Analysis of high-frequency local coupling instability induced by multi- transmitting formula - P-SV wave simulation in a 2D waveguide

Local coupling instability will occur when the numerical scheme of absorbing boundary condition and that of the field wave equation allow energies to spontaneously enter into the computational domain. That is, the two schemes support common wave solutions with group velocity pointed into the computation domain. The key to eliminate local coupling instability is to avoid such wave solutions. For lumped-mass finite element simulation of P-SV wave motion in a 2D waveguide, an approach for stable implementation of high order multi-transmitting formula is provided. With a uniform rectangular mesh, it is proven and validated that high-freqaency local coupling instability can be eliminated by setting the ratio of the element size equal to or greater than x/2 times the ratio of the P wave velocity to the S wave velocity. These results can be valuable for dealing instability problems induced by other absorbing boundary conditions.

Behaviour of Coupled Modes in a Simple Nonlinear Air-Sea Interaction Model

In this article, a model with the simplest framework is constructed and solved analytically. It is shown that intraseasonal wave coexists with interannual variation, and ENSO cycle arises from coupled Kelvin wave destabilisation. Its irregularity can be related to reasonable model parameters. External processes have some important effects on nonlinear air-sea interaction modes.

A Comparative Study on the Dominant Factors Responsible for the Weaker-than-expected El Ni?o Event in 2014

Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014, attracting much attention to the possible occurrence of an extreme E1 Nifio event that year because of its similarity to the situation in early 1997. However, the subsequent variation in sea surface temperature anomalies （SSTAs） during summer 2014 in the tropical Pacific was evidently different to that in 1997, but somewhat similar to the situation of the 1990 aborted E1 Nifio event. Based on NCEP （National Centers for Environmental Prediction） oceanic and atmospheric reanalysis data, the physical processes responsible for the strength of E1 Nifio events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean-atmosphere system and westerly wind bursts （WWBs） with those in 1997 and 1990, separately. Although the unstable ocean-atmosphere system formed over the tropical Pacific in the preceding winter of 2014, the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014, as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific-unfavorable for the subsequent generation of summertime WWB sequences. Moreover, the equatorial western Pacific was not cooled, indicating the absence of positive Bjerknes feedback in early summer 2014. Therefore, the development of E1 Nifio was suspended in summer 2014.

Studies on electron cloud instability in BEPCⅡ

Electron Cloud Instability has been studied in the operation of BEPC. The BEPCⅡ began the commissioning in November 2006 and the positron beam current has reached 500 mA. Because of such a high beam current, some instabilities such as ECI, bunch lengthening et al, have appeared during the operation. The experimental investigation on ECI during the commissioning of BEPCⅡ will be reported in this paper.