Stable Regime Research Articles

Vortex shedding characteristics and hydrodynamic forces of stationary and elastically mounted side-by-side cylinders fitted with small diameter control rods

Vortex shedding behind bluff bodies can induce noise and vibration problems when it engages in a feedback cycle of oscillations with an acoustic or structural mode. Therefore, several passive and active flow control techniques have been developed to suppress vortex shedding behind a single bluff body. However, less attention was given to the case of multiple bluff bodies in cross-flow. Therefore, in this paper, a numerical investigation is performed to study the effect of adding small diameter control rods in the vicinity of two side-by-side cylinders on the vortex shedding characteristics, the hydrodynamic fluid forces, as well as the suppression of vortex-induced vibration. The simulations are conducted at a Reynolds number of Re = 200 and the side-by-side cylinders have a spacing ratio of T∗=2.5. The number of control rods and their angular positions around the main cylinders are varied. Fitting the side-by-side cylinders with control rods has resulted in three different flow regimes compared to the base case with no control rods. These flow regimes depend on the angular positions of the control rods and they are classified as; bistable flow, stable biased flow, and merged wake flow regimes. The stable biased flow regime has resulted in the maximum reduction of the fluctuating lift coefficient of the side-by-side cylinders, especially when the control rods are placed at θ=135° from the flow stagnation point. Increasing the number of control rods around the cylinders has further reduced the fluctuating lift coefficient and produced a more symmetric wake at the rod angles which produce the stable biased flow, and the merged wake flow regimes. The vortex-induced vibration of the side-by-side cylinders is investigated when the control rods are placed at the angular position that results in a stable biased flow regime. It is revealed that fitting the cylinders with control rods at an angular position of θ=135° has resulted in reducing the vibration amplitude over the whole range of the simulated reduced flow velocities, which makes this technique a viable one to control the vortex shedding and the vortex-induced vibration of side-by-side cylinders.

UNVEILING REALIST EXPLANATIONS: MEARSHEIMER'S PERSPECTIVE ON WAR IN UKRAINE

The realist explanations surrounding the origins of Russia's war against Ukraine, specifically John Mearsheimer's assertion of western responsibility, have sparked controversy. Realism, including its more specific offensive sub-school, presents a wide range of contrasting interpretations, depending on the actors studied and the characteristics attributed to them. Similar to classical realism, structural realism is based on underlying assumptions about human nature. To elucidate these assumptions regarding the behavior of states and their leaders, the article explores key components of structural theory, such as power differentials, rational interests, and the perception of states as unified actors. It establishes connections between these concepts and fundamental emotions like fear and anger. The article argues that realists should distinguish between the aspirations of states and the individual leaders' pursuit of power and status. Additionally, the consideration of Russian security concerns may reflect the elite's perceived threat of cultural subordination and Putin's personal apprehension about regime stability. Engaging with realist thought is crucial, not only due to its ongoing influence on policy-making, particularly in Russia, but also to prevent the misappropriation of caricatured versions of realist arguments.

Corrosion Performance of Ti6Al7Nb Alloy in Simulated Body Fluid for Implant Application Characterized Using Macro- and Microelectrochemical Techniques

In this paper, the applicability of Ti6Al7Nb as a more biocompatible alternative for bone and dental implants than Ti6Al4V and pure titanium in terms of corrosion resistance and electrochemical inertness is investigated. The chemical inertness and corrosion resistance of the Ti6Al7Nb biomaterial were characterized by a multi-scale electrochemical approach during immersion in simulated physiological environments at 37 °C comparing its behavior to that of c.p. Ti, Ti6Al4V, and stainless steel. The establishment of a passive regime for Ti6Al7Nb results from the formation of a thin layer of metal oxide on the surface of the material which prevents the action of aggressive species in the physiological medium from direct reaction with the bulk of the alloy. Conventional electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) provide quantified information on the surface film resistance and its stability domain that encompasses the potential range experienced in the human body; unfortunately, these methods only provide an average estimate of the exposed surface because they lack spatial resolution. Although local physiological environments of the human body are usually simulated using different artificial physiological solutions, and changes in the electrochemical response of a metallic material are observed in each case, similar corrosion resistances have been obtained for Ti6Al7Nb in Hank’s and Ringer’s solutions after one week of immersion (with a corrosion resistance of the order of MΩ cm2). Additionally, scanning electrochemical microscopy (SECM) provides in situ chemical images of reactive metal and passive dielectric surfaces to assess localized corrosion phenomena. In this way, it was observed that Ti6Al7Nb exhibits a high corrosion resistance consistent with a fairly stable passive regime that prevents the electron transfer reactions necessary to sustain the metal dissolution of the bulk biomaterial. Our results support the proposition of this alloy as an efficient alternative to Ti6Al4V for biomaterial applications.

Convection of snow: when and why does it happen?

Convection of water vapor in snowpacks is supposed to have a major impact on snow density and microstructure profiles with strong implications for the thermal regime and snow stability. However, the process has never been directly measured and only recently been simulated for idealized conditions. The analysis suggests that natural convection is not likely to happen in typical horizontally homogeneous polar or Alpine snow covers. This paper studies the potential impact of heterogeneity induced, e.g., by shrubs on convection of water vapor. We find that natural convection triggered by buoyancy occurs even with sub-critical Rayleigh number as low as 5 due to heterogeneity in snow density. This leads to complementing contributions of diffusive and convective flux divergence on snow density changes. The combined effect of diffusion and convection helps to generate the often-observed low density foot and high-density top of, e.g., Arctic snowpacks. The strongest effect of convection is not for very thin or thick snow covers but for snow covers with thickness in the order of 0.5 m. This scale facilitates the development of convection cells. Further work should address the additional effects of sub-snow lateral temperature variations and assess the effect of convective vapor fluxes on snow microstructure.

Embrace or repress? Explaining China’s responses to nationalism in international incidents

As China’s power grows, a widespread perception is that China is more willing to respond to nationalist demands and act assertively in international incidents. In reality, China has not supported and accommodated nationalism in all events but has cooled down nationalism in some cases. An important unanswered question is, why does the Chinese government demonstrate selectivity when responding to nationalism and take different foreign policies concerning nationalism in various incidents? This article provides a coherent and testable framework to answer this question and uses five cases to test the congruence and validity of the analytical framework. The core argument is that the primary concern of the Chinese government in dealing with nationalism is its legitimacy. When policymakers perceive severe threats to China’s regime security and stability, they will open a ‘safety valve’ to embrace nationalism, allowing nationalism to unleash its anger under the government’s monitor and escalating disputes to defend national interests and appease nationalism. When there are few threats to the regime, three factors will affect China’s choice: the economic value of the diplomatic relationship, elements of China’s core interests, and the viability of reaching an agreement that sets aside the dispute.

New Insights Into the Role of Atmospheric Transport and Mixing on Column and Surface Concentrations of NO2 at a Coastal Urban Site

AbstractWe use a multi‐year record of Pandora‐derived NO2 total column abundance in Boston to examine the influence of atmospheric transport on column NO2 and its surface concentrations during the warm season in a coastal urban environment. We derive tropospheric NO2 estimates from the total column with a measurement‐model fusion approach using near‐real‐time estimates of stratospheric NO2 from NASA's Goddard Earth Observing System Composition Forecast model system and find the average influence of stratospheric NO2 at this urban site can be 30%–70% depending on season and time of day. Sea breeze days tend to exhibit rapid temporal variability in the column that which can go in the opposite direction of changes in surface NO2 concentrations. By comparing tropospheric NO2 with surface concentrations, we constrain the role of boundary layer entrainment processes in the evolution of surface NO2 concentrations, while highlighting the value of column measurements in identifying sea breeze frontal dynamics. We estimate an apparent equal mixing layer height of NO2 and infer that surface NOx emissions remain concentrated near the surface regardless of atmospheric stability regime. When comparing the Pandora‐ to TROPOMI‐derived column NO2 measurements, we find that sea breeze days present a unique challenge likely due to higher spatial heterogeneity in NO2 and the meteorology involved that is not well represented in retrieval inputs. Our observations provide new insights into column and surface variability of NO2 which will be relevant to interpreting geostationary observations, especially in coastal urban locations.

An Experimental Investigation of Two-Phase Frictional Pressure Drop in Straight-Tube Steam Generator Used in SFR

Due to the presence of sodium, it is a challenging task to achieve the reliable and safe operation of steam generators in a sodium-cooled fast reactor (SFR). Water flow oscillations in a two-phase flow system worsen the tube integrity. An accurate prediction of two-phase pressure drop is essential in designing steam generators to operate in a stable regime. Toward this, experiments have been carried out on an industrial-size 19-tube model sodium-heated steam generator of 5.5-MW capacity to understand two-phase pressure drop characteristics at various operating conditions. The measured data are used to estimate the two-phase frictional pressure drop. The concept of a two-phase friction multiplier has been used in the present study. A significant variation in the two-phase frictional multiplier is seen with steam quality, whereas the variation of the two-phase friction multiplier is insignificant at saturated steam condition. Based on the experiments, complemented by computational model, a correlation has been developed for the two-phase frictional multiplier as a function of steam quality for sodium-heated once-through straight-tube steam generators.

Effects of Vegetation Restoration on the Hydrological Regimes of the Chinese Loess Plateau: A Comparative Analysis of Forested and Less-Forested Catchments

Large-scale vegetation restoration can significantly affect catchment hydrology. Assessing the impact of vegetation restoration on hydrological regimes is important for water resource management. We chose three less-forested catchments (LFCs, forested area <30%) undergoing major vegetation restoration, i.e., Wuqi, Zhidan, and Liujiahe, and two forested reference catchments (FCs, forested area >77%) that have had secondary forests for more than 150 years, i.e., Zhangcunyi and Huangling, of the Beiluo River basin located in the Loess Plateau (LP) to compare and analyze the stationary and variation characteristics of streamflow and its components from 1958 to 2019. Results show that the mean annual streamflows were 25.07~34.21 and 21.62~48.02 mm in the LFCs and FCs, respectively. The mean streamflow in the LFCs decreased by 50% on average from before the year 2000 to after. The decreasing trend of high flows and increasing trend of low flows is represented in the daily flow duration curves of the LFCs. The result of baseflow separation shows that the average percentages of baseflow in the streamflow were 31.89~43.36% in the LFCs and 58.23~60.14% in the FCs. The Mann–Kendall tests showed significant decreasing trends in annual streamflow (−0.27~−0.70 mm/a) and stormflow (−0.29~−0.64 mm/a) in the LFCs from 1958 to 2019, while the baseflow exhibited increasing trends except for in the Zhidan catchment. The seasonal streamflow and stormflow showed significant decreasing trends in the summer and non-flood season, while the winter and non-flood season’s baseflow increased in LFCs. In FCs, however, the streamflow and its components showed only slight fluctuations over the study period in annual trends, decadal variability, and seasonal trends over the study period, suggesting that FCs have stable hydrological regimes. These results indicate that 20 years of large-scale vegetation restoration greatly influenced hydrological regimes by reducing stormflow and increasing baseflow, and played an important role in streamflow regulation. Conversely, areas with well-preserved vegetation can effectively mitigate the effects of weather and other factors on runoff with stable hydrological regimes. This study can help vegetation restoration and water resource management on the LP.

Turbulence regimes in the nocturnal roughness sublayer: Interaction with deep convection and tree mortality in the Amazon

We investigated the influence of seasonality and proximity to the forest canopy on nocturnal turbulence regimes in the roughness sublayer of a Central Amazon forest. Since convective systems of different scales are common in this region, we also analyzed the effect of extreme wind gusts (propagated from convective downdrafts) on the organization of the turbulence regimes, and their potential to cause the mortality of canopy trees. Our data include high-frequency winds, temperature and ozone concentration at different heights during the dry and wet seasons of 2014. In addition, we used critical wind-speed data derived from a tree-winching experiment and a modeling study conducted in the same study site. Two different turbulence regimes were identified at three heights above the canopy: a weakly stable (WS) and a very stable regime (VS). The threshold wind speeds that mark the transition between turbulence regimes were larger during the dry season and increased as a function of the height above the canopy. The turbulent fluxes of sensible heat and momentum during the WS accounted for 88% of the entire nighttime flux. Downdrafts occurred only in the WS and favored a fully coupled state of wind flow along the canopy profile. The destructive potential of winds was four times higher than on nights without downdrafts.

Numerical investigation of air curtain flows in the doorway of a building using RANS and LES

Air curtains are commonly adopted in building flows to facilitate aerodynamic sealing against the exchange flow that occurs across an open doorway due to the density differences owing to buoyancy. This paper reports numerical simulations of air curtain flows using Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) methodologies. Computations are performed for various operating conditions that are described using a deflection modulus (Dm) which ranges from 0 to 1.25. Physically, Dm represents the relative strength of the jet momentum flux compared to the transverse stack effect. The value of this deflection modulus, along with qualitative distribution of buoyancy, demarcates the operating regime of an air curtain into unstable (Dm⪅ 0.1), stable (Dm⪆ 0.2), and strong installation (Dm⪆ 0.6). The quantification of aerodynamic sealing is established using effectiveness, E, which suggests that the optimal operation lies in the stable regime with the poorest performance in the unstable regime. The comparison between various simulation methodologies suggests that 2D RANS computations generally over-predict the effectiveness, whereas both 3D RANS and LES are able to estimate E reasonably well, especially at practical values of Dm. If the employed LES grid is sufficiently fine, the predicted effectiveness falls within the experimental uncertainty limits. Furthermore, we quantify the statistics of turbulence in the flow field using the transport equation of turbulent kinetic energy. This analysis reveals that shear production and turbulent dissipation are dominant processes in the flow. In comparison, the effects of transport terms and buoyancy production are rather small. These quantities are shown to scale well with w3/l where w and l are the characteristic velocity and length scales associated with an air curtain. Visualization of large-scale coherent structures using the Q-criterion depicted the presence of long spanwise vortices in the shear layer with a wide range of scales of motion at higher Dm. These vortical structures break down significantly upon impinging on the wall (floor), resulting in a complex spatiotemporal distribution of vortices.

Different driving mechanisms of spatial heterogeneity at the community and landscape levels in the wetland ecosystem dominated by hydro‐ecological processes

AbstractThe analysis of wetland patterns and their driving mechanisms is the research focus of discussion in wetland ecology. In this study, we tried to analyse the spatial differentiation patterns of community and landscape levels dominated by hydro‐ecological processes. The spatial differentiation patterns of landscape patterns and their driving mechanisms were explored in focus. This study was based on 30‐year land cover data of Baiyangdian Wetland Nature Reserve in North China Plain, China, constructing an ecological response relationship matrix between inundation frequency and vegetation cover frequency for the spatial divide at the community and landscape level. By constructing a landscape index set and analysing the index set using clustering, non‐metric multidimensional scale analysis, discriminant and redundancy analysis, we finally obtained the differentiation characteristics of wetland landscape patterns and their driving mechanisms. The spatial differentiation of community levels along the water level gradient showed a banded distribution pattern of submerged‐floating‐emerge to meadow shrubs, but the stability of the hydrological regime dominated the spatial differentiation of landscape levels. Wetland landscapes along the stability gradient of the hydrological regime were divided into stable and fluctuating zones. The stable zone had the landscape pattern characteristics of low fragmentation, simple and regular shape and high numbers of different patch types; the landscape pattern of the fluctuating zone was characterized by a high fragmentation and complex shape, with a small number of different patch types and a non‐uniform distribution. The analysis of the driving mechanism showed that the stability of the hydrological regime was the only reason for the formation of the landscape pattern of the stable zone. The combined effect of human‐dominated and natural‐supplemented effects jointly drove the formation of the landscape pattern of the fluctuating zone. The influence of human on the landscape pattern of the fluctuating zone showed a trend of spatial expansion from high areas to shallow water areas. The combination of traditional landscape indices and aquatic ecological processes characterized the spatial differentiation of wetland landscape patterns, and also verified the feasibility of this method. This study was a successful example of conducting ecological process‐oriented landscape level spatial differentiation analysis. The results of the study are intended to provide recommendations for the rational and effective use and precise conservation of wetland plant and animal resources and the restoration of wetland ecosystems.

On the constant roll complex scalar field inflationary models

In this paper we wish to point out the possibility of using a complex scalar field in a constant roll inflationary model, as needed for observational viability. We extend the idea of real field inflaton with constant rate of roll to a complex field, showing the feasibility of solving Einstein Klein-Gordon equations constrained by an appropriate form of constant roll definition. As compared to the well known (two-parametric class of) real field models, there is one more degree of flexibility in constant roll inflationary solutions which is represented by an arbitrary function of time, γ(t). We work with an arbitrary but constant function γ (where γ = 0 refers to the corresponding real field model) and find new inflationary class of potentials. In this class of models, the behavior of real and complex field models are similar in some aspects, for example the solutions with large constant roll parameter are not stable and should be considered as early time transients. These field solutions relax at late time on a dual attractor trajectory. However, complex fields phase space trajectories reach this stable regime after real fields. We performed the stability analysis on γ function space solutions and found that dynamically stable trajectories in phase space are stable under γ variations. We extended this study by considering multifield models of constant roll inflation with non-canonical kinetic terms. By enlarging the size of field space, we showed that a multifield constant roll model is dynamically a single field effective theory. If field space is parametrized by N non-canonical fields, there will be N free parameters in the potential that can be attributed to the interaction between the fields.

Separating the Chaff from the Wheat: An Exploration of Latin American Political Regimes

This article seeks to characterize the predominant political regimes in Latin America in the 21^st century, their relative stability and endurance. It points out that even if the differences between them are less clear than in the 20^th century, the main regimes are authoritarian (autocratic), more or less liberal democracies, and populism. It goes on arguing that in contrast to the 20^th century, the role and presence of the military current authoritarian regimes is less direct, which may have created confusion. Of the three regimes, authoritarian are the more stable ones. Regarding liberal democracy, despite ups and downs, democracy has remained as the dominant regime in the region. As established in mainstream political science findings about the region, the sources of this continuity do not depend overall on the economy or social trends such as inequality or poverty levels but on political factors (the normative preferences of political actors over democracy, and on their political moderation or radicalism). Also, Latin American democracies have weathered several storms of widespread protests deriving from inner discontent. In order to make sense of the sources of instability one has to look into strictly political factors such as fragmentation, volatility, acute polarization, coalition breakdowns, rejection of critical government policies, and impeachment of presidents. The third regime type is populism, which has had a strong revival during this century, with important differences with its earlier 20^th century versions. Several scholarly works point out that present populist regimes’ most prominent features are strictly political, which they characterize as a “moment” or a movement to attain power, which may end up giving birth to more stable regimes like competitive authoritarism. I prefer to delve into populism as a regime in its own right, which has emerged frequently in the region, in some cases deriving into fully authoritarian ones (Venezuela) or moving back to liberal democracy (Ecuador, Bolivia, Peru). The paper ends calling for the need to deepen research regarding both differences between the three regimes and the specific factors affecting stability of democracies in the region.

Wetting dynamics of a droplet impact on target-like chemically heterogeneous substrates: The determinations of contact angle

Wetting is a common natural phenomenon with many useful applications. However, some fundamental aspects of wetting theory, such as determining the contact angle, remain unresolved. Two mainstream options are debated: whether the contact angle depends on the contact area or the contact line. To understand this, the present work used many-body dissipative particle dynamics to simulate wetting on chemically heterogeneous substrates. The simulations show that there is a ratio that separates contact angles into two regimes: a stable regime where the contact angles follow the contact-area-based Cassie-Baxter equation, and a fluctuating regime where contact angles are mainly determined by the contact line. The ring/droplet ratio, initial impact velocity, and wettability pair all play a role in the formation of the contact angle. This brief communication provides clear and sound observations to shed light on the determinations of the contact angle.

On the stability of inhomogeneous fluids under acoustic fields

In this work, we present the stability theory for inhomogeneous fluids subjected to standing acoustic fields. Starting from the first principles, the stability criterion is established for two fluids of different acoustic impedance (product of density and speed of sound of the fluid) separated by a plane interface. Through stability theory and numerical simulations, we show that, in the presence of interfacial tension, the relocation of high-impedance fluid from the pressure anti-node to the pressure node occurs when the acoustic force overcomes the interfacial tension force, which is in agreement with recent acoustic relocation experiments in the microchannel. Furthermore, we establish an acoustic Bond number that characterizes stable ( $Bo_{a}<1$ ) and relocation ( $Bo_{a}>1$ ) regimes. Remarkably, it is found that the critical acoustic energy density required for relocation can be significantly reduced by increasing the height of the channel which could help in designing acoustofluidic devices that handle immiscible fluids.

Apatite U-Pb geochronological and geochemical constraints on the Mazhala Au-Sb deposit in South Tibet, China

Due to lack of suitable datable minerals, timing of the Mazhala Au-Sb deposit in South Tibet remains unresolved. Also, previous geochemical studies suggest an ambiguous conclusion on the source of ore-forming fluid, with diverse viewpoints of magmatic water, mixed magmatic–meteoric water or mixed metamorphic-meteoric water, respectively. In this study, hydrothermal apatite U-Pb dating constrains the Mazhala Au-Sb deposits to have formed at ca. 35 Ma, consistent with the timing of Eocene leucogranite intrusions in South Tibet. The Mazhala Au-Sb mineralization occurred during the main collision stage to late collision stage in Eocene to Oligocene, and predates the Cuonadong Sn mineralization in South Tibet which occurred during the crustal extension stage in Miocene. An EPMA analysis shows the apatite belongs to fluorapatite that have F contents in the dominant range of 2–4%. The Mazhala apatite displays an overall MREE enrichment and LREE- and HREE depletion distribution pattern with positive Eu anomalies, and have δ18O values dominantly in the range of 16-19‰ with a mean δ18O‰ value of 17.74 ± 0.15‰ (2σ), which suggest a relatively homogenous chemical composition for the ore forming fluids with a relatively stable temperature regime. In combination with previous temperature estimates, the ore forming fluids are inferred to have a dominant δ18Ofluid value of 13-16‰, which overlap with metamorphic water.The combined age and geochemical data suggest Eocene leucogranite intrusion induced metamorphic fluid circulation within Paleozoic-Mesozoic metamorphic sedimentary rocks around the study area, which prompted precipitation of fluorapatite together with Au-Sb bearing pyrite and arsenopyrite in the Mazhala Au-Sb deposit. This study puts an emphasis on the contribution of metamorphic water as the dominant ore-forming fluid origin in the Mazhala Au-Sb deposit. In contrast, the close spatial and temporal association of the Cuonadong Sn polymetallic mineralization with the Miocene leucogranite intrusion-induced hydrothermal activity in the Cuonadong dome demonstrates that their genetic links, as was also evidenced by C-H-O-S-Pb isotope data that point to a dominant contritution of Miocene leucogranite to ore-forming fluid and materials for the Cuonadong Sn polymetallic ore deposits. As such, from a perspective of ore-forming model in South Tibet it be seen that during the main collision stage to late collision stage ore-forming fluid and materials were mainly derived from metamorphic water, whereas during the crustal extension stage, ore-forming fluid and materials were dominantly sourced from magmatic water.

Investigating the Impact of Inflation and Other Macroeconomic Variables on Stock Returns in Nigeria

This study investigates the impact of inflation and other macroeconomic variables on the stock returns of listed firms across five sectors: In Nigeria. A GARCH model and multivariate panel analysis were used in this study. The results show that inflation negatively and significantly impacts stock returns in the short and long run on an aggregate and sectoral basis. Moreover, money supply has a negative impact on stock returns in the long run and a positive impact in the short run, while the exchange rate has a positive impact. In contrast, interest rate is inversely related to stock returns in the short-run. Therefore, the Nigerian Monetary Authority should introduce a contractionary monetary policy and a stable exchange rate regime in order to achieve moderate inflation commensurate with economic growth and development.

Thermal radiation in asymmetrically driven coupled non-linear photonic cavities

We study thermal radiation from a system of two coupled photonic cavities including the nonlinear Kerr effect, using numerical simulations and analytical methods. The general system, which can be implemented as two cavities in a waveguide, is asymmetrically driven with a monochromatic pumping from a single side. We describe the eigenmodes in the linear and nonlinear regime as well as the transmission of the coupled system. These results are then employed to understand the thermal radiation exhibited from both sides as each part of the system is coupled to a bath at a different temperature. Interestingly, the radiation spectrum is complicated, and can present up to four peaks due to the rich nonlinear coupling features. Furthermore, in certain regimes these spectra can drastically change upon variation of the bath temperatures. In addition, interference between the emitted and reflected radiation can lead to dips in the thermal radiation. Moreover, the system can exhibit self-pulsing, leading to comblike spectra for thermal radiation with peaks of very large amplitude. Our proposed analytical model for thermal radiation in the stable regime fits very well with the numerical results and describes a general class of devices.

Stable stadial and interstadial states of the last glacial's climate identified in a combined stable water isotope and dust record from Greenland

Abstract. During the last glacial interval, the Northern Hemisphere climate was punctuated by a series of abrupt changes between two characteristic climate regimes. The existence of stadial (cold) and interstadial (milder) periods is typically attributed to a hypothesised bistability in the glacial North Atlantic climate system, allowing for rapid transitions from the stadial to the interstadial state – the so-called Dansgaard–Oeschger (DO) events – and more gradual yet still fairly abrupt reverse shifts. The physical mechanisms driving these regime transitions remain debated. DO events are characterised by substantial warming over Greenland and a reorganisation of the Northern Hemisphere atmospheric circulation, which are evident from concomitant shifts in the δ18O ratios and dust concentration records from Greenland ice cores. Treating the combined δ18O and dust record obtained by the North Greenland Ice Core Project (NGRIP) as a realisation of a two-dimensional, time-homogeneous, and Markovian stochastic process, we present a reconstruction of its underlying deterministic drift based on the leading-order terms of the Kramers–Moyal equation. The analysis reveals two basins of attraction in the two-dimensional state space that can be identified with the stadial and interstadial regimes. The drift term of the dust exhibits a double-fold bifurcation structure, while – in contrast to prevailing assumptions – the δ18O component of the drift is clearly mono-stable. This suggests that the last glacial's Greenland temperatures should not be regarded as an intrinsically bistable climate variable. Instead, the two-regime nature of the δ18O record is apparently inherited from a coupling to another bistable climate process. In contrast, the bistability evidenced in the dust drift points to the presence of two stable circulation regimes of the last glacial's Northern Hemisphere atmosphere.

Chaos in coupled heteroclinic cycles and its piecewise-constant representation

We consider two robust heteroclinic cycles rotating in opposite directions, coupled by diffusive terms. A complete synchronization is impossible in this system. Numerical studies show that chaos is abundant at low levels of coupling. With increasing coupling strength, several symmetry-changing transitions are observed, and finally, a stable periodic regime appears via an inverse period-doubling cascade. To reveal the behavior at extremely small couplings, a piecewise constant model for the dynamics is proposed. Within this model, we numerically construct a Poincaré map for a chaotic state, which appears to be an expanding non-invertible circle map, thus confirming the abundance of chaos in the small coupling limit. We also show that within the piecewise constant description, there is a set of periodic solutions with different phase shifts between subsystems due to dead zones in the coupling.