term acceleration and short-term activation), quiescence, doughnut patterns and event migration. The existence of these precursory patterns is however debated. Here, we develop an approach based on the concept of stress accumulation to unify and categorize all claimed seismic precursors in a same physical framework. We first extend the Non-Critical Precursory Accelerating Seismicity Theory (N-C PAST), which already explains most precursors, to additionally include short-term activation. Theoretical results are then compared to the time series observed prior to the 2009 Mw = 6.3 L'Aquila, Italy, earthquake. We finally show that different precursory paths are possible before large earthquakes, with possible coupling of different patterns or non-occurrence of any. This is described by a logic tree defined from the combined probabilities of occurrence of the mainshock at a given stress state and of precursory silent slip on the fault. In the case of the L'Aquila earthquake, the observed precursory path is coupling of quiescence and accelerating seismic release, followed by activation. These results provide guidelines for future research on earthquake predictability.

Beschreibung: Seasonal forecasts of the September 2012 Arctic sea ice thickness and extent are conducted starting from 1 June 2012. An ensemble of forecasts is made with a coupled ice-ocean model. For the first time, observations of the ice thickness are used to correct the initial ice thickness distribution to improve the initial conditions. Data from two airborne campaigns are used: NASA Operation IceBridge and SIZONet. The model was advanced through April and May using reanalysis data from 2012 and for June–September it was forced with reanalysis data from the previous seven summers. The ice extent in the corrected runs averaged lower in the Pacific sector and higher in the Atlantic sector compared to control runs with no corrections. The predicted total ice extent is 4.4 +/− 0.5 M km2, 0.2 M km2 less than that made with the control runs but 0.8 M km2 higher than the observed September extent.

Beschreibung: We investigate the relationship between large-scale atmospheric flow and the evolution of the most extreme windstorms affecting Western Continental Europe. The 25 most destructive Western Continental European wind storms are selected from a 43-year climatology. 22 of these storms are grouped as having a similar trajectory and evolution. We show that these storms typically occur during particularly strong and persistent positive NAO anomalies which peak approximately 2 days before the storms' peak intensity; the NAO pattern then shifts eastward to a position over the European continent when the storms strike Europe. A temporal composite of potential temperature on the 2-PVU surface suggests that this NAO shift is the result of simultaneous cyclonic and anticyclonic wave breaking penetrating further to the east than during a typical high-NAO event. This creates an extremely intense, zonally-orientated jet over the North Atlantic whose baroclinicity favours explosive intensification of storms while steering them into Western Continental Europe.

Beschreibung: The simultaneous measurements of vertical velocity and cloud droplet size distributions in cumuli collected during the RACORO field campaign over the Atmospheric Radiation Measurement Program's Southern Great Plains site near Lamont, Oklahoma, US, are analyzed to determine the effects of vertical velocity on droplet number concentration, relative dispersion (the ratio of standard deviation to mean radius), and their relationship. The results show that with increasing vertical velocity the droplet number concentration increases while the relative dispersion decreases. The data also exhibit a negative correlation between relative dispersion and droplet number concentration. These empirical relationships can be fitted well with power law functions. This observational study confirms the theoretical and numerical expectations of the effects of vertical velocity on cloud microphysics by analyzing the data of vertical velocity directly. The effects of vertical velocity on relative dispersion and its relationship with droplet number concentration are opposite to that associated with aerosol loading, posing a confounding challenge for separating aerosol indirect effects from dynamical effects.

Beschreibung: Proxy records indicate that export production was enhanced at the onset of the last glaciation. We examine how glacial cooling affects marine export production through temperature-dependent phytoplankton productivity and organic carbon remineralization using glacial climate simulation. Compared with the standard assumption of no temperature dependence as applied in existing paleoclimate models, including temperature-dependence enhances export production globally under glacial climate conditions. The near freezing temperatures of Antarctic seawater significantly reduce organic carbon remineralization. Less remineralization than production results in increasing carbon export to the deep ocean. Nutrients remaining near the surface at high latitudes are advected to lower latitudes via Antarctic Intermediate Water, enhancing primary production, and hence export, in temperate and tropical regions as well. Including temperature-dependence improves the model's agreement with the glacial proxy records of export production and stable carbon isotopes even under stably stratified conditions with a weakening of North Atlantic Deep Water.

Beschreibung: Can the magnitude of a giant earthquake be estimated from paleoseismological data alone? Attempts to estimate the size of the Jogan earthquake of AD 869, whose tsunami affected much of the same coast as the 2011 Tohoku tsunami, offers an excellent opportunity to address this question, which is fundamental to assessing earthquake and tsunami hazards at subduction zones. Between 2004 and 2010, examining stratigraphy at 399 locations beneath paddy fields along 180 km of coast mainly south of Sendai, we learned that a tsunami deposit associated with the AD 869 Jogan earthquake had run inland at least 1.5 km across multiple coastal lowlands, and that one of the lowlands had subsided during the Jogan earthquake and an earlier earthquake as well. Radiocarbon ages just below/above sand deposits left by the pre-Jogan tsunamis suggested recurrence intervals in the range of 500 to 800 years. Modeling inundation and subsidence, we estimated size of the Jogan earthquake as moment magnitude 8.4 or larger and a fault rupture area 200 km long. We did not consider a longer rupture, like the one in 2011, because coastal landform and absence of a volcanic ash layer make any Jogan layer difficult to identify along the Sanriku coast. Still, Sendai tsunami geology might have reduced casualties by improving evacuation maps and informing public-awareness campaigns.

Beschreibung: Recurring Slope Lineae (RSL) are small scale seasonal flow features identified by Mars Reconnaissance Orbiter that present several interesting characteristics such as an albedo contrast, seasonal dependence, and a strong preference for equator-facing slopes. All of these characteristics strongly suggest a thermally driven mechanism such as a liquid triggered or dominated flow. Here we investigate the possibility that these features are formed by melting of brines of various compositions via a combination of thermodynamic and kinetic numerical models. Results suggest that a solution with a freezing temperature of ∼223 K can best reproduce the observed seasonality. Relatively high surface evaporation rates at the RSL locations make the flows quickly disappear over a single season. Our model reproduces well the seasonality of RSL and can explain the preference for equator-facing slopes suggesting that brine flows, and therefore liquids, are possible on a small time and space scale today on Mars. However, if the RSL are indeed formed by brines, it may indicate that a recharge mechanism is active in order to maintain a source of brine over even short geological timescales, which would have important implications for the Martian water cycle.

Beschreibung: Sea ice core chlorophyll a data are used to describe the seasonal, regional and vertical distribution of algal biomass in Southern Ocean pack ice. The Antarctic Sea Ice Processes and Climate – Biology (ASPeCt – Bio) circumpolar dataset consists of 1300 ice cores collected during 32 cruises over a period of 25 years. The analyses show that integrated sea ice chlorophyll a peaks in early spring and late austral summer, which is consistent with theories on light and nutrient limitation. The results indicate that on a circum-Antarctic scale, surface, internal and bottom sea ice layers contribute equally to integrated biomass, but vertical distribution shows distinct differences among six regions around the continent. The vertical distribution of sea ice algal biomass depends on sea ice thickness, with surface communities most commonly associated with thin ice (

Beschreibung: Dynamic triggering around the Fangshan Pluton near Beijing is repeatedly identified. Here we report clear triggered events in this region during the surface waves of the 2012 Mw8.6 Sumatra earthquake. However, we do not find any triggered events during the G2 waves of the Mw8.6 event, and the surface waves of the Mw8.2 Sumatra earthquake that occurred two hours later. The peak ground velocities of the 2012 Mw8.2 event are around the apparent triggering threshold of 0.1–0.2 cm/s in this region. Hence, the fact that this event did not trigger does not require an influence of elapsed time since last trigger (the Mw8.6 mainshock), but is consistent with it. The lack of triggering during the G2 wave of the 2012 Mw8.6 mainshock may be caused by relatively weak surface-wave signals in the intermediate period of 100–10 s.

Beschreibung: We use 2006–2009 ALOS Interferometric Synthetic Aperture Radar data over the entire west Sunda arc, Indonesia, home of 13% of the world's active volcanoes, to derive arc-wide time-dependent ground deformation data. We present unambiguous evidence of inflation at six volcanoes, three of which erupted after the observation period. We show that these volcanoes have shallow magma reservoirs at ∼1–3 km depth below the average regional elevation. A global comparison of reservoir depths at arc volcanoes suggests that volcanoes in extensional and strike-slip settings (west Sunda) can develop shallow reservoirs whereas volcanoes in compressional settings may lack them. Thus, magma ascend through the upper crust could be influenced by intra-arc tectonic settings.

Beschreibung: We present visible and thermal infrared observations of the Martian surface acquired during three Phobos transits. Observations show a decrease of up to ∼20% of the reflected solar energy, consistent with the fraction of the Sun disk eclipsed by Phobos, and no measurable surface cooling. Thermal modeling indicates that the top millimeter of the regolith has a thermal inertia larger than 100 J m−2 K−1 s−1/2 regardless of the surface morphology, and is consistent with TES regional thermal inertia values derived from diurnal cycles (e.g. ∼200 J m−2 K−1 s−1/2). The thermophysical properties of the top millimeter of the regolith exclude the presence of widespread thermally-thick dust layers, are consistent with those of the diurnal skin depths at TES and THEMIS spatial resolutions, are in accordance with high-resolution images of the surface showing no surface mantling, with General Circulation Model results, thermally derived rock abundance values, albedo, and spectroscopic data.

Beschreibung: Long-lasting mesoscale convective systems (MCSs) may occur in the outer region of tropical cyclones in the western North Pacific, especially in conjunction with the southwest monsoon as in the case of Typhoon Morakot that caused great flooding and landslides in Taiwan. These “outer-MCSs” are linear convective systems that develop from distant rainbands, have a large cold cloud shield, and last more than six hours. These outer-MCSs are important for typhoon rainfall forecasting because of the torrential rainfall when they interact with land and terrain to produce serious flooding that is separate from the rainfall near the center. A total of 109 outer-MCSs that occurred during 1999–2009 are identified using infrared and passive microwave images. About 22% of all typhoons in the western North Pacific have at least one outer-MCS during their life cycle. In 85% of the QuikSCAT oceanic 10-m wind observations of outer-MCSs, positive shear vorticity on the left side of mesoscale surface jets below the stratiform precipitation regions may be contributing to the continuous formation of new convective cells.

Beschreibung: We analyze 1977–2010 trends in atmospheric mercury (Hg) from 21 ship cruises over the North Atlantic (NA) and 15 over the South Atlantic (SA). We find a steep 1990–2009 decline of −0.046 ± 0.010 ng m−3 a−1 (−2.5% a−1) over the NA (steeper than at Northern Hemispheric land sites) but no significant decline over the SA. Surface water Hg0 measurements in the NA show a decline of −5.7% a−1 since 1999, and limited subsurface ocean data show an ∼80% decline from 1980 to present. We use a coupled global atmosphere-ocean model to show that the decline in NA atmospheric concentrations can be explained by decreasing oceanic evasion from the NA driven by declining subsurface water Hg concentrations. We speculate that this large historical decline of Hg in the NA Ocean could have been caused by decreasing Hg inputs from rivers and wastewater and by changes in the oxidant chemistry of the atmospheric marine boundary layer.

Beschreibung: In Alaska, where many active volcanoes exist without ground-based instrumentation, the use of techniques suitable for distant monitoring is pivotal. In this study we report regional-scale seismic and infrasound observations of volcanic activity at Mt. Cleveland between December 2011 and August 2012. During this period, twenty explosions were detected by infrasound sensors as far away as 1827 km from the active vent, and ground-coupled acoustic waves were recorded at seismic stations across the Aleutian Arc. Several events resulting from the explosive disruption of small lava domes within the summit crater were confirmed by analysis of satellite remote sensing data. However, many explosions eluded initial, automated, analyses of satellite data due to poor weather conditions. Infrasound and seismic monitoring provided effective means for detecting these hidden events. We present results from the implementation of automatic infrasound and seismo-acoustic eruption detection algorithms, and review the challenges of real-time volcano monitoring operations in remote regions. We also model acoustic propagation in the Northern Pacific, showing how tropospheric ducting effects allow infrasound to travel long distances across the Aleutian Arc. The successful results of our investigation provide motivation for expanded efforts in infrasound monitoring across the Aleutians and contributes to our knowledge of the number and style of vulcanian eruptions at Mt. Cleveland.

Beschreibung: For decades, whistlers observed on the ground at mid and high latitudes have been used for diagnostics of Earth's plasmasphere. Whistlers have also been observed at low latitudes however, the propagation characteristics of low latitude whistlers are poorly understood thus they have not been used effectively as a diagnostic for the low latitude ionosphere. One key limitation with past studies has been lack of knowledge of the whistler source lightning location. Here we present the first cases of low latitude ground whistlers most likely linked with their causative lightning discharges in the conjugate zone. The Global Lightning Dataset 360 (GLD360) detected lightning discharges were found to be located close to the conjugate location of the recording stations, providing direct evidence of inter-hemispheric propagation at the low latitudes. A total of 864 whistlers were observed at Allahabad, India (Geomag. lat. 16.05°N; Geomag. long. 155.34°E; L = 1.08) during the night of 26 January 2011. Using GLD360 network data, we show the occurrence of thunderstorm activity between 200 and 450 km from the conjugate point of Allahabad. We also report the distribution of peak currents of whistler-producing lightning, which demonstrate a cutoff at 30 kA.

Beschreibung: This study investigates the statistical significance of the trends of station temperature time series from the European Climate Assessment & Data archive poleward of 60°N. The trends are identified by different methods and their significance is assessed by three different null models of climate noise. All stations show a warming trend but only 17 out of the 109 considered stations have trends which cannot be explained as arising from intrinsic climate fluctuations when tested against any of the three null models. Out of those 17, only one station exhibits a warming trend which is significant against all three null models. The stations with significant warming trends are located mainly in Scandinavia and Iceland.

Beschreibung: We present the first high resolution, approximately ∼4 years sample spacing, precipitation record from northeastern Brazil (hereafter referred to as ‘Nordeste’) covering the last ∼3000 yrs from 230Th-dated stalagmites oxygen isotope records. Our record shows abrupt fluctuations in rainfall tied to variations in the intensity of the South American summer monsoon (SASM), including the periods corresponding to the Little Ice Age (LIA), the Medieval Climate Anomaly (MCA) and an event around 2800 yr B.P. Unlike other monsoon records in southern tropical South America, dry conditions prevailed during the LIA in the Nordeste. Our record suggests that the region is currently undergoing drought conditions that are unprecedented over the past 3 millennia, rivaled only by the LIA period. Using spectral, wavelet and cross-wavelet analyses we show that changes in SASM activity in the region are mainly associated with variations of the Atlantic Multidecadal Oscillation (AMO) and to a lesser degree caused by fluctuations in tropical Pacific SST. Our record also shows a distinct periodicity around 210 years, which has been linked to solar variability.

Beschreibung: Radiatively active water ice clouds (RAC) play a key role in shaping the thermal structure of the Martian atmosphere. In this paper, RAC are implemented in the LMD Mars Global Climate Model (GCM) and the simulated temperatures are compared to Thermal Emission Spectrometer observations over a full year. RAC change the temperature gradients and global dynamics of the atmosphere and this change in dynamics in turn implies large-scale adiabatic temperature changes. Therefore, clouds have both a direct and indirect effect on atmospheric temperatures. RAC successfully reduce major GCM temperature biases, especially in the regions of formation of the aphelion cloud belt where a cold bias of more than 10 K is corrected. Departures from the observations are however seen in the polar regions, and highlight the need for better modeling of cloud formation and evolution.

Beschreibung: The Venus ionospheric response to solar and solar wind variations is most evident in its magnetic field properties. Early Pioneer Venus observations during the solar maximum revealed that the Venus ionosphere exhibits two magnetic states depending on the solar wind dynamic pressure conditions: magnetized ionosphere with large-scale horizontal magnetic field; or unmagnetized ionosphere with numerous small-scale thin structures, so-called flux ropes. Here we report yet another magnetic state of Venus' ionosphere, giant flux ropes in the magnetized ionosphere, using Venus Express magnetic field measurements during solar minimum. These giant flux ropes all have strong core fields and diameters of hundreds of kilometers, which is about the vertical dimension of the ionosphere. This finding represents the first observation of these giant flux ropes at Venus. The cause of these giant flux ropes remains unknown and speculative.

Beschreibung: Recent studies of satellite-derived Chlorophyll concentrations (Chl-a) in the western Arabian Sea (AS) have suggested an increasing temporal trend, but the length of the records used have typically been too short to resolve longer-term trends, if any. Our analysis of a long term satellite ocean color data shows a change of trend in the summer chlorophyll for the western AS before and after 2003; Chl-a concentration was indeed increasing till 2003, but appears to be declining since then, indicating a secular multi-year trend in Chl-a variability. However, this trend is not uniform over the entire region. Analysis of wind, sea surface temperature (SST), Sea Level Anomaly (SLA) and thermocline depth, suggests that the declining summer monsoon chlorophyll-a (Chl-a) concentration may be due to increasing SLA in this region. The earlier observed biological changes in the western AS could be an artifact of the change in local winds and ocean dynamics, which may be a part of the natural long-term variability.

Beschreibung: This study examines the performance of malaria-relevant climate variables from the ENSEMBLES seasonal ensemble re-forecasts for sub-Saharan West Africa, using a dynamic malaria model to transform temperature and rainfall forecasts into simulated malaria incidence and verifying these forecasts against simulations obtained by driving the malaria model with General Circulation Model-derived reanalysis. Two subregions of forecast skill are identified: the highlands of Cameroon, where low temperatures limit simulated malaria during the forecast period and interannual variability in simulated malaria is closely linked to variability in temperature, and northern Nigeria/southern Niger, where simulated malaria variability is strongly associated with rainfall variability during the peak rain months.

Beschreibung: On 2 August 2012 a dramatic storm formed over Siberia, moved into the Arctic, and died in the Canadian Arctic Archipelago on 14 August. During its lifetime its central pressure dropped to 966 hPa, leading it to be dubbed ‘The Great Arctic Cyclone of August 2012’. This cyclone occurred during a period when the sea ice extent was on the way to reaching a new satellite-era low, and its intense behavior was related to baroclinicity and a tropopause polar vortex. The pressure of the storm was the lowest of all Arctic August storms over our record starting in 1979, and the system was also the most extreme when a combination of key cyclone properties was considered. Even though, climatologically, summer is a ‘quiet’ time in the Arctic, when compared with all Arctic storms across the period it came in as the 13th most extreme storm, warranting the attribution of ‘Great’.

Beschreibung: During the Archean (3.8–2.5 billion years ago), the Sun was up to 25% less luminous than today, yet there is strong evidence that the Earth's ocean surface was not completely frozen. The most obvious solutions to this ‘faint young Sun problem’ demand high concentrations of greenhouse gases such as carbon dioxide. Here we present the first comprehensive 3-dimensional simulations of the Archean climate that include processes as the sea-ice albedo feedback and the higher rotation rate of the Earth. These effects lead to CO2 partial pressures required to prevent the Earth from freezing that are significantly higher than previously thought. For the early Archean, we find a critical CO2 partial pressure of 0.4 bar in contrast to 0.06 bar estimated in previous studies with 1-dimensional radiative-convective models. Our results suggest that currently favored greenhouse solutions could be in conflict with constraints emerging for the middle and late Archean.

Beschreibung: Observations of extratropical Southern Hemisphere austral summer precipitation over recent decades show mid-latitude drying and high-latitude moistening. Here we show that the observed precipitation trends in two datasets are inconsistent with simulated internal variability, but are closely consistent with trends simulated in response to historical changes in anthropogenic and natural forcings. Simulations with individual anthropogenic and natural forcings suggest that the observed pattern of precipitation change is substantially forced by anthropogenic greenhouse gas and ozone changes, with an opposing influence from aerosols. Our results demonstrate that human influence had a significant impact on precipitation across the mid and high latitudes of the Southern Hemisphere, changes which are expected to have a profound impact on Southern Ocean stratification and hence on ocean-atmosphere heat and carbon fluxes.

Beschreibung: We study limits of interannual to decadal predictability of sea surface temperature (SST) in the North Pacific sector of the Community Climate System Model version 3 (CCSM3). Using a set of low-frequency and intermittent spatiotemporal SST modes acquired through nonlinear Laplacian spectral analysis (a nonlinear data manifold generalization of singular spectrum analysis), we build a hierarchy of regression models with external factors to determine which modes govern the dynamic evolution and predictability of prominent large-scale patterns, namely the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO). Retaining key triple correlations between prognostic variables and external factors, as well as the seasonality of the data, we find that the PDO and NPGO modes of CCSM3 can be described with remarkably high fidelity as an outcome of forcing by the intermittent modes (with phase demodulation by the seasonal cycle) and cubic interactions between the low-frequency modes. Our results differ from the classical picture of ENSO-driven autoregressive models for North Pacific SST variability, providing evidence that intermittent processes, such as variability of the Kuroshio current, limit long-range predictability in this climate model.

Beschreibung: There is no scientific consensus about how dissolved organic carbon (DOC) in surface waters is regulated. Here we combine recent literature data from 49 catchments with detailed stream and catchment process information from nine well established research catchments at mid- to high latitudes to examine the question of how climate controls stream water DOC. We show for the first time that mean annual temperature (MAT) in the range from −3° to +10° C has a strong control over the regional stream water DOC concentration in catchments, with highest concentrations in areas ranging between 0° and +3° C MAT. Although relatively large deviations from this model occur for individual streams, catchment topography appears to explain much of this divergence. These findings suggest that the long-term trajectory of stream water DOC response to climate change may be more predictable than previously thought.

Beschreibung: Large-scale geometrical complexities along faults are known to be likely endpoints for coseismic rupture, as suggested by analysis of historic ruptures and corroborated by models of rupture on bent or discontinuous faults. However, natural faults also include smaller-scale complexities. We use the 3D finite element method to model dynamic ruptures on strike-slip fault stepovers with a smaller intermediate fault between the main strands. We find that such small faults can have a controlling effect on rupture behavior and ground motion intensity and distribution. In particular, the intermediate fault can either aid or prevent rupture propagation across the stepover, depending on its length and basal depth. The results have important implications for hazard assessment of faults with large- and small-scale geometrical complexities, and also suggest that more site-specific modeling studies may be necessary to develop realistic rupture scenarios for individual complex fault systems.

Beschreibung: Offshore tropical river plumes are associated with areas of high N2 fixation (diazotrophy) and biological carbon drawdown. Episodic blooms of the diatom Hemiaulus hauckii and its diazotrophic cyanobacterial symbiont Richelia intracellularis are believed to dominate that carbon drawdown, but the mechanism is not well understood. We report primary productivity associated with blooms of these diatom-diazotroph assemblages (DDAs) in the offshore plume of the Amazon River using simultaneous measurements of O2/Ar ratios and the triple-isotope composition of dissolved O2. In these blooms, we observe peaks in net community productivity, but relatively small changes in gross primary productivity, suggesting that DDA blooms increase the ecosystem carbon export ratio more than twofold. These events of enhanced export efficiency lead to biological uptake of dissolved inorganic carbon and silicate, whose longer mixed-layer residence times otherwise obscure the differential impact of DDAs. The shorter-term rate estimates presented here are consistent with the results derived from longer-term geochemical tracers, confirming that DDAs drive a significant biological CO2 pump in tropical oceans.

Beschreibung: Under increasing greenhouse gas concentrations, ocean heat uptake moderates the rate of climate change, and thermal expansion makes a substantial contribution to sea level rise. In this paper we quantify the differences in projections among atmosphere-ocean general circulation models of the Coupled Model Intercomparison Project in terms of transient climate response, ocean heat uptake efficiency and expansion efficiency of heat. The CMIP3 and CMIP5 ensembles have statistically indistinguishable distributions in these parameters. The ocean heat uptake efficiency varies by a factor of two across the models, explaining about 50% of the spread in ocean heat uptake in CMIP5 models with CO2 increasing at 1%/year. It correlates with the ocean global-mean vertical profiles both of temperature and of temperature change, and comparison with observations suggests the models may overestimate ocean heat uptake and underestimate surface warming, because their stratification is too weak. The models agree on the location of maxima of shallow ocean heat uptake (above 700 m) in the Southern Ocean and the North Atlantic, and on deep ocean heat uptake (below 2000 m) in areas of the Southern Ocean, in some places amounting to 40% of the top-to-bottom integral in the CMIP3 SRES A1B scenario. The Southern Ocean dominates global ocean heat uptake; consequently the eddy-induced thickness diffusivity parameter, which is particularly influential in the Southern Ocean, correlates with the ocean heat uptake efficiency. The thermal expansion produced by ocean heat uptake is 0.12 m YJ−1, with an uncertainty of about 10% (1 YJ = 1024 J).

Beschreibung: Climate warming will likely cause a shift from snow to rain in midlatitude mountains. Because rain falls faster than snow, it is not advected as far by prevailing winds before reaching the ground. A shift in precipitation phase thus may alter precipitation patterns. Using the Weather Research and Forecasting (WRF) regional climate model at 27-9-3 km resolutions over the California Sierra Nevada, we conducted an idealized experiment consisting of a present climate control run and two additional simulations in which (a) fall speed for snow is similar to rain and (b) all precipitation is constrained to fall as liquid. Rather than simulating future climates directly, these perturbation experiments allow us to test the potential impacts of changing precipitation phase in isolation from other factors such as variable large-scale atmospheric circulation. Relative to the control, both perturbations result in a rain shadow deepened by ∼30–60%, with increased focusing of precipitation on the western Sierra Nevada slopes best resolved at ≤9 km resolutions. Our results suggest that altered precipitation phase associated with climate change will likely affect spatial distributions of water resources, floods, and landslides in the Sierra Nevada and similar midlatitude mountain ranges.

Beschreibung: Three years ago we proposed that the summer Arctic would be nearly sea ice free by the 2030s; “nearly” is interpreted as sea ice extent less than 1.0 million km2. We consider this estimate to be still valid based on projections of updated climate models (CMIP5) and observational data. Similar to previous models (CMIP3), CMIP5 still shows a wide spread in hindcast and projected sea ice loss among different models. Further, there is no consensus in the scientific literature for the cause of such a spread in results for CMIP3 and CMIP5. While CMIP5 model mean sea ice extents are closer to observations than CMIP3, the rates of sea ice reduction in most model runs are slow relative to recent observations. All CMIP5 models do show loss of sea ice due to increased anthropogenic forcing relative to pre-industrial control runs. Applying the same technique of model selection and extrapolation approach to CMIP5 as we used in our previous paper, the interval range for a nearly sea ice free Arctic is 14 to 36 years, with a median value of 28 years. Relative to a 2007 baseline, this suggests a nearly sea ice free Arctic in the 2030s.

Beschreibung: Projected changes in the extra-tropical wintertime storm tracks are investigated using the multi-model ensembles from both the third and fifth phases of the World Climate Research Programme's Coupled Model Intercomparison Project (CMIP3 and CMIP5). The aim is to characterize the magnitude of the storm track responses relative to their present-day year-to-year variability. For the experiments considered, the ‘middle-of-the-road’ scenarios in each CMIP, there are regions of the Northern Hemisphere where the responses of up to 40% of the models exceed half of the inter-annual variability, and for the Southern Hemisphere there are regions where up to 60% of the model responses exceed half of the inter-annual variability.

Beschreibung: Using a global climate model with fully predictive aerosol life cycle, we investigate the fast and slow responses of the South Asian monsoon system to anthropogenic aerosol forcing. Our results show that the feedbacks associated with sea surface temperature (SST) change caused by aerosols play a more important role than the aerosol's direct impact on radiation, clouds and land surface (rapid adjustments) in shaping the total equilibrium climate response of the monsoon system to aerosol forcing. Inhomogeneous SST cooling caused by anthropogenic aerosols eventually reduces the meridional tropospheric temperature gradient and the easterly shear of zonal winds over the region, slowing down the local Hadley cell circulation, decreasing the northward moisture transport, and causing a reduction in precipitation over South Asia. Although total responses in precipitation are closer to the slow responses in general, the fast component dominates over land areas north of 25°N. Our results also show an east-west asymmetry in the fast responses to anthropogenic aerosols causing increases in precipitation west of 80°E but decreases east of it.

Beschreibung: Cluster analysis offers a simple visual exploratory tool for the initial investigation of regional Global Positioning System (GPS) velocity observations, which are providing increasingly precise mappings of actively deforming continental lithosphere. The deformation fields from dense regional GPS networks can often be concisely described in terms of relatively coherent blocks bounded by active faults, although the choice of blocks, their number and size, can be subjective and is often guided by the distribution of known faults. To illustrate our method, we apply cluster analysis to GPS velocities from the San Francisco Bay Region, California, to search for spatially coherent patterns of deformation, including evidence of block-like behavior. The clustering process identifies four robust groupings of velocities that we identify with four crustal blocks. Although the analysis uses no prior geologic information other than the GPS velocities, the cluster/block boundaries track three major faults, both locked and creeping.

Beschreibung: The transient crater is an important impact cratering concept. Its volume and diameter can be used to predict impact energy and momentum, impact melt volume, and maximum depth and volume of ejected material. Transient crater sizes are often estimated using scaling laws based on final crater rim diameters. However, crater rim estimates, especially for lunar basins, can be controversial. Here, we use numerical modeling of lunar basin-scale impacts to produce a new, alternative method for estimating transient crater radius using the annular bulge of crust observed beneath most lunar basins. Using target thermal conditions appropriate for the lunar Imbrian and Nectarian periods, we find this relationship to be dependent on lunar crust and upper mantle temperatures. This result is potentially important when analyzing lunar basin subsurface structures inferred from the GRAIL mission.

Beschreibung: Slow slip events (SSE) in many subduction zones incrementally stress the adjacent locked megathrust, suggesting that they could potentially either trigger or evolve into damaging earthquakes. We explore this with 2D quasi-dynamic simulations with rate-state friction, dilatancy, and coupled 1D pore-fluid and heat transport. Steady-state weakening friction allows transient slip to nucleate, but is inhibited by dilatant strengthening and destabilized by thermal pressurization. SSE spontaneously nucleate in Low Effective-Stress Velocity-Weakening (LESVW) regions. If the dimension of the LESVW is relatively small the SSE are trapped at its updip end, imparting a strong stress concentration in the locked zone. After several centuries SSE penetrate into the region of higher effective stress, where thermal pressurization eventually leads to dynamic rupture. For larger LESVW regions SSE tend to increase in length with time; ultimately higher slip speeds enhance thermal weakening, leading to dynamic instability within the SSE zone. In both cases the onset of the ultimate SSE is essentially indistinguishable from preceding events.

Beschreibung: Images of Vesta taken by the Dawn spacecraft reveal large-scale linear structural features on the surface of the asteroid. We evaluate the morphology of the Vesta structures to determine what processes caused them to form and what implications this has for the history of Vesta as a planetary body. The dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets, not fractures or grooves such as are found on smaller asteroids. As graben, their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults. Linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events. While the orientation of the large-scale Vesta structures does imply that their formation is related to the impact events that formed the Rheasilvia and Veneneia basins, their size and morphology is greatly different from impact-formed fractures on the smaller bodies. This is consistent with new analyses that suggest that Vesta is fully differentiated, with a mantle and core. We suggest that impact into a differentiated asteroid such as Vesta could result in graben, while grooves and fractures would form on undifferentiated asteroids.

Beschreibung: In this work, waveform variations in repeating volcano-tectonic earthquakes occurring from 2001–2009 in the north-eastern flank of Mt. Etna were studied. Changes in waveform were found mainly during 2002–2003; and consisted of a decreasing similarity in the coda of events in earthquake families, as revealed by cross-correlation analysis, and delays, increasing proportionally to the lapse time, detected by coda wave interferometry. Such variations, mainly evident at stations located in the north-eastern flank of the volcano, were likely due to medium changes taking place within this region. Localized medium velocity decreases were inferred to occur in 2002–2003, followed by successive increases. The velocity decrease was interpreted as being caused by the opening or enlargement of cracks, produced by intruding magma bodies, intense ground deformation, and/or VT earthquake activity that accompanied the 2002–2003 Mt. Etna eruption. On the other hand, subsequent velocity increases were interpreted as resulting from healing processes.

Beschreibung: The influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water-mass transformation, stratification maintenance, and the overturning circulation. However, the distribution of ocean mixing is not well described by sparse ship-based observations since this mixing is both spatially patchy and temporally intermittent. We use strain information from Argo float profiles in the upper 2,000 m of the ocean to generate over 400,000 estimates of the energy dissipation rate, indicative of ocean mixing. These estimates rely on numerous assumptions, and do not take the place of direct measurement methods. Temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans. They corroborate previous observations linking elevated dissipation rates to regions of rough topography. We also observe heightened estimated dissipation rates in areas of high eddy kinetic energy, as well as heightened diffusivity in high latitudes where stratification is weak. The seasonal dependence of mixing is observed in the Northwest Pacific, suggesting a wind-forced response in the upper ocean.

Beschreibung: Turbulence in the surf zone, the shallow region adjacent to the shoreline, has a key role in beach erosion, fertilization, dispersal, and larval settlement of marine invertebrates, and microbial contamination dilution in beach waters. Breaking-wave generated (the dominant source) surf zone turbulence is understood poorly. A new surf zone turbulent dissipation rate ϵ scaling is derived, that collapses new field surf zone ϵ observations with relatively high skill compared to other scalings. The vertically-uniform length-scale is 1/6 the water depth, and 15% of the wave-energy flux gradient is dissipated below the mean surface. Field and laboratory surf zone turbulence observations are shown to be consistent using the scaling. The non-dimensional surf zone diffusivity and suspended sediment profile can be applied to sediment transport and a range of biological processes including microbial pathogen contamination of beach waters.

Beschreibung: The future of large-scale coastline evolution will be strongly coupled to human manipulations designed to prevent erosion. We explore the consequences of this coupling using a numerical model for large-scale coastline evolution to compare the long-term, non-local effects of two generalized classes of shoreline stabilization: 1) beach nourishment (the addition of dredged sand to an eroding beach), and 2) hard-structures (e.g., seawalls, groynes, etc.) which fix the position of the shoreline without adding sand. In centurial model experiments where localized stabilization is maintained in the context of changing climate forcing, both forms of stabilization are found to significantly alter patterns of erosion and accretion at distances up to tens of kilometers. On a cuspate-cape coastline similar to the North and South Carolina coast, USA, with stabilization applied to the eroding updrift flank of a single cape, perturbations to coastline evolution are qualitatively similar within ∼20 km for each stabilization scenario, though they differ in magnitude both updrift and downdrift of the stabilized shoreline. The “human” signal in coastline change can extend as far as a neighboring cape (approximately 100 km away), but these long-range effects differ for each scenario. Nourishment resulted in seaward growth of the stabilized cape, increasing the extent that it blocked sediment flux in downdrift regions of the coast through wave shadowing. When stabilized with a hard structure the cape's initial position remain fixed, decreasing wave shadowing.

Beschreibung: We investigate whether the observed surface specific humidity (q) trends over the Mediterranean region in the period 1974–2003 are consistent with climate model (CMIP3, CMIP5) simulations of q in response to anthropogenic forcing (Greenhouse gas and sulphate aerosols). The natural (internal) variability is estimated using 6,000-year of pre-industrial control simulations. With the exception of winter, the increases in annual and seasonal q over this region are very unlikely (with less than 1%chance) due to natural (internal) variability or natural forcing alone. Using several climate models and ensemble means, we demonstrate that the large-scale component (spatial-mean trend) of the anthropogenic forcing is detectable (at 1% level) in the annual and seasonal trends of q (except winter). However, the smaller-scale component (spatial anomalies about the mean trend) of the anthropogenic signal is detectable only in warm seasons (spring and summer). We further show that the spread of projected trends based on the A1B scenario derived from 13 CMIP3 models encompasses the observed area-averaged trend in q. This may imply that the observed trends of surface humidity, which is an important factor in human thermal comfort, serves as an illustration of plausible future expected change in the region.

Beschreibung: A method for post-processing decadal predictions from global climate models that accounts for model deficiencies in representing climate trends is proposed and applied to decadal predictions of annual global mean temperature from the Canadian Centre for Climate Modelling and Analysis climate model. The method, which provides a time-dependent trend adjustment, reduces residual drifts that remain after applying the standard time-independent bias correction when the modelled and observed long-term trends differ. Initialized predictions and uninitialized simulations that share common specified external forcing are analyzed. Trend adjustment substantially reduces forecast errors in both cases and initialization further enhances skill, particularly for the first forecast year.

Beschreibung: One potential way to geologically sequester captured CO2 emissions is to inject them below the seafloor into marine sedimentary strata where pressures and temperatures would trap the CO2 through “self-sealing” gravitational and hydrate-formation mechanisms. Here we map out the worldwide distribution and thicknesses of such self-sealing strata using a comprehensive, global dataset of deep-sea sediment cores in combination with digital grids of ocean floor heat flow, bathymetry, and sediment thickness. Based on our mapping, we estimate that the total bulk sediment volume of self-sealing strata is 63 million cubic kilometers, 0.8–1.4 km3 (or ∼1.3–2.7%) of which are sands with intrinsic permeability suitable for storing CO2. This is enough storage capacity to hold between 1,260–28,500 gigatonnes of CO2, or about 40–1,000 y of total global CO2 emissions. However, the storage capacity is unevenly distributed where it lies within the Exclusive Economic Zones (EEZ) of the world's largest CO2 emitting economies. The United States and India respectively release 16% and 62% of their annual CO2 emissions (or 1 Bt/y and 800 Mt/y) within 500 km of self-sealing sands located in their EEZs, while only 6% of the annual emissions from China and the European Union (or 330 Mt/y and 250 Mt/y, respectively) occur within this distance.

Beschreibung: We compare historical global temperature time series, based on bias-adjusted sea-surface temperatures with independent temperature time series, for the upper 20 meter layer of the ocean based on the latest update of an historical hydrographic profile data set. Despite the two underlying data sets being different in number of data points, instrumentation and applied adjustments, both of the time series are consistent in showing an overall warming since 1900. We also extend records of temperature change in the upper 400 m back to 1900. Noting that the geographic coverage is limited prior to 1950, the temperature change in the 0–400 m layer is characterized by two periods of temperature increase between 1900 and 1940–45 and between 1970 and 2003, separated by a period of little change.

Beschreibung: Observed North Atlantic Ocean surface temperatures have changed in a non-monotonic and non-uniform fashion over the last century. Here we assess the relative roles of greenhouses gases, anthropogenic aerosols, natural forcings and internal variability to the North Atlantic surface temperature decadal fluctuations using multi-model climate simulations driven by estimates of observed external forcings. While the latter are the main source of decadal variability in the tropics and subtropics, there is a large contribution from the unforced component to subpolar Atlantic variations. Reconstruction of forced response patterns suggests that anthropogenic forcings are the main causes of the accelerated warming of the last three decades while internal variability has a dominant contribution to the early 20th-century temperature multi-decadal swings and recent abrupt changes in the subpolar Atlantic. Significant inter-model spread with regard to the spatial response patterns to anthropogenic forcing leads to substantial uncertainty as to robust attribution statements for the mid-to-late 20th century North Atlantic warm and cold periods.

Beschreibung: Knowledge of seismic properties in an earthquake rupture zone is essential for understanding the factors controlling rupture dynamics. We use data from aftershocks following the Maule earthquake to derive a three-dimensional seismic velocity model of the central Chile forearc. At 36°S, we find a high vp (〉7.0 km/s) and high vp/vs (∼1.89) anomaly lying along the megathrust at 25 km depth, which coincides with a strong forearc Bouguer gravity signal. We interpret this as a subducted topographic high, possibly a former seamount on the Nazca slab. The Maule earthquake nucleated at the anomaly's updip boundary; yet high co-seismic slip occurred where the megathrust is overlain by lower seismic velocities. Sparse aftershock seismicity occurs within this structure, suggesting that it disrupts normal interface seismogenesis. These findings imply that subducted structures can be conducive to the nucleation of large megathrust earthquakes, even if they subsequently hinder co-seismic slip and aftershock activity.

Beschreibung: Detailed observations of the 2010 Eyjafjallajökull eruptions in Iceland show seismic activity propagating vertically through the entire crust during a ten-week period of volcanic unrest comprising multiple eruption episodes. Systematic changes in magma chemistry suggest a complex magmatic plumbing system, tapping several accumulation zones at different depths containing magma of differing ages and compositions. During the eruption, a systematic downward propagation of seismicity through the crust and into the upper mantle to ∼30 km depth occurred in a series of steps, each of which preceded an explosive surge in eruption rate. Here we show that the sequence of seismicity and eruptive activity may be explained by the downward propagation of a decompression wave that triggers magma release from progressively deeper sills in the crust. Comparing observations of the downward-propagating seismicity with the decompression of a series of model elastic sills suggests that each sill was 1–10 km3 in size.

Beschreibung: Auroral radio emissions reveal physics of beam-plasma interactions and provide possibilities to remotely sense ionospheric plasma processes. Sato et al. (2012) recently discovered that auroral roar emissions, long known to occur at two and three times the electron gyrofrequency (fce), also occur at 4fce. Using data from wave receivers in the British Antarctic Survey Automatic Geophysical Observatories, we confirm the existence of 4fce-roars and observe for the first time 5fce-roars. A search at higher frequencies did not find higher harmonics. Both 4fce- and 5fce-roars only occur in sunlit conditions near summer solstice. Harmonic roar emission frequencies scale with the strength of the geomagnetic field, and combining data from four observatories suggests that the 4fce-roar source height lies around 245 km, lower than the 275 km estimated for 2fce-roar. These observations show that the auroral roar generation mechanism acts under a broader set of plasma conditions than previously considered.

Beschreibung: Surface warming and steric sea level rise over the global ocean nearly linearly increase with cumulative carbon emissions for an atmosphere-ocean equilibrium, reached many centuries after emissions cease. Surface warming increases with cumulative emissions with a proportionality factor, ΔTsurface:2×CO2/(IB ln 2), ranging from 0.8 to 1.9 K (1000 PgC)−1 for surface air temperature, depending on the climate sensitivity ΔTsurface:2×CO2 and the buffered carbon inventory IB. Steric sea level rise similarly increases with cumulative emissions and depends on the climate sensitivity of the bulk ocean, ranging from 0.4 K to 2.7 K; a factor 0.4 ± 0.2 smaller than that for surface temperature based on diagnostics of two Earth System models. The implied steric sea level rise ranges from 0.7 m to 5 m for a cumulative emission of 5000 PgC, approached perhaps 500 years or more after emissions cease.

Beschreibung: Numerical simulations are performed to investigate the generation of the wave-4 longitude variation in the low-latitude ionosphere due to the diurnal eastward propagating nonmigrating tide with zonal wavenumber 3 (DE3), semidiurnal eastward propagating nonmigrating tide with zonal wavenumber 2 (SE2), and stationary planetary wave 4 (SPW4). From a fixed local time perspective, the DE3, SE2, and SPW4 all appear as wave-4 structures in longitude, and thus each of these waves must be considered as a potential source of the wave-4 variation in the ionosphere. Both the DE3 and SPW4 are found to produce significant wave-4 variations in the equatorial vertical E × B drift velocity, and in the ionospheric peak density (NmF2) at 15°N magnetic latitude. The daytime wave-4 variation in NmF2 is driven by the combination of vertical E × B drift variability and in-situ effects due largely to meridional neutral winds. The simulation results indicate that the SE2 is not a contributor to the wave-4 longitude variation. Our results further demonstrate that the actual wave-4 longitude variation is due to a combination of the DE3 and SPW4. We therefore conclude that, in addition to the DE3, the SPW4 also needs to be considered as an important driver of the wave-4 longitude variation in the low-latitude ionosphere. We additionally present evidence for the generation of the SPW4 due to the nonlinear interaction between the migrating diurnal tide and the DE3, and demonstrate the impact of DE3 variability on the amplitude of the SPW4.

Beschreibung: The forces acting on solar Coronal Mass Ejections (CMEs) in the interplanetary medium have been evaluated so far in terms of an empirical drag coefficient CD ∼ 1 that quantifies the role of the aerodynamic drag experienced by a typical CME due to its interaction with the ambient solar wind. We use a microphysical prescription for viscosity in the turbulent solar wind to obtain an analytical model for the drag coefficient CD. This is the first physical characterization of the aerodynamic drag experienced by CMEs. We use this physically motivated prescription for CD in a simple, 1D model for CME propagation to obtain velocity profiles and travel times that agree well with observations of deceleration experienced by fast CMEs.

Beschreibung: The influence of the cloud representation in global climate models on the accuracy of the North Atlantic tropical cyclone simulations is investigated. The North Atlantic tropical cyclone activity is simulated with a standard climate model, CCSM. The conventional parameterization of cloud processes in CCSM is replaced by the “super-parameterization” and the simulation is run again. The comparison of tropical cyclone statistics reveals that the model with explicit representation of cloud processes produces a larger number of events, with stronger intensity and longer life-cycle. The results show that clouds have a significant impact on the mechanisms associated with tropical cyclone activity such as surface temperature and ocean subsurface processes, vertical wind shear, and the transport of moisture in the lower troposphere.

Beschreibung: We reconstructed new migrated images from autocorrelation functions of ambient noise at seismic stations in northeastern Japan. During the analysis period, the autocorrelation functions, having a frequency range of 0.5–2 Hz, showed some coherent signals at large lapse times of 10–50 s as well as at small lapse times of less than 10 s. These signals were also coherent in space along cross sections across the Japan subduction zone. The migrated images show seismic scatterers within the subducting Pacific Slab at the depth range of 50–150 km. The distribution of the scatterers within the subducting slab is comparable to the double seismic zone in the subduction zone; in particular, the thickness of the distribution of the seismic scatterers is consistent with that of two planes of the double seismic zone. This indicates inhomogeneities in the seismic velocity structure around the upper and lower planes of the seismicity. The new migrated images suggest that seismic scatterers, such as fluid from dehydration reactions of the hydrated metamorphosed mantle, exist in the lower as well as upper plane of the seismicity in the double seismic zone; this fluid may act as a scatterer within the subducting slab.

Beschreibung: We conducted 228-year long, three-member ensemble simulations using a high resolution (60 km grid size) global atmosphere model, MRI-AGCM3.2, with prescribed sea surface temperature and greenhouse gases and aerosols from 1872 to 2099. We found a clear decreasing trend of global tropical cyclone (TC) frequency throughout the 228 years of the simulation. We also found a significant multidecadal variation (MDV) in the long term variation of Northern Hemisphere and Southern Hemisphere TC count in addition to the decreasing trend. The decreasing trend and MDV in the long term variation of TC count correspond well to a similar decreasing trend and MDV of upward mass flux averaged over the TC genesis region and active TC season. It has been shown that the upward mass flux decreases primarily because the rate of increase of dry static stability, which is close to that of surface specific humidity, is much larger than the rate of increase of precipitation, which is nearly the same as that of atmospheric radiative cooling. Thus, it is suggested that the decreasing trend of TC count is mainly caused by the decreasing trend of upward mass flux associated with the increasing dry static stability.

Beschreibung: The last six years (2007–2012) show a persistent change in early summer Arctic wind patterns relative to previous decades. The persistent pattern, which has been previously recognized as the Arctic Dipole (AD), is characterized by relatively low sea-level pressure over the Siberian Arctic with high pressure over the Beaufort Sea, extending across northern North America and over Greenland. Pressure differences peak in June. In a search for a proximate cause for the newly persistent AD pattern, we note that the composite 700 hPa geopotential height field during June 2007–2012 exhibits a positive anomaly only on the North American side of the Arctic, thus creating the enhanced mean meridional flow across the Arctic. Coupled impacts of the new persistent pattern are increased sea ice loss in summer, long-lived positive temperature anomalies and ice sheet loss in west Greenland, and a possible increase in Arctic-subarctic weather linkages through higher-amplitude upper-level flow. The North American location of increased 700 hPa positive anomalies suggests that a regional atmospheric blocking mechanism is responsible for the presence of the AD pattern, consistent with observations of unprecedented high pressure anomalies over Greenland since 2007.

Beschreibung: Closures of the Iapetus Ocean and the Tornquist Sea lead to the collision of the paleocontinents of Laurentia, Baltica and Eastern Avalonia during the Caledonian orogeny. It has been speculated that relicts of these two closures may be preserved within the crust or upper mantle. Over the past decades, numerous wide-angle seismic profiles were gathered in northwestern Europe to search for related subsurface features. Although active source studies revealed detailed crustal structures across the Iapetus and Tornquist suture zones, there are relatively few clear three-dimensional upper mantle images beneath this region. We use a new European crust and upper mantle model, EU30, determined based on continental scale, nonlinear adjoint tomography, to explore upper mantle structures across these two suture zones. Model EU30 reveals two fast anomalies within the upper mantle: one dips in a northwesterly direction down to approximately 400 km beneath the North Sea, and the other dips in a southwesterly direction down to nearly 250 km across the Tornquist Suture Zone. In addition, we observe a “gap” between the lithospheres of Laurentia and Eastern Avalonia across the Iapetus Suture Zone beneath the central British Isles. These seismic images suggest that heterogeneity related to the closures of the Iapetus Ocean and the Tornquist Sea have been preserved within the upper mantle over hundreds of millions of years.

Beschreibung: Ocean wind waves follow shifts in atmospheric circulation and wind regimes caused by climate change and variability. We use a wave model (WAM) forced by winds from an Earth system model (EC-Earth) to study the evolution of global wave climate in terms of trends in wind speed and wave height, their rates and anomalies over the 250 years. Historical period and future scenarios (RCP8.5 and RCP4.5) are calculated within the fifth phase of the Coupled Model Intercomparison Project. Both scenarios indicate a future increase of wind speed and wave height in the Arctic and Southern Ocean and a decrease in the Pacific Ocean. Some regions, e.g., the North Atlantic show a change of sign from currently positive to negative trends in this century. Swell dominated regions, connected to the Southern Ocean show opposite trends for winds and waves. Calculated trends are evaluated taking into account natural variability estimated from a simulation with pre–industrial climate. We conclude that projected wave heights are still within natural variability, whereas wind speed exceeds it in the Arctic and Southern Ocean. Probability estimates show that extreme winds and waves become more dominant in the Southern Hemisphere, Arctic and Indian Ocean. Low and medium winds and waves are highly probable in the North and Equatorial Atlantic and Pacific.

Beschreibung: The relationship between the quality of a general circulation model's (GCM's) representation of present climate and its predictive skill on seasonal time scales is investigated by a series of GCM experiments. A novel procedure is developed to improve the quality of a GCM's present-day control climate by applying cyclostationary annually varying run-time “bias corrections”. Application of this procedure to the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM (AGCM3) is shown to result in a significant reduction of time-mean biases in wind, temperature and humidity fields in its simulation of present-day climate. Furthermore, it is found that this cyclostationary correction leads to improved variability on seasonal time scales. The ability to improve a GCM's properties in this way allows a careful assessment of the relationship between a model's fidelity and its predictive skill. In this study, the potential predictive skill on seasonal time scales is assessed by performing ensemble simulations with the observed sea surface temperatures and sea-ice distribution. The analysis indicates that the increase in model fidelity associated with the application of the bias correction results in a general increase in predictive skill on seasonal times scales. To investigate this result further, an additional set of ensemble forecasts are performed with the sign of the bias correction reversed, thereby degrading model fidelity. In this additional experiment the corresponding predictive skill is also degraded. The results of this study have implications with regard to the application and interpretation of model metrics for climate GCMs.

Beschreibung: We examine long tide gauge records in every ocean basin to examine whether a quasi 60-year oscillation observed in global mean sea level (GMSL) reconstructions reflects a true global oscillation, or an artifact associated with a small number of gauges. We find that there is a significant oscillation with a period around 60-years in the majority of the tide gauges examined during the 20th Century, and that it appears in every ocean basin. Averaging of tide gauges over regions shows that the phase and amplitude of the fluctuations are similar in the North Atlantic, western North Pacific, and Indian Oceans, while the signal is shifted by 10 years in the western South Pacific. The only sampled region with no apparent 60-year fluctuation is the Central/Eastern North Pacific. The phase of the 60-year oscillation found in the tide gauge records is such that sea level in the North Atlantic, western North Pacific, Indian Ocean, and western South Pacific has been increasing since 1985–1990. Although the tide gauge data are still too limited, both in time and space, to determine conclusively that there is a 60-year oscillation in GMSL, the possibility should be considered when attempting to interpret the acceleration in the rate of global and regional mean sea level rise.

Beschreibung: This study investigated variations in outgoing longwave radiation (OLR) in response to changes in sea surface temperature (SST) over the Pacific warm pool area (20°N–20°S, 130°E–170°W). OLR values were obtained from recent (January 2008–June 2010) geostationary window channel imagery at hourly resolution, which resolves processes associated with tropical convective clouds. We used linear regression analysis with the domain-averaged OLR and SST anomalies (i.e., ΔOLR, ΔSST; deviations from their 90-day moving averages). Results show that the regression slope appears to be significant only with SST least-affected by cloud radiative forcing, for which SST needs to be obtained as daily average over cloud-free regions (ΔSSTclear). The estimated value of ΔOLR/ΔSSTclear is 15.72 W m−2 K−1, indicating the presence of strong outgoing longwave radiation in response to surface warming. This atmospheric cooling effect is found to be primarily associated with reduced areal coverage of clouds (−14.4% K−1).

Beschreibung: Global trends in a new multi-satellite surface soil moisture dataset were analyzed for the period 1988–2010. 27% of the area covered by the dataset showed significant trends (p = 0.05). Of these, 73% were negative and 27% positive. Subtle drying trends were found in the Southern US, central South America, central Eurasia, northern Africa and the Middle East, Mongolia and northeast China, northern Siberia, and Western Australia. The strongest wetting trends were found in southern Africa and the subarctic region. Intra-annual analysis revealed that most trends are not uniform among seasons. The most prominent trend patterns in remotely sensed surface soil moisture were also found in GLDAS-Noah and ERA Interim modeled surface soil moisture and GPCP precipitation, lending confidence to the obtained results. The relationship with trends in GIMMS-NDVI appeared more complex. In areas of mutual disagreement more research is needed to identify potential deficiencies in models and/or remotely sensed products.