Variations In External Forcing Research Articles

The direction of landscape erosion

Abstract. The rate of erosion of a landscape depends largely on local gradient and material fluxes. Since both quantities are functions of the shape of the catchment surface, this dependence constitutes a mathematical straitjacket, in the sense that – subject to simplifying assumptions about the erosion process, and absent variations in external forcing and erodibility – the rate of change of surface geometry is solely a function of surface geometry. Here we demonstrate how to use this geometric self-constraint to convert a gradient-dependent erosion model into its equivalent Hamiltonian, and explore the implications of having a Hamiltonian description of the erosion process. To achieve this conversion, we recognize that the rate of erosion defines the velocity of surface motion in its orthogonal direction, and we express this rate in its reciprocal form as the surface-normal slowness. By rewriting surface tilt in terms of normal slowness components and deploying a substitution developed in geometric mechanics, we extract what is known as the fundamental metric function of the model phase space; its square is the Hamiltonian. Such a Hamiltonian provides several new ways to solve for the evolution of an erosion surface: here we use it to derive Hamilton's ray-tracing equations, which describe both the velocity of a surface point and the rate of change of the surface-normal slowness at that point. In this context, gradient-dependent erosion involves two distinct directions: (i) the surface-normal direction, which points subvertically downwards, and (ii) the erosion ray direction, which points upstream at a generally small angle to horizontal with a sign controlled by the scaling of erosion with slope. If the model erosion rate scales faster than linearly with gradient, the rays point obliquely upwards, but if erosion scales sublinearly with gradient, the rays point obliquely downwards. This dependence of erosional anisotropy on gradient scaling explains why, as previous studies have shown, model knickpoints behave in two distinct ways depending on the gradient exponent. Analysis of the Hamiltonian shows that the erosion rays carry boundary-condition information upstream, and that they are geodesics, meaning that surface evolution takes the path of least erosion time. Correspondingly, the time it takes for external changes to propagate into and change a landscape is set by the velocity of these rays. The Hamiltonian also reveals that gradient-dependent erosion surfaces have a critical tilt, given by a simple function of the gradient scaling exponent, at which ray-propagation behaviour changes. Channel profiles generated from the non-dimensionalized Hamiltonian have a shape entirely determined by the scaling exponents and by a dimensionless erosion rate expressed as the surface tilt at the downstream boundary.

The Evolving Role of External Forcing in North Atlantic SST Variability over the Last Millennium

AbstractAtlantic multidecadal variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic Ocean basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea surface temperature (SST) anomalies that have similar characteristics to the observed AMV. However, there is an active debate as to whether these internal fluctuations driven by coupled atmosphere–ocean variability remain influential to the AMV on multidecadal time scales in our modern, anthropogenically forced climate. Here we provide evidence from multiple large ensembles of climate models, paleoreconstructions, and instrumental observations of a growing role for external forcing in the AMV. Prior to 1850, external forcing, primarily from volcanoes, explains about one-third of AMV variance. Between 1850 and 1950, there is a transitional period, where external forcing explains one-half of AMV variance, but volcanic forcing only accounts for about 10% of that. After 1950, external forcing explains three-quarters of AMV variance. That is, the role for external forcing in the AMV grows as the variations in external forcing grow, even if the forcing is from different sources. When forcing is relatively stable, as in earlier modeling studies, a higher percentage of AMV variations are internally generated.

Force Dependent Quantum Phase Transition in the Hybrid Optomechanical System

The optomechanics shows a great potential in quantum control and precise measurement due to appropriate mechanical control. Here we theoretically study the quantum phase transition in a hybrid atom-optomechanical cavity with an external force. Our study shows, in the thermodynamic limit, the critical value of quantum phase transition between the normal phase and super-radiant phase can be controlled and modified by the external force via the tunable frequency of optomechanics, then a force dependent quantum phase transition can be achieved in our system. Moreover, this force dependent quantum phase transition can be employed to detect the external force variation. In addition, our numerical simulations illustrate the sensitivity of the external force measurement can be improved by the squeezing properties of the quantum phase transition.

A statistical method for simulation of boiling flow inside a Platinum microchannel

This paper presents molecular dynamics simulations to study on the effects of external driving force and boundary wall temperature on the density, velocity and temperature profiles of Argon fluid atoms, flowed in a platinum microchannel with square section. The Argon atoms are structured in three regions. Two thin liquid film of Argon sandwich central vapor zone. Applying wall temperatures in the range of 84K to 133K prepares boiling condition which causes liquid atoms move from lateral layers to central bins of microchannel. Afterward, density of central layers of microchannel increases which is highlighted by sampling data after equilibrium condition of system energy in 4 time steps. It is concluded that augmentation of external driving forces on the Argon atoms from 0.002 to 0.01 and 0.02 eV/Angstrom can increase flow temperature from 180K to 1920K and 7570K, respectively which is noticeable for practical application such as medical especially in Cryosurgery. Moreover, augmentation of external forces can increase velocity profiles of Argon fluid flow. Generally, density was independent from variation of external force and boundary wall temperature in the studied limits. Also, alteration of boundary wall temperature does not play very important role on the velocity and temperature of Argon fluid inside microchannel.

Diagnosis of Middle-Atmosphere Climate Sensitivity by the Climate Feedback–Response Analysis Method

Abstract The authors present a new method to diagnose the middle-atmosphere climate sensitivity by extending the climate feedback–response analysis method (CFRAM) for the coupled atmosphere–surface system to the middle atmosphere. The middle-atmosphere CFRAM (MCFRAM) is built on the atmospheric energy equation per unit mass with radiative heating and cooling rates as its major thermal energy sources. MCFRAM preserves CFRAM’s unique feature of additivity, such that partial temperature changes due to variations in external forcing and feedback processes can be added to give a total temperature change for direct comparison with the observed temperature change. In addition, MCFRAM establishes a physical relationship of radiative damping between the energy perturbations associated with various feedback processes and temperature perturbations associated with thermal responses. In this study, MCFRAM is applied to both observations and model output fields to diagnose the middle-atmosphere climate sensitivity. The authors found that the largest component of the middle-atmosphere temperature response to the 11-yr solar cycle (solar maximum vs solar minimum) is the partial temperature change due to the variation of the solar flux. Increasing CO2 cools the middle atmosphere, whereas the partial temperature change due to changes in O3 can be either positive or negative. The application of MCFRAM to model dynamical fields reconfirms the advantage of introducing the residual circulation to characterize middle-atmosphere dynamics in terms of the partial temperature changes. The radiatively driven globally averaged partial temperature change is approximately equal to the observed temperature change, ranging from −0.5 K near 25 km to −1.0 K near 70 km between solar maximum and solar minimum.

An extended shakedown theory on an elastic–plastic spherical shell

The article analyses one of possible optimization methodologies for ideal elastic–plastic structures at shakedown and its application for shallow spherical shells having prescribed geometry and affected by a variable repeated load (VRL) – a system of external forces that may vary independently of each other. The paper accepts that only time-independent upper and lower bounds of variations in external forces are given. The pronounced effect of external forces, i.e. in this context, possible histories of variations in forces, is not examined (the unloading phenomenon of cross-sections is ignored in the course of plastic deformation). The discussed concept of the structure at shakedown refers to the Melan theorem related to statically allowable admissible internal forces. Thus, for the discretization of the spherical shell, with the help of an assumption about small displacements, the equilibrium finite element method based on internal force approximation is applied. The limit axial force of the cross-section is supposed to be constant within the bounds of the finite element, and only the optimal distribution of limit internal forces among elements, according to the selected criterion, is in need of search. The article presents a discrete mathematical model for determining the optimal allocation problem with strength and stiffness requirements. The model conforms to the limit axial force of the shallow spherical shell of the variable repeated load and takes into account ultimate and serviceability limit states of EC3 with corresponding reliability levels. Structural optimization methods refer to extreme energy principles of mechanics and are illustrated with the numerical examples of spherical shell optimization.

The freshwater balance of polar regions in transient simulations from 1500 to 2100 AD using a comprehensive coupled climate model

The ocean and sea ice in both polar regions are important reservoirs of freshwater within the climate system. While the response of these reservoirs to future climate change has been studied intensively, the sensitivity of the polar freshwater balance to natural forcing variations during preindustrial times has received less attention. Using an ensemble of transient simulations from 1500 to 2100 AD we put present-day and future states of the polar freshwater balance in the context of low frequency variability of the past five centuries. This is done by focusing on different multi-decadal periods of characteristic external forcing. In the Arctic, freshwater is shifted from the ocean to sea ice during the Maunder Minimum while the total amount of freshwater within the Arctic domain remains unchanged. In contrast, the subsequent Dalton Minimum does not leave an imprint on the slow-reacting reservoirs of the ocean and sea ice, but triggers a drop in the import of freshwater through the atmosphere. During the twentieth and twenty-first century the build-up of freshwater in the Arctic Ocean leads to a strengthening of the liquid export. The Arctic freshwater balance is shifted towards being a large source of freshwater to the North Atlantic ocean. The Antarctic freshwater cycle, on the other hand, appears to be insensitive to preindustrial variations in external forcing. In line with the rising temperature during the industrial era the freshwater budget becomes increasingly unbalanced and strengthens the high latitude’s Southern Ocean as a source of liquid freshwater to lower latitude oceans.

Seasonal variability of the Caspian Sea three-dimensional circulation, sea level and air-sea interaction

Abstract. A three-dimensional primitive equation model including sea ice thermodynamics and air-sea interaction is used to study seasonal circulation and water mass variability in the Caspian Sea under the influence of realistic mass, momentum and heat fluxes. River discharges, precipitation, radiation and wind stress are seasonally specified in the model, based on available data sets. The evaporation rate, sensible and latent heat fluxes at the sea surface are computed interactively through an atmospheric boundary layer sub-model, using the ECMWF-ERA15 re-analysis atmospheric data and model generated sea surface temperature. The model successfully simulates sea-level changes and baroclinic circulation/mixing features with forcing specified for a selected year. The results suggest that the seasonal cycle of wind stress is crucial in producing basin circulation. Seasonal cycle of sea surface currents presents three types: cyclonic gyres in December–January; Eckman south-, south-westward drift in February–July embedded by western and eastern southward coastal currents and transition type in August–November. Western and eastern northward sub-surface coastal currents being a result of coastal local dynamics at the same time play an important role in meridional redistribution of water masses. An important part of the work is the simulation of sea surface topography, yielding verifiable results in terms of sea level. The model successfully reproduces sea level variability for four coastal points, where the observed data are available. Analyses of heat and water budgets confirm climatologic estimates of heat and moisture fluxes at the sea surface. Experiments performed with variations in external forcing suggest a sensitive response of the circulation and the water budget to atmospheric and river forcing.

Temporal variation in phytoplankton in two lakes with contrasting disturbance regimes

Ecological communities are structured by multiple biotic and abiotic forces. Temporal variations in external forces result in communities that vary temporally at multiple scales. Aim of this study was to examine temporal variations in phytoplankton communities in two small adjacent eutrophic lakes with contrasting disturbance regimes; one is shallow and constantly mixing and the other one is stratified and thus more stable. Analyses of classification strength showed that in the more stable lake, community composition differed more among the years than in the more disturbed lake. However, according to Multi-Response Procedures, communities were more variable within years in the more disturbed lake. According to Canonical Correspondence Analysis, community composition was most strongly controlled by variation in nitrogen and iron concentrations, pH, Secchi-depth and nitrogen to phosphorus ratio. We also found a strong negative relationship between diversity and biomass production in both lakes. Our results support the view that under very disturbed conditions, the assembly follows more the same trajectory among the years, as only a limited number of species persists and the number of species occupying the lake throughout the years may be lower. This gives evidence that physical forcing is a highly important driver for the phytoplankton assembly resulting in clear differences in communities between the adjacent lakes.

The force is with α‐actinin: dynamic regulation of the extracellular matrix‐cytoskeletal connection in airway smooth muscle

The force is with α‐actinin: dynamic regulation of the extracellular matrix‐cytoskeletal connection in airway smooth muscle

Interannual variability of stratospheric trace gases: The role of extratropical wave driving

AbstractThe interannual variability of methane and ozone from a 35‐year middle atmosphere climate model simulation with no interannual variations in external forcing or chemistry is examined. The internal dynamics in the model produces large tracer interannual variability, particularly in polar regions. During winter and spring the interannual standard deviation in the polar lower‐middle stratosphere is about 30% of the climatological mean for methane and 15% for ozone. Global‐scale, coherent interannual variations in temperature, residual circulation, and tracers are correlated with variability in the extratropical wave forcing. Statistically significant positive correlations between wave driving and polar tracer tendencies, including column ozone, occur from autumn to spring in both hemispheres. These positive correlations imply that interannual variations in polar tracers are dominated by variations in the horizontal eddy transport and not by variations in residual mean descent rates. Copyright © 2004 Royal Meteorological Society

A Theoretical Analysis on the Deformation of Rectangular Plate Under Out-of-Plane Tearing

A theoretical analysis is proposed to predict the out-of-plane tearing response of rectangular plates with two or three legs. The nonlinear differentiate equation for deformation and the double linear strain-hardening constitutive relation for bending moment against curvature are used in the analysis. The external work is assumed dissipated through the bending deformation of the legs and the crack propagation at the tip of the crack between the legs. A good agreement has been achieved between the theoretical predictions and the experimental results for the variation of external force with displacement. It has been found that the variation of bending force with displacement significantly depends on the strain-hardening modulus and the tearing force.

Quantifying Predictability Variations in a Low-Order Occan-Atmosphere Model: A Dynamical Systems Approach

A dynamical systems approach is used to quantify the predictability of weather and climatic states of a low order, moist general circulation model. The effects on predictability of incorporating a simple oceanic circulation are evaluated. The predictability and structure of the model attractors are compared using Lyapunov exponents, local divergence rates, and the correlation and Lyapunov dimensions. Lyapunov exponents quantify global, or time-averaged predictability, by measuring the mean rate of growth of small perturbations on an attractor, while local divergence rates quantify temporal variations of this error growth rate and thus measure local, or instantaneous, predictability. Activating an oceanic circulation increases the average error doubling time of the atmosphere and the coupled ocean-atmosphere system by 10% while decreasing the variance of the largest local divergence rate by 20% . The correlation dimension of the attractor decreases slightly when an oceanic circulation is activated, while the Lyapunov dimension decreases more significantly because it depends directly on the Lyapunov exponents. The average predictability of annually averaged states is improved by 25% when an oceanic circulation develops, and the variance of the largest local divergence rate also decreases by 25%. One-third of the yearly averaged states have local error doubling times larger than 2 years, indicating that annual averages may, at times, be predictable, even without predictable variations in external forcing. The dimensions of the attractors of the yearly averaged states are not significantly different than the dimensions of the attractors of the original model. Arguably the most important contribution of this article is the demonstration that the local divergence rates provide a concise quantification of the variations of predictability on attractors and an efficient basis for comparing their local predictability characteristics. From a practical standpoint, local divergence rates might he computed to provide a real-time estimate of local predictability to accompany an operational forecast.

Decadal oscillations of the air‐ice‐ocean system in the Northern Hemisphere

Abstract Feedback mechanisms among decadal oscillations of Northern Hemisphere atmospheric circulation, Arctic‐subarctic ice cover and ocean circulation are studied. Decadal oscillations of the ice cover have been observed since 1950, most significantly in the Barents Sea and on the Labrador Shelf. The extreme southward extent of Labrador ice is explained by local weather conditions, such as low air temperatures and strong northwesterly winds. This weather pattern is attributed to an atmospheric circulation with low sea‐level pressure anomalies over the Eurasian Basin in the Arctic. Barents Sea ice is also highly (but inversely) correlated with this atmospheric circulation, i.e. Barents Sea ice is inversely correlated with Labrador ice. Barents Sea ice is weakly correlated with local air temperature. The following air‐ice‐ocean coupling is suggested for Barents Sea ice and atmospheric circulation: the increased cyclonic wind‐stress curl associated with the atmospheric circulation enhances the exchange of cold Arctic water, including sea ice, and warm Atlantic water, and reduces the amount of Barents Sea ice. The reduced ice cover encourages heat flux from the Barents Sea to the atmosphere, tending to reinforce the low pressure in the Arctic. This positive feedback amplifies the oscillations of the air‐ice‐ocean system driven by external forcing, such as solar activity with fairly weak variability. A mechanistic, 2‐level ocean model, which is driven by prescribed buoyancy flux and wind stresses, suggests that the Arctic‐Atlantic system resonates with the variable wind stresses at a decadal time‐scale. A conceptual air‐ice‐ocean model that includes the fundamental oceanic response found by the mechanistic ocean model as well as positive feedback between air and ice‐ocean systems shows that weak decadal variations in external forcing can be amplified in the air‐ice‐ocean system.

Intermittent baroclinic instability and fluctuations in geophysical circulations

The presence of blocking highs in atmospheric circulation, bringing periods of stable weather alternating with more variable weather patterns, is an example of two distinct modes of behaviour occurring under similar conditions in a natural system. The occurrence of two or more different flow regimes under a given external forcing is a consequence of nonlinear dynamics that allows the system to equilibrate in different modes. Each mode can be stable over a range of conditions and yet there may be sudden transitions between modes. We have now discovered in a laboratory experiment using rotating fluids a case of at least two modes of behaviour under identical conditions. In one mode, a weak continuous forcing drives a mean circulation that is steady but on which are superimposed waves whose amplitudes vacillate under the influence of viscous dissipation. Under suitable conditions there is an alternative mode in which the mean flow fluctuates and unstable waves occur only in short bursts at periodic intervals separated by relatively long periods of stable flow with no waves. The existence of multiple equilibrium states is of interest in the climatology of all geophysical and astrophysical fluid bodies. Our discovery of a mode involving intermittent bursts of instability and long-period fluctuations of mean flow (resulting purely from internal dynamics rather than from variations in external forcing) may have implications for the variability of oceanic circulation or solar radiation on timescales of a few years to centuries.