Search

Transport and the spread of heat in Hamiltonian one dimensional momentum conserving nonlinear systems is commonly thought to proceed anomalously. Notable exceptions, however, do exist of which the coupled rotator model is a prominent case. Therefore, the quest arises to identify the origin of manifest anomalous energy and momentum transport in those low dimensional systems. We develop the theory for both, the statistical densities for momentum- and energy-spread and particularly its momentum-/heat-diffusion behavior, as well as its corresponding momentum/heat transport features. We demonstrate that the second temporal derivative of the mean squared deviation of the momentum spread is proportional to the equilibrium correlation of the total momentum flux. Subtracting the part which corresponds to a ballistic momentum spread relates (via this integrated, subleading momentum flux correlation) to an effective viscosity, or equivalently, to the underlying momentum diffusivity. We next put forward the intriguing hypothesis: normal spread of this so adjusted excess momentum density causes normal energy spread and alike normal heat transport (Fourier Law). Its corollary being that an anomalous, superdiffusive broadening of this adjusted excess momentum density in turn implies an anomalous energy spread and correspondingly anomalous, superdiffusive heat transport. This hypothesis is successfully corroborated within extensive molecular dynamics simulations over large extended time scales. Our numerical validation of the hypothesis involves four distinct archetype classes of nonlinear pair-interaction potentials: (i) a globally bounded pair interaction (the noted coupled rotator model), (ii) unbounded interactions acting at large distances (the coupled rotator model amended with harmonic pair interactions), (iii) the case of a hard point gas with unbounded square-well interactions and (iv) a pair interaction potential being unbounded at short distances while displaying an asymptotic free part (Lennard–Jones model). We compare our findings with recent predictions obtained from nonlinear fluctuating hydrodynamics theory.

Previous studies have shown different impacts of eastern (EP) and central (CP) Pacific El Niños on the East Asian precipitation. This study reveals distinct precipitation anomalies over East Asia in coupled and uncoupled El Niños. The uncoupled warm events in 1979, 2004, 2014, 2018, and 2019, which occurred in the autumn–winter transition, represent an unusual form of El Niño diversity. The sea surface temperature anomalies (SSTA) in the western North Pacific (WNP) appear to be an important factor leading to the different distribution of precipitation anomalies over East Asia and the WNP. In coupled El Niños, anomalous lower-level anticyclone over the WNP is a result of combined effects of warm SSTAs in the equatorial CP and EP and cold SSTAs in the WNP. The cold SSTAs are attributed to surface heat flux anomalies that in turn are associated with convection and wind anomalies. The lack of cold SSTAs in the WNP in the uncoupled El Niños leads to a weaker anomalous anticyclone with a northwestward location. The different features of an anomalous anticyclone over the WNP induce discrepancy in the distribution of precipitation anomalies and the associated predictability over East Asia between coupled and uncoupled El Niños. The prediction skill tends to be lower for the uncoupled El Niño events than the coupled ones, which further indicates the necessity to distinguish uncoupled from coupled warm events.

The thermal conductance of a homogeneous 1D nonlinear lattice system with neareast-neighbor interactions has recently been computationally studied in detail by Li et al. (Eur. Phys. J. B, 88 (2015) 182), where its power-law dependence on temperature T for high temperatures is shown. Here, we address its entire temperature dependence, in addition to its dependence on the size N of the system. We obtain a neat data collapse for arbitrary temperatures and system sizes, and numerically show that the thermal conductance curve is quite satisfactorily described by a fat-tailed q-Gaussian dependence on with . Consequently, its asymptotic behavior is given by with . [ABSTRACT FROM AUTHOR]