Sensitivity of coastal polynyas and high-salinity shelf water production in the Ross Sea,Antarctica, to the atmospheric forcing

Coastal polynyas around Antarctica are the place of intense air–sea exchanges which eventually lead to the formation of high-salinity shelf waters (HSSW) over continental shelves. Here, the influence of atmospheric forcing on coastal polynyas in the Ross Sea is studied by contrasting the response of a regional ocean/sea-ice circulation model to two different atmospheric forcing sets. A first forcing (DFS3) is based on ERA40 atmospheric surface variables and satellite products. A second forcing (MAR) is produced on the basis of ERA40 with a dynamical downscaling procedure. As compared to DFS3, MAR forcing is shown to improve substantially the representation of small-scale patterns of coastal winds with stronger katabatic winds along the coast. The response of the ocean/sea-ice model to the two forcing sets shows that the MAR forcing improves substantially the geographical distribution of polynyas in the Ross Sea. With the MAR forcing, the polynya season is also shown to last longer with a greater ice-production rate. As a consequence, a greater flow of dense water out of the polynyas is found with the MAR forcing and the properties of HSSW are notably improved as compared to the DFS3 forcing. The factors contributing to the activity of Terra Nova Bay and Ross Ice Shelf polynyas in the model are studied in detail. The general picture that emerges from our simulations is that the properties of HSSW are mostly set by brine rejection when the polynya season resume. We found that coastal polynyas in the Ross Sea export about 0.4 Sv of HSSW which then flows along three separate channels over the Ross Shelf. A 6-month time lag is observed between the peak of activity of polynyas and the maximum transport across the sills in the channels with a maximum transport of about 1 Sv in February. This lag corresponds to the time it takes to the newly formed HSSW to spread from the polynya to the sills (at a speed of nearly 2 cm s⁻¹).     

A partially non-ergodic ground-motion prediction equation for Europe and the Middle East

A partially non-ergodic ground-motion prediction equation is estimated for Europe and the Middle East. Therefore, a hierarchical model is presented that accounts for regional differences. For this purpose, the scaling of ground-motion intensity measures is assumed to be similar, but not identical in different regions. This is achieved by assuming a hierarchical model, where some coefficients are treated as random variables which are sampled from an underlying global distribution. The coefficients are estimated by Bayesian inference. This allows one to estimate the epistemic uncertainty in the coefficients, and consequently in model predictions, in a rigorous way. The model is estimated based on peak ground acceleration data from nine different European/Middle Eastern regions. There are large differences in the amount of earthquakes and records in the different regions. However, due to the hierarchical nature of the model, regions with only few data points borrow strength from other regions with more data. This makes it possible to estimate a separate set of coefficients for all regions. Different regionalized models are compared, for which different coefficients are assumed to be regionally dependent. Results show that regionalizing the coefficients for magnitude and distance scaling leads to better performance of the models. The models for all regions are physically sound, even if only very few earthquakes comprise one region.     

Life strategy traits of the liana Sericostachys scandens spreading in the montane forests in the Kahuzi-Biega national park (DR Congo)

Sericostachys scandens is a monocarpic and heliophilous liana, native in tropical African forests. In the montane forests of the Kahuzi-Biega National Park (KBNP) (East of DR Congo), it has been expanding very strongly for a decade, and is currently considered as having negative impacts on biodiversity conservation. In this paper, we test if S. scandens differs from three co-occurring, native, non spreading lianas (Gouania longispicata, Tacazzea apiculata and Adenia bequaertii) for functional traits which might influence plant expansion. For leaf traits (SLA, dry matter content, nitrogen concentration), S. scandens did not show extreme values compared to those of the three other lianas. In contrast, S. scandens had much higher biomass allocation to sexual reproduction. It also differs from the three other lianas for its reproductive strategy that combines both vegetative propagation and sexual reproduction, and propagule dispersal by wind. Moreover, S. scandens has larger leaves and a greater number of lateral branches per unit stem length. It is argued that the particular combination of functional traits exhibited by S.scandens may in part explain its propensity to behave as an opportunistic weed in the disturbed areas in the montane forests of Kahuzi-Biega.     

Qualitative and Quantitative Description of Multibeam Echosounder Systematic Errors on Rocky Areas

This paper deals with the problem of systematic depth errors made in surveying dumped rocks with multibeam echosounder. These errors may induce dangers for navigation in very shallow water areas or huge costs for coastal engineering contractors who perform rock dumping operation and surveying. We analyze results from four different multibeam echosounder systems and compare those data to a reference digital terrain model of a dumped rocks area obtained from a 3D terrestrial laser scanner. The systematic depth errors are statistically described and analysed on local areas. Finally, we propose a look-up table linking the error amplitude with the rock size, resulting from our error analysis.     

Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data

Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as ‘opportunistic’ since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts.     

Assessing the Nonlinearity of Karst Response Function under Variable Boundary Conditions

The hydraulic and transfer response of karst aquifers is complex and often highly nonlinear: due to their high transmissivity and connection with the surface, such systems are very sensitive to modifications of their boundary conditions. The aim of this study was to assess the variation of the response depending on both upstream and downstream parameters, and propose a methodology to simulate the response of the karst system depending on those parameters. The impact of the variations of multiple environmental parameters on the response of a karstic system submitted to tidal variations (Normandy, France) was investigated after a campaign of artificial tracer tests acquired in very different hydrologic conditions (rainfall events, low tide, high tide, low/high piezometric level, and low/high waters). Principal components analysis and hierarchical clustering were applied on both environmental variables and karstic system response variables (parameters of the residence time distribution [RTD] curves). Equations between the RTD parameters and the most relevant variables were established using a symbolic regression algorithm. It appeared that the variations of the RTD parameters depend mainly on the cumulated rainfall preceding the injection, the piezometric level of the aquifer, and on the tide parameters. The hydraulic conditions downstream of the aquifer have a strong influence on the hydraulic and transfer response of the aquifer. The response of the aquifer in various and extreme conditions has been simulated using the equations resulting from the symbolic regression algorithm. Such relationships can be useful for management of water resources in karst media, and support decision making.     

An estimate of the influence of loading effects on tectonic velocities in the Pyrenees

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we investigated the effect of loading on estimates of tectonic velocity computed from campaign-style GNSS observations. The study region is in the Pyrenees mountain range between France and Spain (ResPyr campaigns). In this area, seismic activity is continuous and moderate and the expected amplitude of the horizontal tectonic velocity is less than 0.5 mm/yr. In order to determine the velocity, 4 sparse GNSS campaigns were carried out from 1995 to 2010. Considering this small rate of deformation, loading phenomena can contribute a non-negligible artifact to the velocity computation that could affect our geodynamical interpretation. In this investigation, we specifically considered the atmospheric, hydrological, and non-tidal ocean loading phenomena. The computed loading deformations for this region show the horizontal displacements are dominated by the non-tidal ocean loading (maximum 4 mm for the North and 3.1 mm for the East components); the main vertical contributions are due to the atmospheric and continental water storage loading (maximum 14.3 for the atmosphere and 8.1 mm for the hydrology, respectively). We have found that the dominant loading effect on the horizontal velocity is the non-tidal ocean loading (mean of 0.11 mm/yr), whereas the vertical component is dominated by the hydrological loading (mean of 0.21 mm/yr). Since the study area is in a mountainous region, we also analyzed the difference between the atmospheric and the topography dependent atmospheric loading models at our GNSS campaign sites. We did not find any significant difference between the two atmospheric loading models in terms of horizontal velocity. Finally, we performed simulations to identify the optimum timing and frequency of future GNSS campaigns in this area that would minimize the loading effects on tectonic velocity estimates.     

Fourier spectral- and duration models for the generation of response spectra adjustable to different source-, propagation-, and site conditions

One of the major challenges related with the current practice in seismic hazard studies is the adjustment of empirical ground motion prediction equations (GMPEs) to different seismological environments. We believe that the key to accommodating differences in regional seismological attributes of a ground motion model lies in the Fourier spectrum. In the present study, we attempt to explore a new approach for the development of response spectral GMPEs, which is fully consistent with linear system theory when it comes to adjustment issues. This approach consists of developing empirical prediction equations for Fourier spectra and for a particular duration estimate of ground motion which is tuned to optimize the fit between response spectra obtained through the random vibration theory framework and the classical way. The presented analysis for the development of GMPEs is performed on the recently compiled reference database for seismic ground motion in Europe (RESORCE-2012). Although, the main motivation for the presented approach is the adjustability and the use of the corresponding model to generate data driven host-to-target conversions, even as a standalone response spectral model it compares reasonably well with the GMPEs of Ambraseys et al. (Bull Earthq Eng 3:1–53, 2005), Akkar and Bommer (Seismol Res Lett 81(2):195–206, 2010) and Akkar and Cagnan (Bull Seismol Soc Am 100(6):2978–2995, 2010).