A study on the Antarctic circumpolar wave mode-A coexistence system of standing and traveling wave

The Antarctic circumpolar wave(ACW)has become a focus of the air-sea coupled Southern Ocean study since 1996,when it was discovered as an air-sea coupled interannual signal propagating eastward in the region of the Antarctic C i rcumpolar Current(ACC).In order to analyze the mechanism of discontinuity alon g the latitudinal propagation,a new idea that ACW is a system with a traveling wave in the Southern Pacific and Atlantic Ocean and with a concurrent standing w ave in the southern Indian Ocean is proposed in this paper.Based on the ideal w ave principle,the average wave parameters of ACW is achieved using a non-linea r approximation method,by which we find that the standing part and the travelin g part possess similar radius frequency,proving their belonging to an integral s ystem.We also give the latitudinal distribution of wave speed with which we cou ld tell the reason for steady propagation during the same period.The spatial di stribution of the propagation reveals complex process with variant spatial and t emporal scales—The ENSO scale oscillation greatly impacts on the traveling process,while the result at the south of Australia indicates little connection betw een the Indian Ocean and the Pacific,which may be blocked by the vibration at t he west of the Pacific.The advective effect of ACC on the propagation process s hould be examined clearly through dynamical method.     

Sigmoid wave transitions at alpine treeline

Ecotones that are controlled by a single environmental gradient or a set of correlated environmental factors are hypothesized to show a sigmoid wave form in percentage cover as the ecotone is crossed. This study investigates the sigmoid wave hypothesis at the alpine treeline in Glacier National Park, Montana, USA. We find that approximately half of the sampled locations exhibit a sigmoid wave form in percentage cover across the ecotone. These results indicate that in some cases treeline is controlled by a single environmental gradient or a set of correlated environmental factors. However, in other cases, additional factors that are not correlated with the primary controlling gradient act to change the form of the transition. We investigated surficial geology as a possible controlling factor that would not be correlated with the primary climatic gradients found along transects across the alpine treeline. In at least some of the cases, surficial geology type was shown to be significantly associated with differences between sigmoidal and non‐sigmoidal transitions. These results support previous evidence for sigmoid wave transitions at the boreal forest—tundra ecotone, but also show that the alpine treeline case is more complex. We argue that in cases where ecotones are used as sites for monitoring the effects of climate change on vegetation, care should be taken to utilize only sites that exhibit a sigmoidal transition so as to minimize the effects of non‐climate‐related controlling factors on the interpretations.     

Teleseismic shear wave splitting measurements in noisyenvironments

High noise levels hamper teleseismic shear wave splitting measurements, which bandpass filtering does not always help. To investigate how robust splitting measurements are to noise, we analysed a set of synthetic records with known splitting parameters and added fixed levels of noise. In the presence of weak anisotropy, single-waveform splitting measurements are unreliable when operating with noisy data sets. A practical rule in terms of S/N ratio and splitting delay time parameters is that splitting is confidently detectable at S/N > 8, regardless of the wave's original polarization orientation. However, for the evidence of weak anisotropy to be detectable and measurable at an S/N value of 4, the backazimuth separation of the phases from the fast polarization direction needs to be higher than 20°. Stacks of individual measurements consistently yield reliable results down to S/N values of 4. Applying stacking to data from DSB (Dublin, Ireland), the fast polarization direction φ and lag time δt are 58° and 0.95  s. This orientation reflects surface trends of deformation in the area, as found elsewhere in the UK. Our result thus reinforces the proposed model that the detected anisotropy in the British Isles originates from lithospheric coherent deformation preserved from the last main tectonic episode.