scholarly journals Twentieth Century Winter Changes in Southern Hemisphere Synoptic Weather Modes

2011 ◽  
Vol 2011 ◽  
pp. 1-16 ◽  
Author(s):  
Jorgen S. Frederiksen ◽  
Carsten S. Frederiksen
Keyword(s):  
Growth Rate ◽  
Southern Hemisphere ◽  
Western Australia ◽  
Three Dimensional ◽  
Storm Track ◽  
Intraseasonal Oscillation ◽  
Low Frequency ◽  
Winter Rainfall ◽  
Cloud Band ◽  
Eastern Australia

During the last sixty years, there have been large changes in the southern hemisphere winter circulation and reductions in rainfall particularly in the southern Australian region. Here we examine the corresponding changes in dynamical modes of variability ranging from storm tracks, onset-of-blocking modes, northwest cloud-band disturbances, Antarctic low-frequency modes, intraseasonal oscillations, and African easterly waves. Our study is performed using a global two-level primitive equation instability-model with reanalyzed observed July three-dimensional basic states for the periods 1949–1968, 1975–1994, and 1997–2006. We relate the reduction in the winter rainfall in the southwest of Western Australia since the mid-1970s and in south-eastern Australia since the mid-1990s to changes in growth rate and structures of leading storm track and blocking modes. We find that cyclogenesis and onset-of-blocking modes growing on the subtropical jet have significantly reduced growth rates in the latter periods. On the other hand there is a significant increase in the growth rate of northwest cloud-band modes and intraseasonal oscillation disturbances that cross Australia and are shown to be related to recent positive trends in winter rainfall over northwest Western Australia and central Australia, in general. The implications of our findings are discussed.

scholarly journals Science with the Murchison Widefield Array

2013 ◽  
Vol 30 ◽  
Author(s):  
Judd D. Bowman ◽  
Iver Cairns ◽  
David L. Kaplan ◽  
Tara Murphy ◽  
Divya Oberoi ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Early Universe ◽  
Western Australia ◽  
Space Weather ◽  
Time Domain ◽  
Angular Resolution ◽  
Low Frequency ◽  
Performance Properties ◽  
Square Kilometre Array ◽  
Murchison Widefield Array

AbstractSignificant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Taxonomy and biogeography of Australian Rozites species mycorrhizal with Nothofagus and Myrtaceae

1994 ◽  
Vol 7 (4) ◽  
pp. 353 ◽  
Author(s):  
NL Bougher ◽  
BA Fuhrer ◽  
E Horak
Keyword(s):  
New Species ◽  
Southern Hemisphere ◽  
Ectomycorrhizal Fungi ◽  
Western Australia ◽  
Forest Tree ◽  
Soil Conditions ◽  
Relict Species ◽  
South Eastern ◽  
Eastern Australia ◽  
Northern Portion

Seven species of the putatively obligately ectomycorrhizal fungal genus Rozites are described from Australian Nothofagus and myrtaceaeous forests. Rozites metallica, R. armeniacovelata, R. foetens, and R. occulta are new species associated with Nothofagus in south eastern Australia. Rozites fusipes, previously known only from New Zealand, is reported from Tasmanian Nothofagus forests. Rozites roseolilacina and R. symea are new species associated with Eucalyptus in south eastern and south western Australia respectively. The significance of these Rozites species to mycorrhizal and biogeographical theories, such as the origin of ectomycorrhizal fungi associated with myrtaceous plants in Australia are discussed. The diversity of Rozites species in Australia, which equals or exceeds that of other southern regions, furthers the notion that many species of the genus co-evolved with Nothofagus in the Southern Hemisphere. Rozites symea in Western Australia occurs well outside the current geographic range of Nothofagus. It is considered to be a relict species that has survived the shift in dominant ectomycorrhizal forest tree type from Nothofagus to Myrtaceae (local extinction of Nothofagus 4–5 million years ago), and is most likely now confined to the high rainfall zone in the south west. Data on Rozites in Australia support the concept that at least some of the present set of ectomycorrhizal fungi associated with Myrtaceae in Australia are those which successfully completed a host change from Nothofagus, and adapted to changing climate, vegetation and soil conditions during and since the Tertiary. We suggest that the ancient stock of Rozites arose somewhere within the geographical range of a Cretaceous fagalean complex of plant taxa. By the end of the Cretaceous, Rozites and the fagalean complex may have spanned the Asian–Australian region including perhaps many Southern Hemisphere regions. A northern portion of the ancestral Rozites stock gave rise to extant Northern Hemisphere Rozites species and a southern portion speciated as Nothofagus itself speciated.

scholarly journals Variability of the Southern Hemisphere Winter Split Flow—A Case of Two-Way Reinforcement between Mean Flow and Eddy Anomalies

2006 ◽  
Vol 63 (2) ◽  
pp. 634-650 ◽  
Author(s):  
Xiaosong Yang ◽  
Edmund K. M. Chang
Keyword(s):  
Southern Hemisphere ◽  
High Frequency ◽  
Diabatic Heating ◽  
Temperature Anomaly ◽  
Storm Track ◽  
Low Frequency ◽  
Mean Flow ◽  
Split Flow ◽  
Frequency Split ◽  
Vorticity Flux

Abstract A new split-jet index is defined in this study, and composites based on this index show that the split-flow regime is characterized by a cold–warm–cold tripolar temperature anomaly in the South Pacific that extends equatorward from the Southern Hemisphere (SH) high latitudes, while nonsplit flow occurs when the phase of the tripolar temperature anomaly is reversed. Analyses of the heat budget reveal that the temperature anomalies associated with the split/nonsplit flow are mainly forced by mean flow advection instead of local diabatic heating or convergence of eddy heat fluxes. Localized Eliassen–Palm (E–P) flux diagnostics suggest that the zonal wind anomalies are maintained by the eddy vorticity flux anomalies. These diagnostic results are confirmed by numerical experiments conducted using a stationary wave model forced by observed eddy forcings and diabatic heating anomalies. The model results show that the effects of the vorticity flux dominates over those of the heat flux, which tend to dampen the flow anomalies, and that tropical diabatic heating anomalies are not important in maintaining the split-/nonsplit-flow anomalies. The organization of high-frequency eddies by the low-frequency split/nonsplit jet is also studied. Two sets of experiments using a linear storm-track model initialized with random initial perturbations superposed upon the split- and nonsplit-jet basic state, respectively, have been conducted. Model results show that the storm-track anomalies that are organized by the split/nonsplit jet are consistent with observed storm-track anomalies, thus demonstrating that the low-frequency split/nonsplit jet acts to organize the high-frequency eddies. The results of this paper directly establish that there is a two-way reinforcement between eddies and mean flow anomalies in the low-frequency variability of the SH winter split jet.

scholarly journals First results of low frequency electromagnetic wave detector of TC-2/Double Star program

2005 ◽  
Vol 23 (8) ◽  
pp. 2803-2811 ◽  
Author(s):  
J. B. Cao ◽  
Z. X. Liu ◽  
J. Y. Yang ◽  
C. X. Yian ◽  
Z. G. Wang ◽  
...  
Keyword(s):  
Electromagnetic Wave ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Geomagnetic Storms ◽  
Low Frequency ◽  
Wave Detector ◽  
Ion Cyclotron Waves ◽  
Satellite Platform ◽  
Ion Cyclotron ◽  
First Results

Abstract. LFEW is a low frequency electromagnetic wave detector mounted on TC-2, which can measure the magnetic fluctuation of low frequency electromagnetic waves. The frequency range is 8 Hz to 10 kHz. LFEW comprises a boom-mounted, three-axis search coil magnetometer, a preamplifier and an electronics box that houses a Digital Spectrum Analyzer. LFEW was calibrated at Chambon-la-Forêt in France. The ground calibration results show that the performance of LFEW is similar to that of STAFF on TC-1. The first results of LFEW show that it works normally on board, and that the AC magnetic interference of the satellite platform is very small. In the plasmasphere, LFEW observed the ion cyclotron waves. During the geomagnetic storm on 8 November 2004, LFEW observed a wave burst associated with the oxygen ion cyclotron waves. This observation shows that during geomagnetic storms, the oxygen ions are very active in the inner magnetosphere. Outside the plasmasphere, LFEW observed the chorus on 3 November 2004. LFEW also observed the plasmaspheric hiss and mid-latitude hiss both in the Southern Hemisphere and Northern Hemisphere on 8 November 2004. The hiss in the Southern Hemisphere may be the reflected waves of the hiss in the Northern Hemisphere.

scholarly journals Evolution of a narrow-band spectrum of random surface gravity waves

2003 ◽  
Vol 478 ◽  
pp. 1-10 ◽  
Author(s):  
KRISTIAN B. DYSTHE ◽  
KARSTEN TRULSEN ◽  
HARALD E. KROGSTAD ◽  
HERVÉ SOCQUET-JUGLARD
Keyword(s):  
Three Dimensional ◽  
Low Frequency ◽  
Three Dimensions ◽  
Band Spectrum ◽  
Nls Equation ◽  
Random Surface ◽  
Narrow Bandwidth ◽  
Quasi Stationary State ◽  
The Stability ◽  
Three Dimensional Simulations

Numerical simulations of the evolution of gravity wave spectra of fairly narrow bandwidth have been performed both for two and three dimensions. Simulations using the nonlinear Schrödinger (NLS) equation approximately verify the stability criteria of Alber (1978) in the two-dimensional but not in the three-dimensional case. Using a modified NLS equation (Trulsen et al. 2000) the spectra ‘relax’ towards a quasi-stationary state on a timescale (ε2ω0)−1. In this state the low-frequency face is steepened and the spectral peak is downshifted. The three-dimensional simulations show a power-law behaviour ω−4 on the high-frequency side of the (angularly integrated) spectrum.

scholarly journals Resolution of low‐frequency attenuation anomalies in the Southern Hemisphere

1987 ◽  
Vol 81 (S1) ◽  
pp. S11-S11
Author(s):  
R. N. Denham ◽  
R. W. Bannister ◽  
K. M. Guthrie ◽  
R. H. Mellen ◽  
D. G. Browning
Keyword(s):  
Southern Hemisphere ◽  
Low Frequency

Scattering of electromagnetic waves by hybrid waves in a two-electron-temperature plasma

1991 ◽  
Vol 46 (1) ◽  
pp. 99-106 ◽  
Author(s):  
S. K. Sharma ◽  
A. Sudarshan
Keyword(s):  
Growth Rate ◽  
Electron Temperature ◽  
High Frequency ◽  
Electromagnetic Waves ◽  
Low Frequency ◽  
Lower Hybrid ◽  
Stimulated Scattering ◽  
Coupled Modes ◽  
Temperature Plasma ◽  
Electrostatic Perturbation

In this paper, we use the hydrodynamic approach to study the stimulated scattering of high-frequency electromagnetic waves by a low-frequency electrostatic perturbation that is either an upper- or lower-hybrid wave in a two-electron-temperature plasma. Considering the four-wave interaction between a strong high-frequency pump and the low-frequency electrostatic perturbation (LHW or UHW), we obtain the dispersion relation for the scattered wave, which is then solved to obtain an explicit expression for the growth rate of the coupled modes. For a typical Q-machine plasma, results show that in both cases the growth rate increases with noh/noc. This is in contrast with the results of Guha & Asthana (1989), who predicted that, for scattering by a UHW perturbation, the growth rate should decrease with increasing noh/noc.

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