Order Of Several Years Research Articles

Nutrients against adipogenesis : aim to cheating age or weight reduction

Obesity is associated with increased risk of conditions such as type 2 diabetes, CHD, hypertension, dyslipidaemia (which are often included in the so-called metabolic syndrome), gall stones, certain types of cancer (breast and colon), osteoarthritis, non-alcoholic steatohepatitis, sleep apnoea, infertility, and many psychological conditions. Moreover, there is a marked reduction in life expectancy of the order of several years.

Promoting zero-emissions vehicles using robust multi-period tradable credit scheme

This study designs a robust multi-period tradable credit scheme (TCS) to incentivize travelers to shift from internal combustion engine vehicles (ICEVs) to zero-emissions vehicles (ZEVs) over a long-term planning horizon to reduce vehicular emissions. The need for robust design arises because of uncertainty in forecasting travel demand over a planning horizon in the order of several years. The robust multi-period TCS design is formulated as a bi-level model. In the upper level, the central authority (CA) determines the TCS parameters (credit allocation and charging schemes) by vehicle type to minimize the worst-case vehicular emissions rate, i.e. the maximum vehicular emissions rate under the possible travel demand scenarios. The upper-level model is a mixed-integer nonlinear program. In the lower level, travelers minimize their generalized travel costs under the TCS parameters obtained in the upper level. These parameters are used to determine the vehicle type choice, between ICEVs and ZEVs, using a binomial logit function, and influence route selection based on the difference in credits charged on links for these two vehicle types. The lower-level model is a mathematical program with equilibrium constraints. The bi-level model is solved using a cutting plane method. Numerical experiments illustrate that the incentive to shift to ZEVs is fostered by allocating more credits and charging fewer credits to ZEV travelers compared to ICEV travelers. Further, the proposed TCS design reduces volatility in the realized vehicular emissions rates under different travel demand scenarios compared to a TCS design that does not consider demand uncertainty.

A global mass budget for positively buoyant macroplastic debris in the ocean

Predicted global figures for plastic debris accumulation in the ocean surface layer range on the order of hundreds of thousands of metric tons, representing only a few percent of estimated annual emissions into the marine environment. The current accepted explanation for this difference is that positively buoyant macroplastic objects do not persist on the ocean surface. Subject to degradation into microplastics, the major part of the mass is predicted to have settled below the surface. However, we argue that such a simple emission-degradation model cannot explain the occurrence of decades-old objects collected by oceanic expeditions. We show that debris circulation dynamics in coastal environments may be a better explanation for this difference. The results presented here suggest that there is a significant time interval, on the order of several years to decades, between terrestrial emissions and representative accumulation in offshore waters. Importantly, our results also indicate that the current generation of secondary microplastics in the global ocean is mostly a result of the degradation of objects produced in the 1990s and earlier. Finally, we propose a series of future emission scenarios until 2050, discussing the necessity to rapidly reduce emissions and actively remove waste accumulated in the environment to mitigate further microplastic contamination in the global ocean.

Characterization of Biostable Atomic Layer Deposited (ALD) Multilayer Passivation Coatings for Active Implants.

The next generation of flexible, electrically active implants, such as brain implants or retina chips require a flexible, biostable as well as biocompatible passivation, ensuring a degradation-free usage for long time periods on the order of several years. Until today, these passivations are prepared mostly by polyimides or parylene, both of which are water vapor permeable to a certain degree. To remedy this deficiency, Atomic Layer Deposited (ALD) thin films are characterized regarding their electrical passivating features under conditions of accelerated aging, such as elevated temperatures in a liquid environment. The initial electrical passivation by various ALD deposited multilayers, combining alternating thin Al2O3 and TiO2 layers is the goal of this research as well as the stability of these layers under induced degradation. Such layers, in combination with a parylene passivation, would ensure a water vapor impermeable and biocompatible coating.

On the cause of variability of the cosmic ray spectrum in the knee region

Cosmic ray (CR) energy spectra for H, He, Si, and Fe nuclei with energy-to-charge number ratios ℰ/Z in the range from 10 to 5 × 107 GeV are studied using observational data obtained at different times in different energy ranges: AMS-02, CREAM, Tibet ASγ, Tibet (hybrid), GRAPES-3, KASCADE, and KASCADE-Grande. Comparison of the H and He CR fluxes according to the KASCADE and KASCADE-Grande data (for different models of deconvolving CR spectra) with the Tibet ASγ and Tibet (hybrid) data obtained at another time in the range of ℰ/Z ∼ 3 × 106 GeV demonstrates space weather-caused variability of the CR flux. This feature of CR energy spectra in the Tibet ASγ data is most clearly observed in the spectra of heavier nuclei (Si and Fe) according to the KASCADE-Grande and GRAPES-3 data. The variability in the energy spectra of all CRs in the vicinity of the “knee” is shown in the data of Yakutsk EAS, CASA-BLANCA, and Tibet-III experiments. The variability of the CR flux on a time scale on the order of several years exists only if the source corresponding to the peak in the energy spectrum is situated at a distance of no more than 1 pc from the Sun. Rapid surfatron acceleration of CRs may result from colliding interstellar clouds nearest to the Sun (LIC and G). This acceleration mechanism allows one to explain the variability of the CR spectrum in the range 103 GeV < ℰ/Z < 108 GeV. Conditions for the trapping of strongly relativistic Fe nuclei by an electromagnetic wave, the dynamics of the components of the particle velocity and momentum, and the dependence of the particle acceleration rate on the initial parameters of the problem are analyzed using numerical calculations. The structure of the phase plane of the accelerated Fe nuclei is examined. Optimal conditions for the implementation of ultrarelativistic surfatron acceleration of Fe nuclei by an electromagnetic wave are formulated.

A coupled wave–current–sediment transport model for an estuarine system: Application to the Río de la Plata and Montevideo Bay

Modelling the hydrodynamics and fine-sediment dynamics in estuarine environments is important for coastal engineering design and environmental assessment. This paper presents the application of a coupled wave–current–sediment transport model to a complex estuarine system with high spatial resolution in a harbour area. The case study was the Río de la Plata, with a focus on the Montevideo Bay area. We used a bidimensional depth-averaged approach, and the sediment transport modelling focused on fine cohesive sediments. The model considers realistic forcings, allows for the simulation of complex geometries such as those present in harbour basins and is capable of providing long-term environmental simulations (on the order of several years) within reasonable computational times. The model results are in good agreement with the measured data and satisfactorily represent the main features of the flow and sediment dynamics of the Río de la Plata. The effect of the internal coupling on the hydrodynamic results is analysed, and the computational times with various coupling alternatives are discussed. The dynamics of fine sediment in Montevideo Bay were analysed based on the model results. The current-induced bottom shear stress results are relevant for representing the permanent suspended sediment concentrations, whereas the wave-induced bottom shear stress is fundamental for reproducing the main resuspension events during storm conditions. The suspended fine-sediment dynamics in the estuary are strongly controlled by the sediment exchange between the bed and water column, whereas inside Montevideo Bay, the dynamics are controlled mainly by advection of sediment originating from the nearby coastal area.

Generation of large-scale intrusions at baroclinic fronts: an analytical consideration with a reference to the Arctic Ocean

Abstract. Analytical solutions are found for the problem of instability of a weak geostrophic flow with linear velocity shear accounting for vertical diffusion of buoyancy. The analysis is based on the potential-vorticity equation in a long-wave approximation when the horizontal scale of disturbances is considered much larger than the local baroclinic Rossby radius. It is hypothesized that the solutions found can be applied to describe stable and unstable disturbances of the planetary scale with respect, in particular, to the Arctic Ocean, where weak baroclinic fronts with typical temporal variability periods on the order of several years or more have been observed and the β effect is negligible. Stable (decaying with time) solutions describe disturbances that, in contrast to the Rossby waves, can propagate to both the west and east, depending on the sign of the linear shear of geostrophic velocity. The unstable (growing with time) solutions are applied to explain the formation of large-scale intrusions at baroclinic fronts under the stable–stable thermohaline stratification observed in the upper layer of the Polar Deep Water in the Eurasian Basin. The suggested mechanism of formation of intrusions can be considered a possible alternative to the mechanism of interleaving at the baroclinic fronts due to the differential mixing.

Towards the Development of Practical Solar-Fuels Devices: Silicon-Based Solar-Hydrogen Generators with 14.2% Solar-to-Hydrogen Efficiency

The implementation of practical solar-fuel technologies can trigger a significant increase in the use of renewables in our energy ecosystem. These technologies can directly capture and store solar energy in the form of energy rich molecules, which could be used at a later stage as fuels for transportation or electricity generation. Storing solar-energy into fuels can also serve as a mean of seasonal energy storage, so that excess solar energy in the summer can be used during the periods of low solar irradiation in the winter. Also, solar-fuel generators can be operated in centralized facilities without disrupting the operation of the electricity grid. The spatio-temporal decoupling of the energy capture and utilization that solar-fuel technologies provide makes them an attractive renewable energy storage solution. For solar-hydrogen generators to be implemented, they need to be able to operate robustly, stably and continuously for prolonged periods of time – in the order of several years to decades. They also need to produce fuels in a cost-effective way and the energy produced over their lifetime needs to exceed the energy inputted into their manufacturing and operation. In this presentation, we describe a system agnostic approach to assess promising designs and components of solar-hydrogen generators from a technoeconomic perspective. Using this framework, we evaluate the effects of device design and material selection on the cost of the generated hydrogen. The results presented here provide insights and device engineering directions that would lead to cost-optimized solar-hydrogen generators. Our findings demonstrate that in order to minimize hydrogen production costs, optimized devices will need to be manufactured with water splitting components that are significantly smaller in area than photovoltaic units (by a factor of 10-100’s). Additionally, the analysis points out that devices based on silicon (Si) photovoltaics (either thin-film or crystalline Si cells) could reach hydrogen production costs that approach those of hydrogen from fossil fuels resources. As the cost of hydrogen is primarily driven by the light-absorbing units, further economic gains can be obtained by improving the overall solar-to-hydrogen efficiencies of devices. These efficiency improvements can be achieved by the implementation of Silicon heterojunction (SHJ) solar cells into solar-hydrogen devices. SHJ cells present higher open-circuit voltages (VOC) than crystalline Si cells and can reach levels above 700 mV. Their high VOC values are mainly due to an interfacial passivation with a thin (~5 nm) film of hydrogenated intrinsic amorphous silicon between the c-Si wafer and the oppositely doped emitter, forming the p-n junction. Thanks to the high VOC of SHJ cells, we demonstrate that modules of three cells in series can provide enough potential to drive the water splitting reaction at high current densities in electrolysis units. Devices based on SHJ modules and Nafion-based membrane-electrode assemblies with platinum and iridium oxide electrocatalysts show stable performance over 100 hrs of operation at an unprecedented SHE of 14.2%. The same efficiency level is demonstrated for devices operated under alkaline conditions using microstructured nickel electrodes; representing the highest SHE achieved with earth-abundant components to date. As both the SHJ cells and electrolysis units implemented in this study are commercially viable, easily scalable and have long lifetimes, the devices presented here have the potential to disruptively accelerate the deployment of cost effective solar-hydrogen generators.

The predominance of post‐wildfire erosion in the long‐term denudation of the Valles Caldera, New Mexico

AbstractWildfires can dramatically increase erosion rates over time scales on the order of several years, yet few data firmly constrain the relative importance of post‐wildfire erosion in the long‐term denudation of landscapes. We tested the hypothesis that wildfire‐affected erosion is responsible for a large majority of long‐term denudation in the uplands of the Valles Caldera, New Mexico, by quantifying erosion rates in wildfire‐affected and non‐wildfire‐affected watersheds over short (~100–101 years) time scales using suspended sediment loads, multitemporal terrestrial laser scanning, and airborne laser scanning and over long (~103–106 years) time scales using 10Be inventories and incision into a dated paleosurface. We found that following the Las Conchas fire in 2011, mean watershed‐averaged erosion rates were more than 1000 µm yr−1, i.e., ~103–105 times higher than nearby unburned watersheds of similar area, relief, and lithology. Long‐term denudation rates are on the order of 10–100 µm yr−1. Combining data for wildfire‐affected and non‐wildfire‐affected erosion rates into a long‐term denudation rate budget, we found that wildfire‐affected erosion is responsible for at least 90% of denudation over geologic time scales in our study area despite the fact that such conditions occur only at a small fraction of the time. Monte Carlo analyses demonstrate that this conclusion is robust with respect to uncertainties in the rates and time scales used in the calculations.

Fishery consequences of marine reserves: short-term pain for longer-term gain.

Marine reserves are often established in areas that support fisheries. Larval export from reserves is argued to help compensate for the loss of fishable habitat; however, previous modeling studies have focused on long-term equilibrium outcomes. We examined the transient consequences of reserve establishment for fished metapopulations, considering both a well-mixed larval pool and a spatially explicit model based on a coral trout (Plectropomus spp.) metapopulation. When fishing pressure was reallocated relative to the area protected, yields decreased initially, then recovered, and ultimately exceeded pre-reserve levels. However, recovery time was on the order of several years to decades. If fishing pressure intensified to maintain pre-reserve yields, reserves were sometimes unable to support the increased mortality and the metapopulation collapsed. This was more likely when reserves were small, or located peripherally within the metapopulation. Overall, reserves can achieve positive conservation and fishery benefits, but fisheries management complementary to reserve implementation is essential.

Porewater geochemistry of inland Acid sulfate soils with sulfuric horizons following postdrought reflooding with freshwater.

Following the break of a severe drought in the Murray-Darling Basin, rising water levels restored subaqueous conditions to dried inland acid sulfate soils with sulfuric horizons (pH <3.5). Equilibrium dialysis membrane samplers were used to investigate in situ changes to soil acidity and abundance of metals and metalloids following the first 24 mo of restored subaqueous conditions. The rewetted sulfuric horizons remained severely acidified (pH ∼4) or had retained acidity with jarosite visibly present after 5 mo of continuous subaqueous conditions. A further 19 mo of subaqueous conditions resulted in only small additional increases in pH (∼0.5-1 pH units), with the largest increases occurring within the uppermost 10 cm of the soil profile. Substantial decreases in concentrations of some metal(loid)s were observed with time most likely owing to lower solubility and sorption as a consequence of the increase in pH. In deeper parts of the profiles, porewater remained strongly buffered at low pH values (pH <4.5) and experienced little progression toward anoxic circumneutral pH conditions over the 24 mo of subaqueous conditions. It is proposed that low pH conditions inhibited the activity of SO-reducing bacteria and, in turn, the in situ generation of alkalinity through pyrite production. The limited supply of alkalinity in freshwater systems and the initial highly buffered low pH conditions were also thought to be slowing recovery. The timescales involved for a sulfuric horizon rewetted by a freshwater body to recover from acidic conditions could therefore be in the order of several years.

Vadose zone lag time and potential 21st century climate change effects on spatially distributed groundwater recharge in the semi-arid Nebraska Sand Hills

Summary Deep drainage of water below plant root zones (potential groundwater recharge) will become groundwater recharge (GR) after a delay (or lag time) in which soil moisture traverses the vadose zone before reaching the water table. Depending on the thickness of the vadose zone, the magnitude of deep drainage, and soil hydraulic properties, lag times will vary broadly, exceeding decades to centuries in semi-arid and arid environments. Yet, studies of future climate change impacts to GR have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone. We investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of GR throughout the semi-arid Nebraska Sand Hills (NSH) region (∼50,000 km2). We propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate. Understanding of such effects is needed for groundwater modeling, analysis of climate change impacts on groundwater, and the effective management and sustainability of future water resources. Lag time estimates are made using the pressure-based vertical velocity of soil moisture changes, equivalent to a kinematic wave approximation of Richards’ equation. The underlying assumptions (unit hydraulic gradient and relatively slow changes in climate) are supported by observations in the High Plains aquifer region, encompassing the NSH. The analysis relies on four sources of input data, including: Spatially distributed high resolution (1.1-km) GR rates (as the difference in 2000–2009 mean precipitation and evapotranspiration—estimated using a novel MODIS-based approach); thickness of the vadose zone—based on 30-m digital elevation models of the land surface and water table elevations; statistically downscaled estimates of future (2010–2099) potential GR rates from two hydroclimate model projections (WRCP CMIP3) with opposing GR trends; and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van Genuchten–Mualem equation. We found that vadose zone lag times (with a mean GR rate of 78 mm/yr for 2000–2009) have an estimated spatial mean of 5.28 yrs (vertical velocity of 5.68 m/yr). End-of-century lag times (with mean potential GR rates of 128 and 61 mm/yr for 2090–2099) have estimated spatial means of 3.78 and 6.55 yrs (vertical velocities of 8.13 and 4.69 m/yr) for wet and dry scenarios, respectively. Findings suggest that the highly conductive materials (primarily sand deposits) and high potential GR rates in the NSH lead to relatively quick drainage of vadose zone water on the order of several years (less than a decade) for the entire 21st century. However, considerable spatial variations of lag times exist due to gradients in potential GR rates and the highly variable vadose zone thickness in the NSH.

Cathodic protection with localised galvanic anodes in slender carbonated concrete elements

A combined experimental and numerical investigation was carried out with the aim of determining whether few localised galvanic anodes per unit length could protect the reinforcement of slender carbonated concrete elements, exposed to atmospheric conditions, which could not be repaired with traditional methods. Initially, the cathodic behaviour of steel under galvanostatic polarisation was determined on small-size specimens obtained from a real element. A correlation of potential versus applied current was obtained. The current distribution in slender elements was then determined through finite elements simulations, considering various scenarios of carbonation and humidity. Results showed that, in spite of the high electrical resistivity of carbonated concrete, anodes with spacing of 0.45 m are enough to protect corroding reinforcement in most exposure conditions, even in thin parts of element. Estimated anode durations were of the order of several years or even decades; however, it was shown that also reinforcement in dry (carbonated or alkaline) concrete, which does not need to be protected, contributes to anode consumption. Although other aspects play a role on the performance of a cathodic protection system (such as the effectiveness of anode-encasing material and of electrical connection to reinforcement), the results obtained are supportive of a repair strategy based on the use of localised galvanic anodes and can be generalised to slender elements exposed to atmospheric conditions suffering carbonation induced corrosion.

A RADIAL VELOCITY STUDY OF COMPOSITE-SPECTRA HOT SUBDWARF STARS WITH THE HOBBY–EBERLY TELESCOPE

Many hot subdwarf stars show composite spectral energy distributions indicative of cool main sequence companions. Binary population synthesis (BPS) models demonstrate such systems can be formed via Roche lobe overflow or common envelope evolution but disagree on whether the resulting orbital periods will be long (years) or short (days). Few studies have been carried out to assess the orbital parameters of these spectroscopic composite binaries; current observations suggest the periods are long. To help address this problem, we selected fifteen moderately-bright (V~13) hot subdwarfs with F-K dwarf companions and monitored their radial velocities (RVs) from January 2005 to July 2008 using the bench-mounted Medium Resolution Spectrograph on the Hobby-Eberly Telescope (HET). Here we describe the details of our observing, reduction, and analysis techniques and present preliminary results for all targets. By combining the HET data with recent observations from the Mercator telescope, we are able to calculate precise orbital solutions for three systems using more than 6 years of observations. We also present an up-to-date period histogram for all known hot subdwarf binaries, which suggests those with F-K main sequence companions tend to have orbital periods on the order of several years. Such long periods challenge the predictions of conventional BPS models, although a larger sample is needed for a thorough assessment of the models' predictive success. Lastly, one of our targets has an eccentric orbit, implying some composite-spectrum systems might have formerly been hierarchical triple systems, in which the inner binary merged to create the hot subdwarf.

Photometric observations of three high mass X-ray binaries and a search for variations induced by orbital motion

We searched for long period variation in V-band, IC-band and RXTE X-ray light curves of the High Mass X-ray Binaries (HMXBs) LS 1698 / RX J1037.5-5647, HD 110432 / 1H 1249-637 and HD 161103 / RX J1744.7-2713 in an attempt to discover orbitally induced variation. Data were obtained primarily from the ASAS database and were supplemented by shorter term observations made with the 24- and 40-inch ANU telescopes and one of the robotic PROMPT telescopes. Fourier peri-odograms suggested the existence of long period variation in the V-band light curves of all three HMXBs, however folding the data at those periods did not reveal convincing periodic variation. At this point we cannot rule out the existence of long term V-band variation for these three sources and hints of longer term variation may be seen in the higher precision PROMPT data. Long term V-band observations, on the order of several years, taken at a frequency of at least once per week and with a precision of 0.01 mag, therefore still have a chance of revealing long term variation in these three HMXBs.

A novel accelerated moisture absorption test and characterization

Moisture plays a significant role in influencing the mechanical behavior and long-term durability of composites. With the current available testing techniques the time required for environmental qualification of polymer composites can be on the order of several years and any interruption in the test can result in a significant cost and schedule penalty. Because of these long environmental conditioning times there is a strong desire to accelerate the process to advance environmental qualification of materials for use in commercial and military aerospace structures. An accelerated humidity test technique has been developed where moisture ingression was obtained by increasing the pressure in the test chamber. A hygrothermal humidity chamber was used in combination with D 2O water to subsequently characterize the diffusion of D 2O in a carbon/epoxy composite using Nuclear Reaction Analysis (NRA). Moisture content was also measured as a function of through-thickness depth using NRA. The accelerated technique decreased the time to saturation by 80% as compared to conventional diffusion without pressure. Moisture uptake from both conventional and accelerated diffusion exposures followed typical Fickian diffusion response.

The Effect of a Stellar Magnetic Variation on the Jet Velocity

Stellar jets are normally constituted by chains of knots with some periodicity in their spatial distribution, corresponding to a variability of order of several years in the ejection from the protostar/disk system. A widely accepted theory for the presence of knots is related to the generation of internal working surfaces due to variations in the jet ejection velocity. In this paper we study the effect of variations in the inner disk-wind radius on the jet ejection velocity. We show that a small variation in the inner disk-wind radius produces a variation in the jet velocity large enough to generate the observed knots. We also show that the variation in the inner radius may be related to a variation of the stellar magnetic field.

A brief overview of plan-shape disequilibrium in embayed beaches: Tangier bay (Morocco)

Embayed beaches commonly exhibit long-term equilibrium plan shape configurations that reflect either swash-alignment or drift-alignment (Davies, 1980) relative to the wave field, involving, respectively, virtually no longshore drift, and a balanced longshore drift over space and time. Within the context of an overall sediment budget in equilibrium, imbalances in longshore drift in embayed beaches are essentially generated by medium-term (order of several years) wave modifications in the incident wave field due to: (1) cyclic changes in deepwater wave approach directions caused by climatic oscillations; (2) human constructions, notably port breakwater structures. The latter type of engineered modification is perfectly exemplified by Tangier Bay. The beach in the eastern part of this bay has been steadily eroding over the past 50 years as a result of important port development in the western end of the bay that has perturbed waves from the Atlantic, generating, in particular, destabilization of a hitherto equilibrium drift-aligned system. The long-term resolution of problems generated by such perturbations requires a clear integrated management approach.

Conceptual Models of Vadose Zone Flow and Transport beneath the Pajarito Plateau, Los Alamos, New Mexico

The Pajarito Plateau in northern New Mexico, on which the Los Alamos National Laboratory is situated, is characterized by a thick vadose zone overlying the regional aquifer of the western Espanola Basin. In this study, conceptual models of vadose zone flow and transport processes are presented and then supported through the interpretation of field data, including synthesis with numerical models. The conceptual models differentiate the rate of percolation by their location and surface hydrologic setting, including wet and dry canyons, and wet, dry, and disturbed mesas. Net infiltration beneath wet canyons is the highest, with rates on the order of a meter per year (100–1000 mm yr−1). Transport to the regional aquifer beneath the wettest canyons is likely on the order of several years to several decades, depending on the thicknesses of the various hydrostratigraphic layers. Perched water is sometimes found beneath wetter canyons and is associated with near‐surface alluvial systems and at intermediate depths along low‐permeability interfaces such as buried soils or unfractured regions of basalt flows. Percolation through the volcanic tuffs is generally considered to be via matrix‐dominated flow, whereas fracture flow may play a key role in contaminant transport through densely welded tuffs or basalt units beneath wet canyons. Infiltration beneath dry canyons and dry mesas is much slower (10 mm yr−1 or less), yielding transport times to the aquifer of hundreds to several thousands of years. However, long‐term surface disturbances at mesa‐top locations may alter infiltration rates such that at a local scale, the infiltration rates temporarily approach those of wetter canyons.

The Role of Air–Sea Interaction in Controlling the Optimal Perturbations of Low-Frequency Tropical Coupled Ocean–Atmosphere Modes

In this paper the structure and dynamics of the optimal perturbations of tropical low-frequency coupled ocean–atmosphere oscillations relevant to El Niño–Southern Oscillation (ENSO) are explored. These optimal perturbations yield information about potential precursors for ENSO events, and about the fundamental dynamical processes that may control perturbation growth and limit the predictability of interannual variability. The present study uses a hierarchy of hybrid coupled models. Each model is configured for the tropical Pacific Ocean and shares a common ocean general circulation model. Three different atmospheric models are used: a statistical model, a dynamical model, and a combination of a dynamical model and boundary layer model. Each coupled model possesses a coupled ocean–atmosphere eigenmode oscillation with a period of the order of several years. The properties of these various eigenmodes and their corresponding adjoint eigenmodes are explored. The optimal perturbations of each coupled model for two different perturbation growth norms are also examined, and their behavior can be understood in terms of the properties of the aforementioned eigenmode oscillations. It is found that the optimal perturbation spectrum of each coupled model is primarily dominated by one member. The dominant optimal perturbation evolves into the most unstable eigenmode of the system. The structure of the optimal perturbations of each model is found to be controlled by the dynamics of the atmospheric model and air–sea interaction processes. For the coupled model with a statistical atmosphere, the optimal perturbation center of action is spread across the entire tropical Pacific in the form of a dipole. For the coupled models that include deep atmospheric convection, the optimal perturbation center of action is primarily confined to the western Pacific warm pool. In addition, the degree of nonnormality of the eigenmodes is controlled by the atmospheric model dynamics. These findings are in general agreement with the results obtained from intermediate coupled models. In particular, the atmospheric models used here have also been used in intermediate coupled models that have been employed extensively in previous studies of the optimal perturbations of El Niño–Southern Oscillation. Thus, a direct comparison of the optimal perturbation behavior of those intermediate models and the optimal perturbations of the hybrid models used here can be made.