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N. A. Streten

Abstract

The computer-generated digital mosaics of daily cloud photographs obtained by the ESSA 3 satellite were studied for the Southern Hemisphere south of 40S for the period of the southern summer, December 1966 to February 1967. The distribution and movement of the large-scale vortex patterns enabled a number of regions of high frequency of cyclones, cyclogenesis and cyclolysis to be located over the ocean areas surrounding Antarctica and the principal trajectories of cyclones over the southern oceans to be examined. These results are in substantial agreement with earlier synoptic studies. Little evidence was found for frequent cyclogenesis at latitudes close to the Antarctic coast and mid-latitude cyclogenesis was frequently observed without classical frontal wave cloud sequences. The areas of high cyclonic frequency and of cycloysis close to the Antarctic coast for the period of the study are found to be consistent with some known features of mean annual precipitation over the continent. Particular regions of frequent low cloudiness were observed close to the coast of Antarctica.

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N. A. Streten

Abstract

Analyses of daily and multi-day satellite mosaic sequences are used to obtain evidence regarding the atmospheric circulation pattern over the South Pacific from the winter of 1971 to the summer of 1972–73. Interseason comparisons indicate a strong increase in cyclonic activity in the autumn, winter, and spring of 1972 over the ocean to the west of the coast of Chile accompanied by an apparent eastward trend in the movement of the quasipermanent South Pacific cloud band. Such a pattern is consistent with available conventional observations and it suggests an anomalously high frequency of northerly component in the surface winds over the higher latitude course of the Peru current. The latter would thus be retarded, presenting conditions favorable to a Niño season.

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N. A. Streten

Abstract

A description is given of a quasi-periodic variation averaging some 20–25 days in the westward motion of the mid-South Pacific cloud band as using 5-day averaged satellite imagery.

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N. A. Streten

Abstract

A description is given of an Indian Ocean cloud band oriented quasi-meridionally across the equator. The feature is some 3000 km in length and only 10–20 km in core width, but exhibiting more extensive blow-off cirrus. Synoptically, the band is associated with the longitudinal coincidence of outflow from a Northern Hemisphere tropical cyclone with high-level flow eastward of a major Southern Hemisphere trough; however, the detailed dynamics of the phenomenon is obscure.

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N. A. Streten

Abstract

An analysis is made of the geographical variations in the magnitude of, and the relation between, the ranges of monthly and seasonal mean temperatures for longer established stations over the Southern Hemisphere from the beginning of observations in the mid-nineteenth century to 1970. The latitude of maximum variation between the same seasons of different years varies with the region; but over the lower latitude continents it occurs, in general, equatorward of the subtropical anticyclones. An extreme variation exists in winter at islands on the fringe of the pack ice zone of the Weddell Sea. The temporal frequency of occurrence of maximum and minimum seasonal temperatures over the past century displays considerable regional differences and no clear hemispheric trends are apparent; however, some evidence exists of fewer extreme warm seasons after 1950 over Australasia and Africa in summer. Differing circulation patterns associated with extremes of temperature in winter months over the pack ice zone in the southwest Atlantic, over southern Africa and over the Australian region are described primarily using sequences of indices of zonal and meridional flow. Some indication is given of the accuracy that would be required in a numerical prognosis of monthly mean pressure field over the hemisphere to permit inferences to regional anomalies of mean temperature.

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N. A. STRETEN

Abstract

Synoptic chart data for the Southern Hemisphere south of 30°S (excluding the Pacific sector) are examined for the winter of 1962 in relation to the frequency of cyclones and anticyclones at the 700- and 500-mb levels. Regions of high cyclonic frequency are noted and aspects of those in the eastern Ross Sea and in Prydz Bay are discussed. Cyclonic frequency over east Antarctica is very low, but clearly defined anticyclones at 500 mb are apparent on 1day in 6 over the highlands close to 80°S, 60°E. The longitudinal frequency of prominent troughs and ridges and some aspects of the pattern of cyclogenesis for this period are described.

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N. A. Streten

Abstract

A set of 10 synoptic indices for the Southern Hemisphere is presented based on the first five years of numerical analysis by the Australian Bureau of Meteorology. The indices describe the characteristics and variability of the monthly and seasonal mean patterns obtained by the time averaging of the daily analyses, referring specifically to (i) the extension of the subtropical anticyclones to higher and lower latitudes, the latitude of the axis of highest pressure, the location of the anticyclonic centers and their longitudinal separation; (ii) the strength and variability of the midlatitude zonal westerlies; (iii) the latitude of the sub-Antarctic trough and the longitudinal spacing of its pressure minima; and (iv) the location of the Antarctic quasi-anticyclone. The indices are intended for maintenance as a historical series. They may provide a medium for monitoring the monthly and seasonal circulation of the hemisphere, for investigating its anomalies, and for validating numerical simulations of its gross features.

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N. A. Streten

Abstract

The intensity and duration of strong wind events at three Australian stations in East Antarctica are discussed using four years of records. The longer term events are examined in relation to the pressure, temperature, wind speed and direction, and weather fluctuations occurring during the periods. The mean event differs little in intensity between the three stations, but at the station which experiences high frequencies of katabatic winds, the events show a slight tendency to longer life. A characteristic temperature change is apparent at all the stations, differing between the colder and the warmer seasons. Some inferences are made regarding the cause of observed variations in some elements both seasonally at all stations and between individual locations. Possible features of the general circulation of the Southern Hemisphere which may be active in producing the observed frequency distribution of strong winds are also outlined.

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A. J. Troup and N. A. Streten

Abstract

A classification scheme for cloud vortices recorded on hemispheric digital mosaics southward of 20S during a period of some 550 days is used to construct models based on observations of associated atmospheric structure. Such models for various vortex types are expressed in terms of anomaly patterns of surface pressure, and 500- and 300-mb geopotential. Hopefully such models may find application in the development of analysis techniques over the data-void regions of the hemisphere.

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N. A. Streten and W. R. Kellas

Abstract

Satellite-observed cloud signatures associated with a large sample of mature and decaying depressions over middle and high southern latitudes are described in terms of a revision of an earlier classification Associated model anomaly patterns of surface pressure and upper geopotential are obtained, and sequential behavior patterns described. Some evidence is found for higher frequencies of particular signatures in regions to the cast of preferred locations of long-wave ridges.

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