Three-dimensional structure of flow at a confluence of river channels with discordant beds

This paper presents three-dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to (1) describe the three-dimensional flow field of a natural junction of channels; (2) assess the role of changes in bed morphology occurring during transport-effective events on the structure of flow at a confluence; and (3) examine how the three-dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams. Three-dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four, two-component electromagnetic current meters. Six detailed velocity profiles were taken at five cross-sections in a wide range of flow conditions. The mean field of flow is characterised by (1) the acceleration of flow in the downstream portion of the post-confluence channel, but by lower velocities upstream in the mixing layer area; (2) a stagnation zone at the apex of the junction; (3) a zone of flow deviation, and strong fluid upwelling, close to the avalanche face and at the margin of the tributary mouth bar; and (4) reduced velocities over the depositional bar at the downstream junction corner. The position and extent of these zones vary with changes in the ratio of momentum flux. Very high intensity of turbulence (peaks up to 50%) and turbulent kinetic energy were observed in the mixing layer region. Distortion of the mixing layer, characteristic of flow where bed discordance is present between the two tributary channels, was evident from mean and turbulent flow data. This field study suggests that the effects of bed discordance on flow, sediment transport, and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow.     

The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in a small forested watershed

The relationship between stream discharge and the concentration of different solutes reflects the hydrological processes occurring in a catchment, but is also strongly affected by antecedent moisture conditions. In this study, the effects of antecedent conditions were examined for a small forested watershed near Montréal (Québec) by comparing data from sequences of fall events for two years of contrasting antecedent moisture conditions. The relationship between discharge and stream water chemistry was also quantified using cross‐correlation analysis. Results show that concentrations of dissolved organic carbon, nitrate and calcium increased during storm events with dry antecedent conditions but that the relationships were weaker under wet conditions. For both years, concentrations of sulfate, silicon and sodium and the pH were negatively correlated with discharge (cross‐correlations from −0·20 to −0·53). With dry antecedent conditions, there was a general decrease in the concentration of all solutes with time, whereas concentrations remained at about the same level under wet conditions. The concentration–discharge relationships for sulfate shifted frequently from higher concentrations during the rising limb to higher concentrations during the falling limb from one event to another as a result of changing antecedent conditions. Although the contrast in antecedent moisture conditions between the two years was moderate in comparison with that reported in other studies (25% dryer than average in 1995 and 12% wetter than average in 1996), the hydrochemistry of the stream was markedly different. Copyright © 1999 John Wiley & Sons, Ltd.     

Impacts of river bank stabilization using riprap on fish habitat in two contrasting environments

Riverbank stabilization using rock riprap is commonly used for protecting road and bridge structures from fluvial erosion. However, little is known about how streams adjust to such perturbation or how this can affect fish habitat in different fluvial environments, particularly for non‐salmonid species in small streams. The objective of this study is to assess impacts of riprap on fish habitat quantity and quality through a pairwise comparison of 27 stabilized and non‐stabilized stream reaches in two physiographic regions, the Saint Lawrence Lowlands and the Appalachian highlands of Montérégie‐Est (Quebec, Canada). Both quantitative (Hydro‐morphological Index of Diversity, HMID) and qualitative (Qualitative Habitat Evaluation Index, QHEI) fish habitat assessment techniques are applied in order to compare results between methods. For each stream reach depth and velocity were measured to calculate HMID. In‐stream cover (woody debris, overhanging vegetation, undercut banks, aquatic macrophytes) and habitat units (pools, riffles, runs, glides) were also documented and used to determine QHEI. Results show that overall bank stabilization using riprap at bridge and stream crossings alters fish habitat characteristics. Loss of in‐stream covers and riparian vegetation lower QHEI scores at stabilized reaches, especially in more pristine Appalachian streams, but has less impact on already altered straightened Lowlands streams. In this latter context, some positive alterations of fish habitat were observed in riprapped reaches due to the coarsening of the substrate and an induced increase of slope. The two metrics (HMID and QHEI) revealed similar differences between stabilized and non‐stabilized sites for Lowlands sites, but their level of agreement was much less in the Appalachian streams, suggesting caution when interpreting habitat quality results based on a single metric. Copyright © 2016 John Wiley & Sons, Ltd.     

Contrasted turbulence intensities in the Indonesian Throughflow: a challenge for parameterizing energy dissipation rate

Microstructure measurements were performed along two sections through the Halmahera Sea and the Ombai Strait and at a station in the deep Banda Sea. Contrasting dissipation rates (??) and vertical eddy diffusivities (K _z ) were obtained with depth-averaged ranges of \(\sim [9 \times 10^{-10}-10^{-5}]\) W kg^??1 and of \(\sim [1 \times 10^{-5}-2 \times 10^{-3}]\) m² s^??1, respectively. Similarly, turbulence intensity, \(I={\epsilon }/(\nu N^{2})\) with ν the kinematic viscosity and N the buoyancy frequency, was found to vary seven orders of magnitude with values up to \(10^{7}\). These large ranges of variations were correlated with the internal tide energy level, which highlights the contrast between regions close and far from internal tide generations. Finescale parameterizations of ?? induced by the breaking of weakly nonlinear internal waves were only relevant in regions located far from any generation area (“far field”), at the deep Banda Sea station. Closer to generation areas, at the “intermediate field” station of the Halmahera Sea, a modified formulation of MacKinnon and Gregg (2005) was validated for moderately turbulent regimes with 100 < I < 1000. Near generation areas marked by strong turbulent regimes such as “near field” stations within strait and passages, ?? is most adequately inferred from horizontal velocities provided that part of the inertial subrange is resolved, according to Kolmogorov scaling.     

Sea surface temperature associations with the late Indian summer monsoon

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.     

Chemical weathering and CO2 consumption rate in a multilayered‐aquifer dominated watershed under intensive farming: The Orgeval Critical Zone Observatory,France

The impact of intensive farming on chemical weathering in the Critical Zone is still an open question. Extensively instrumented and monitored over the last 50 years, the Orgeval Critical Zone Observatory (CZO) in France is an observation site impacted by intensive farming since the 1960s. The Orgeval observatory represents an ideal place to study the response and resilience capability of the Critical Zone under agricultural stress. This paper investigates the chemical composition of different water bodies in two nested catchments of the Orgeval CZO, including rainfall, springs, rivers, and rocks, over one and half hydrological year. We show that elemental and strontium isotopic ratios are powerful to constrain the origin of the elements. The results show that the river chemistry at the outlet of the two nested catchments is dominated by rain inputs (particularly atmospheric dust dissolution) and the chemical weathering of limestone and gypsum. Fertilizer input is clearly visible, although the distinction between gypsum dissolution and fertilizer inputs needs more investigation. The mixtures of water masses inferred from our data are in good agreement with the hydrological context of the watershed, that is, a multilayered aquifer structure. At the main outlet of the CZO, we estimate that the input of ocean‐derived solutes through rainfall represents 7 t km^?2 year^?1, on the same order of magnitude as the net fertilizer input (10 t km^?2 year^?1), and that rock weathering releases 50 t km^?2 year^?1. Including previously published physical erosion rates, we estimate that the total denudation rate (physical and chemical) of the Orgeval CZO is 20 mm (1,000 year)^?1, which, along with the entire Seine watershed, is among the lowest chemical denudation rates for carbonate terrains under temperate climate. Chemical denudation is about 10 times higher than physical erosion in the Orgeval CZO. The consumption of CO₂ by rock weathering is estimated to be between 265.10³ and 360.10³ molC km² year^?1, similar for the two nested catchments. Compared with the rivers, the springs show a higher CO₂ consumption rate that suggests, as pointed out earlier, a enhancement of carbonate dissolution linked to nitrification and thus fertilizer application. The hyporheic zone appears to be a hot spot in the carbon cycle at the Orgeval CZO. This study sheds light on the complex, anthropocenic, interplay between geology, climate, and human activities that characterize and that take place in intensive agriculture regions.     

The role of the ocean feedback on Asian and African monsoon variations at 6 kyr and 9.5 kyr BP

The role of ocean feedback on monsoon variations at 6 and 9.5 kyr Before Present (BP) compared to present-day is investigated by using sets of simulations computed with the IPSL–CM4 ocean–atmosphere coupled model and simulations with the atmospheric model only with the SST prescribed to the present-day simulation for the coupled model. This work is complementary to the study by Marzin and Braconnot (2009) who have analyzed in detail the response of Indian and African monsoons to changes in insolation at 6 and 9.5 kyr BP using the IPSL–CM4 coupled model. The monsoon rainfall was intensified at 6 and 9.5 kyr BP compared to 0 kyr BP as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. In this paper, the impact of the ocean feedback is analysed for the Indian, East-Asian and African monsoons. The response of the ocean to the 6 and 9.5 kyr BP insolation forcing shares similarities between the two periods, but we highlight local differences and a delay in the response of the surface ocean between 6 and 9.5 kyr BP. The ocean feedback is shown to be positive for the early stage of the African monsoon. A dipole of SST in the tropical Atlantic favouring the earlier build-up of the monsoon in the 6 and 9.5 kyr BP coupled simulations. However, it is strongly negative for the Indian and East Asian monsoons, and of stronger amplitude at 9.5 than at 6 kyr BP over India. In these Asian regions, the convection is more active over the ocean than over the continent during the late monsoon season due to the ocean feedback. The results are consistent with previous studies about 6 kyr BP climate. In addition, it is shown that the ocean feedback is not sufficient to explain the relative amplifications of the different monsoon systems within the three periods of the Holocene, but that the mechanisms such as the effect of the precession on the seasonal cycle of monsoons as discussed in Marzin and Braconnot (2009) are more plausible.     

Observations of the temperature and polarization anisotropies with Boomerang 2003

The Boomerang experiment completed its final long duration balloon (LDB) flight over Antarctica in January 2003. The focal plane was upgraded to accommodate four sets of 145 GHz polarization sensitive bolometers (PSBs), identical to those to be flown on the Planck HFI instrument. Approximately, 195 hours of science observations were obtained during this flight, including 75 hours distributed over 1.84% of the sky and an additional 120 hours concentrated on a region covering 0.22% of the sky. We derive the angular power spectra of the cosmic microwave background (cmb) temperature and polarization anisotropies from these data. The temperature anisotropies are detected with high signal to noise on angular scales ranging from several degrees to 10 arcminutes. The curl-free (EE) component is detected at 4.8σ, and a two-sigma upper limit on the curl (BB) component of 8.6 μK² is obtained on scales corresponding to 0.5°. Both the temperature and polarization anisotropies are found to be consistent with a concordance ΛCDM cosmology that is seeded by adiabatic density perturbations. In addition to the cmb observations, Boomerang03 surveyed a 300 square degree region centered on the Galactic plane. These observations represent the first light for polarization sensitive bolometers, which are currently operational in two South-Pole based polarimeters, as well as Planck HFI, at frequencies ranging from 100 to 350 GHz (3 mm to 850 μm).     

Internal and forced variability along a section between Greenland and Portugal in the CLIPPER Atlantic model

Numerical models are used to estimate the meridional overturning and transports along the paths of two hydrographic cruises, carried out in 1997 and 2002 from Greenland to Portugal. We have examined the influence of the different paths of the two cruises and found that it could explain 0.4 to 2 Sv of difference in overturning (the precise value is model-dependent). Models show a decrease in the overturning circulation between 1997 and 2002, with different amplitudes. The CLIPPER ATL6 model reproduces well the observed weakening of the overturning in density coordinates between the cruises; in the model, the change is due to the combination of interannual and high-frequency forcing and internal variability associated with eddies and meanders. Examination of the -coordinate overturning reveals model–data discrepancies: the vertical structure in the models does not change as much as the observed one. The East Greenland current variability is mainly wind-forced in the ATL6 model, while fluctuations due to eddies and instabilities explain a large part of the North Atlantic Current variability. The time-residual transport of dense water and heat due to eddy correlations between currents and properties is small across this section, which is normal to the direction of the main current.