Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

The cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

Mitigation of Antisymmetric Mode and Drag with Base Cavities

Experiments were carried out to examine the impact of base cavities on the base pressure fluctuations and total drag of a cylindrical afterbody for freestream Mach numbers 0.6-1.5. Significant improvement in the base pressure and a substantial reduction in the afterbody drag was noticed in the presence of a base cavity at subsonic Mach numbers. However, on increasing the cavity length beyond a certain value its performance deteriorates. At supersonic Mach numbers their effectiveness drops drastically. Tones in the spectra can be classified into two types depending on the dominant azimuthal mode which is either 0 or 1 and are referred to as symmetric and an antisymmetric mode, respectively. Spectra at subsonic Mach numbers exhibit tones which are related either to mode 0 or 1. However, at supersonic Mach numbers only tones related to mode 0 exist. The base cavity either, effectively suppress the antisymmetric mode or modify it into a symmetric mode resulting in mitigation of the tones related to antisymmetric mode.

In-Situ X-ray Photoelectron Spectroscopy and Raman Microscopy of Roselite Crystals, Ca2(Co2+,Mg)(AsO4)2 2H2O, from the Aghbar Mine, Morocco

Roselite from the Aghbar Mine, Morocco, [Ca2(Co2+,Mg)(AsO4)2 2H2O], was investigated by X-ray Photoelectron and Raman spectroscopy. X-ray Photoelectron Spectroscopy revealed a cobalt to magnesium ratio of 3:1. Magnesium, cobalt and calcium showed single bands associated with unique crystallographic positions. The oxygen 1s spectrum displayed two bands associated with the arsenate group and crystal water. Arsenic 3d exhibited bands with a ratio close to that of the cobalt to magnesium ratio, indicative of the local arsenic environment being sensitive to the substitution of magnesium for cobalt. The Raman arsenate symmetric and antisymmetric modes were all split with the antisymmetric modes observed around 865 and 818 cm−1, while the symmetric modes were found around 980 and 709 cm−1. An overlapping water-libration mode was observed at 709 cm−1. The region at 400–500 cm−1 showed splitting of the arsenate antisymmetric mode with bands at 499, 475, 450 and 425 cm−1. The 300–400 cm−1 region showed the corresponding symmetric bending modes at 377, 353, 336 and 304 cm−1. The bands below 300 cm−1 were assigned to lattice modes.

Mach Number Effect on Symmetric and Antisymmetric Modes of Base Pressure Fluctuations

An experimental study aimed at evaluating the influence of Mach number on the base pressure fluctuations of a cylindrical afterbody was performed over a wide range of Mach numbers from subsonic to supersonic speeds. Time-averaged results indicate that the coefficient of base pressure drops with the increase in the freestream Mach number at subsonic speeds and increases at supersonic Mach numbers. The coefficient of root-mean-square of the pressure fluctuations follows a decreasing trend with the increase in the Mach number. Examination of the spectra reveals different mechanisms dominate the pressure fluctuations from the center to the periphery of the base as well as with the change in the Mach number. Analysis of the azimuthal coherence indicates that all the dominant tones in the spectra can be classified either into a symmetric or an antisymmetric mode at subsonic Mach numbers. However, at supersonic Mach numbers, all the dominant tones in the spectra are symmetric in nature. The results from the cross-correlation suggest that two possible mechanisms of recirculation bubble pulsing and convective motions/vortex shedding are driving the dynamics on the base at subsonic Mach numbers. However, at supersonic Mach numbers, only single mechanism of the recirculation bubble pulsing dominates. Moreover, it indicates that the symmetric mode is associated with the dynamics of the recirculation bubble and the antisymmetric mode is related to the convective motions/vortex shedding.