Forecasting the El Ni\~no type well before the spring predictability barrier

The El Ni\~no Southern Oscillation (ENSO) is the most important driver of interannual global climate variability and can trigger extreme weather events and disasters in various parts of the globe. Depending on the region of maximal warming, El Ni\~no events can be partitioned into 2 types, Eastern Pacific (EP) and Central Pacific (CP) events. The type of an El Ni\~no has a major influence on its impact and can even lead to either dry or wet conditions in the same areas on the globe. Here we show that the zonal difference $\Delta T_{WP-CP}$ between the sea surface temperature anomalies (SSTA) in the equatorial western Pacific and central Pacific gives an early indication of the type of an upcoming El Ni\~no: When at the end of a year, $\Delta T_{WP-CP}$ is positive, an event in the following year will be probably an EP event, otherwise a CP event. Between 1950 and present, 3/4 of the EP forecasts and all CP forecasts are correct. When combining this approach with a previously introduced climate-network approach, we obtain reliable forecasts for both the onset and the type of an event: at a lead time of about one year, 2/3 of the EP forecasts and all CP forecasts in the regarded period are correct. The combined model has considerably more predictive power than the current operational type forecasts with a mean lead time of about 1 month and should allow early mitigation measures.Comment: 16 pages, 10 figure

Evaluation of the real-time El Niño forecasts by the climate network approach between 2011 and present

El Niño episodes are part of the El Niño-Southern Oscillation (ENSO), which is the strongest driver of interannual climate variability, and can trigger extreme weather events and disasters in various parts of the globe. Previously we have described a network approach that allows to forecast El Niño events about 1 year ahead. Here we evaluate the real-time forecasts of this approach between 2011 and 2022. We find that the approach correctly predicted (in 2013 and 2017) the onset of both El Niño periods (2014-2016 and 2018-2019) and generated only 1 false alarm in 2019. In June 2022, the approach correctly forecasted the onset of an El Niño event in 2023. For determining the p-value of the 12 real-time forecasts, we consider 2 null hypotheses: (a) random guessing where we assume that El Niño onsets occur randomly, and (b) correlated guessing where we assume that in the year an El Niño ends, no new El Niño will start. We find pa≅0.005 and pb≅0.015, this way rejecting both the null hypotheses that the same quality of the forecast can be obtained by chance. We also discuss how the network algorithm can be further improved by systematically reducing the number of false alarms. For 2024, the method indicates the absence of a new El Niño event

Herding model and 1/f noise

We provide evidence that for some values of the parameters a simple agent based model, describing herding behavior, yields signals with 1/f power spectral density. We derive a non-linear stochastic differential equation for the ratio of number of agents and show, that it has the form proposed earlier for modeling of 1/f^beta noise with different exponents beta. The non-linear terms in the transition probabilities, quantifying the herding behavior, are crucial to the appearance of 1/f noise. Thus, the herding dynamics can be seen as a microscopic explanation of the proposed non-linear stochastic differential equations generating signals with 1/f^beta spectrum. We also consider the possible feedback of macroscopic state on microscopic transition probabilities strengthening the non-linearity of equations and providing more opportunities in the modeling of processes exhibiting power-law statistics

Setting the tree-ring record straight

Funder: Potsdam-Institut f├╝r Klimafolgenforschung (PIK) e.V. (3500)AbstractTree-ring chronologies are the main source for annually resolved and absolutely dated temperature reconstructions of the last millennia and thus for studying the intriguing problem of climate impacts. Here we focus on central Europe and compare the tree-ring based temperature reconstruction with reconstructions from harvest dates, long meteorological measurements, and historical model data. We find that all data are long-term persistent, but in the tree-ring based reconstruction the strength of the persistence quantified by the Hurst exponent is remarkably larger ($$h\cong 1.02$$ h ≅ 1.02 ) than in the other data ($$h=$$ h = 0.52–0.69), indicating an unrealistic exaggeration of the historical temperature variations.We show how to correct the tree-ring based reconstruction by a mathematical transformation that adjusts the persistence and leads to reduced amplitudes of the warm and cold periods. The new transformed record agrees well with both the observational data and the harvest dates-based reconstructions and allows more realistic studies of climate impacts. It confirms that the present warming is unprecedented.</jats:p

Practice Considerations for Adapting In-Person Groups to Telerehabilitation

The Coronavirus-2019 (COVID-19) pandemic has shifted research and healthcare system priorities, stimulating literature on implementation and evaluation of telerehabilitation for a variety of patient populations. While there is substantial literature on individual telerehabilitation, evidence about group telerehabilitation remains limited despite its increasing use by rehabilitation providers. Therefore, the purpose of this manuscript is to describe our expert team’s consensus on practice considerations for adapting in-person group rehabilitation to group telerehabilitation to provide rapid guidance during a pandemic and create a foundation for sustainability of group telerehabilitation beyond the pandemic’s end. &nbsp

Element-specific spin and orbital momentum dynamics of Fe/Gdmultilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an x-ray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers

Element-specific spin and orbital momentum dynamics of Fe/Gd multilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an xray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers. 1 Author to whom correspondence should be addressed; electronic mail: [email protected]. 2 Ultrafast magnetic storage and processing is founded on our ability to control magnetism on picosecond and femtosecond time scales. Magnetic phase transitions conserve the total angular momentum and usually involve the crystal lattice as a quasi-infinite reservoir of angular momentum. A prototypical ultrafast magnetic phenomenon is the demagnetization after excitation by an intense laser pulse The Fe/Gd multilayer consists of two metals of very different electronic structure. Fe has exchange-split 3d spin bands which intersect the Fermi surface, allowing both low-energy spin-flip (Stoner) and spin wave excitations (magnons). The spin momentum dominates the total angular momentum while the orbital momentum is quenched by the strong ligand field and only partially restored by the spin-orbit interaction. The coupling of the orbital momentum to the anisotropic ligand field enables the flow of angular momentum from the spin system to the lattice during the demagnetization. A direct photon-driven exchange of spin and orbital momentum as proposed by Hübner 3 Early experiments on Gd suggested a slow laser-induced demagnetization in tens of picoseconds Our experiments were performed on a stack of 20 alternating 0.5-nm Fe and Gd layers grown on top of a 200-nm Al heat sink, protected by a thin Al cap layer, and supported by a 100 nm silicon nitride membrane. At and above room temperature the easy magnetization direction was out-of-plane. The thin layers were antiferromagnetically coupled with a common Curie temperature of about 230°C. In order to separate the transient dynamics of the Fe 3d and Gd 4f spin and orbital momenta in the Fe/Gd multilayer, we extended time-resolved XMCD [12, 13] into a laser pump -x-ray probe technique. XMCD has the unique ability to separate and quantify spin and orbital momenta with element specificity Also, XMCD avoids laser pump-induced state-filling effects since the spin-dependent band occupation is determined by recording the absorption cross-section for circularly polarized x-rays, exciting electrons from a spin-orbit-split core level into the valence states. Integration over the absorption resonances accounts for all unoccupied states. The demagnetization dynamics was initiated by heating the sample above the Curie temperature with 60-fs (full width at half maximum (FWHM)) long 800-nm laser pulses at an intensity of 20 mJ/cm 2 and 5 kHz, and probed with 60-ps (FWHM) x-ray pulses from the elliptically polarizing undulator beamline 4.0 at the Advanced Light Source Two representative streaked x-ray pulses are shown in Starting from the integrated transient Fe L 3,2 and Gd M 4,5 dichroism, the transient spin momentum m s (t) = &lt;S z &gt; and orbital momentum m l (t) = &lt;L z &gt; were determined by using sum rules orbital momentum m l (t) before and after t=0 is zero-within experimental errors-in agreement with our expectation that the Gd 4f demagnetization occurs indirectly via exchange with Gd 5d states. The Fe 3d orbital momentum m l (t) decays simultaneously with the Fe spin momentum m s (t). This can be seen more clearly when 5 m l (t) and m s (t) are normalized to their values before t=0, The ultrafast dynamics in Fe/Gd is a true demagnetization as angular momentum is transferred from spin and orbital momentum to the lattice, which acts as a sink. The demagnetization is not primarily the result of a rearrangement of angular momentum between spin and orbit, which would be visible as a change in the orbital to spin momentum ratio. Note that a partial demagnetization in a non-equilibrium situation may be possible without coupling to the lattice because of the different g-factors of electron spin and orbital momenta. It is clear that the Fe spin-orbit interaction does not constitute a bottleneck in the demagnetization of Fe/Gd because the Fe spin and orbital momenta are in or close to equilibrium. The slow dynamics in Fe/Gd, compared to N

Local Difference Measures between Complex Networks for Dynamical System Model Evaluation

Acknowledgments We thank Reik V. Donner for inspiring suggestions that initialized the work presented herein. Jan H. Feldhoff is credited for providing us with the STARS simulation data and for his contributions to fruitful discussions. Comments by the anonymous reviewers are gratefully acknowledged as they led to substantial improvements of the manuscript.Peer reviewedPublisher PD

Effects of long memory in the order submission process on the properties of recurrence intervals of large price fluctuations

Understanding the statistical properties of recurrence intervals of extreme events is crucial to risk assessment and management of complex systems. The probability distributions and correlations of recurrence intervals for many systems have been extensively investigated. However, the impacts of microscopic rules of a complex system on the macroscopic properties of its recurrence intervals are less studied. In this Letter, we adopt an order-driven stock market model to address this issue for stock returns. We find that the distributions of the scaled recurrence intervals of simulated returns have a power law scaling with stretched exponential cutoff and the intervals possess multifractal nature, which are consistent with empirical results. We further investigate the effects of long memory in the directions (or signs) and relative prices of the order flow on the characteristic quantities of these properties. It is found that the long memory in the order directions (Hurst index $H_s$) has a negligible effect on the interval distributions and the multifractal nature. In contrast, the power-law exponent of the interval distribution increases linearly with respect to the Hurst index $H_x$ of the relative prices, and the singularity width of the multifractal nature fluctuates around a constant value when $H_x<0.7$ and then increases with $H_x$. No evident effects of $H_s$ and $H_x$ are found on the long memory of the recurrence intervals. Our results indicate that the nontrivial properties of the recurrence intervals of returns are mainly caused by traders' behaviors of persistently placing new orders around the best bid and ask prices.Comment: 6 EPL pages including 6 figure