South American Sector Research Articles

High-frequency patterns of the atmospheric circulation over the southern hemisphere and South America

Daily 500-hPa geopotential height and 250-hPa meridional wind reanalyzed data obtained from the National Centers for Environmental Prediction are used to document austral winter (May to September) and summer (November to March) high-frequency variability in the Southern Hemisphere (SH) midlatitudes for the 1990–1994 period. Empirical orthogonal function (EOF) technique is used to determine the high-frequency patterns for these variables in selected areas. The high-frequency anomalous 500-hPa geopotential height patterns for two areas in the SH midlatitudes (the zonally global domain and the western hemisphere) and the high-frequency anomalous 250-hPa meridional wind patterns in the western hemisphere between 15° N and 70° S are discussed. The high-frequency winter and summer patterns for both variables feature a wavetrain structure in the SH midlatitudes which is related to synoptic-scale systems, such as cyclones and anticyclones associated with frontal zones. The dominant high-frequency patterns in the SH midlatitudes manifest in the eastern hemisphere while the secondary ones appear in the southeastern Pacific. Analysis of the western hemisphere data reveal that the wavetrain in the South American sector extends northeastward over the continent, thus affecting the regional weather conditions. An important result presented here concerns the preference of the intense synoptic systems in the eastern hemisphere and in the southeastern Pacific to occur in a sequential instead of an intermittent fashion. This result might have a potential for being used in weather monitoring.

The Pacific–South American Modes and Tropical Convection during the Southern Hemisphere Winter

Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation. A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.

Dynamical influence on the equatorial airglow observed from the South American sector

The upper atmospheric airglow emissions, OI 557.7 nm, NaD 589.3 nm, OH (9,4) and O 2 atmospheric (0,1) bands and their rotational temperatures have been measured using ground-based multichannel airglow photometers, one located near the equator at Fortaleza (3.9 S, 38.4 W) and the other at lowmiddle latitude Cachoeira Paulista (22.7 S, 45.0 W). Monthly averaged nocturnal variations calculated from the data from 1987 to 1993 were used to study the influence of atmospheric dynamical processes on these emissions. Harmonic analysis revealed that the Cachoeira Paulista data are mainly represented by a 12 hour period oscillation. Phase differences between the different emissions reveal that the vertical phase velocity is about 4 km/h and the vertical wavelength about 50 km. Fortaleza data on the other hand showed longer period, 24 hours, slow vertical propagation velocity, 1.1 km/h, and short vertical wavelength, about 30 km. These facts lead us to conclude that the Cachoeira Paulista data are mainly controlled by the semidiurnal tidal oscillation and the Fortaleza data are controlled by the diurnal tide.

Post-sunset wintertime 630.0 nm airglow perturbations associated with gravity waves at low latitudes in the South American sector

Abstract We report photometric southern hemisphere wintertime perturbations of the nocturnal F-region atomic oxygen 630 nm red line intensity which are observed to occur concurrently with apparent ionogram signatures of F-region gravity waves. Such airglow events were detected on a total of 40 days of experiments. The airglow experiments were run at Cachoeira Paulista (CP, geographic 22 ° 41′ S, 45 ° 00′ W, dip 28 ° S). The events are characterized by south-to-north travelling airglow valleys, or depletion events (SNE). They were obtained from a set of 299 experiments performed during the period of 10 August 1977 to 4 July 1984. Their equatorwards velocities, wavelengths and periods vary within the ranges of 150–300 m s −1 , 100–200 km and 15 min — 3 h, respectively. They appear during the post-sunset period and remain within the field of view of the meridional scanning photometer from about ten minutes to a few hours. The SNE were seen to occur more frequently in July and were absent from September to March. The photometer scanned ± 75 ° around zenith in the magnetic meridional and zonal planes. Ionogram spread-F and VHF polarimeter data that showed no phase or amplitude scintillations during the SNE indicated no concurrent existence of equatorial plasma depletions. It is shown that the SNE are not necessarily related to geomagnetic disturbed conditions. The present results are, to the authors' knowledge, the first showing gravity wave effects, near the sunset side of the solar terminator, on the low-latitude F-region atomic oxygen red nightglow under undisturbed geomagnetic conditions. The characteristics of these waves point to the solar terminator as their source of generation. A series of physical and morphological features of the phenomena are presented.

Sea-level pressure variability around Antarctica since A.D. 1750 inferred from subantarctic tree-ring records

A tree-ring chronology network recently developed from the subantarctic forests provides an opportunity to study long-term climatic variability at higher latitudes in the Southern Hemisphere. Fifty long (1911–1985), homogeneous records of monthly mean sea-level pressure (MSLP) from the southern latitudes (15–65 °S) were intercorrelated on a seasonal basis to establish the most consistent, long-term Trans-Polar teleconnections during this century. Variations in summer MSLP between the South America-Antarctic Peninsula and the New Zealand sectors of the Southern Ocean are significantly correlated in a negative sense (r=−0.53, P<0.001). Climatically sensitive chronologies from Tierra del Fuego (54–55°) and New Zealand (39–47°) were used to develop verifiable reconstructions of summer (November to February) MSLP for both sectors of the Southern Ocean. These reconstructions, which explain between 37 and 43% of the instrumentally recorded pressure variance, indicate that inverse trends in MSLP from diametrically opposite sides of Antarctica have prevailed during the past two centuries. However, the strength of this relationship varies over time. Differences in normalized MSLP between the New Zealand and the South America-Antarctic Peninsula sectors were used to develop a Summer Trans-Polar Index (STPI), which represents an index of sea-level pressure wavenumber one in the Southern Hemisphere higher latitudes. Tree-ring based reconstructions of STPI show significant differences in large-scale atmospheric circulation between the nineteenth and the twentieth centuries. Predominantly-negative STPI values during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the New Zealand sector during the 1800s. In contrast, cyclonic activity appears to have been stronger in the mid-twentieth than previously for the South American sector of the Southern Ocean. Recent variations in MSLP in both regions are seen as part of the long-term dynamics of the atmosphere connecting opposite sides of Antarctica. A detailed analysis of the MSLP and STPI reconstructions in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency bands with a period around 3.3–3.6 y. Cross spectral analysis between the STPI reconstruction and the Southern Oscillation Index suggests that teleconnections between the tropical ocean and extra-tropical MSLP variations may be influencing climate fluctuations at southern latitudes.

Ionospheric F2 layer seasonal and semiannual variations

An extensive series of computations, using the Coupled Thermosphere‐Ionosphere‐Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the F2‐layer, particularly at low latitudes and in the southern hemisphere at mid‐latitudes. Results from the model reveal such a variation, most prominently, at mid‐latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4‐month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter solstice, leading to diminished ion production, a minimum in NmF2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.

A new family of monotremes feom the Creataceous of Australia

AUSTRALIA'S second Mesozoic mammal, Kollikodon ritchiei (Monotremata, Kollikodontidae, new family) has an extreme bunodont molar morphology. The existence of two distinctively different families of monotremes in the Early Cretaceous suggests that the order originated long before the Cretaceous and was very diverse in at least the Australian portion of eastern Gondwana. With four families now known from Australia, it is probable that monotremes originated and diversified in the Australian/Antarctic sector of Gondwana, followed by a single dispersal (ornithorhyn-chid) to the South American sector before or during the early Paleocene. It is probable that kollikodontids are the sister-group of a steropodontid/ornithorhynchid/tachyglossid clade. K. ritchiei and Steropodon galmani are among the largest Mesozoic mammals known.

Interannual variability of deep convection over the tropical South American sector as deduced from ISCCP C2 data

AbstractThe interannual variability of deep convection over the South American sector and regional hydrometeorological anomalies are studied using International Satellite Cloud Climatology Project (ISCCP) data for 1984–1988. Satellite‐derived convection anomalies are compared with regional rainfall and river runoff anomalies.At the height of the austral summer, the deep convective clouds (DCC) and ISCCP depict the centres of intense convection over central Amazonia and the mouth of the River Amazon, as well as the enhanced convection over north‐east Brazil, which is related to the Atlantic Intertropical Convergence Zone (ITCZ). Positive/negative regional hydrometeorological anomalies are consistent with positive/negative anomalies in the deep convection over northern Amazonia, the mouth of the River Amazon and north north‐east Brazil, whereas this is not always true over southern Amazonia and southern Brazil. Over north‐east Brazil and the mouth of the River Amazon, abundant rainfall and large DCC are both in agreement with variations in the latitudinal position and intensity of the Atlantic ITCZ, which extends between 4°N and 5°S along the Atlantic coast, where the precipitation maximum is found. The related rainfall anomalies over Amazonia and north‐east Brazil during years with anomalously warm or cool surface waters in the central equatorial Pacific (e.g. the El Niño event in 1987), are consistent with the convection anomalies as depicted by the ISCCP deep convective clouds. For the Amazon Basin and north‐east Brazil, DCC based on 2.5°×2.5° horizontal resolution explained about 50 per cent of the variance of rainfall at individual surface stations.A new finding is the enhanced convective activity located at the east side of the central Andes and over central Amazonia around 5°S, with a region of reduced convection in between. These two subcentres are oriented west‐east and are part of the summertime semi‐permanent centre of convection over the Amazon Basin, and have not been identified previously from the analysis of outgoing longwave radiation (OLR) or highly reflective clouds. These subcentres of large DCC exhibit interannual variations in intensity and location, where the centre located near the east flank of the Andes shows a wider range of latitudinal variations than that located over central Amazonia. South of this area convection and rainfall decreases rapidly.

Principal Modes of Outgoing Longwave Radiation and 250-mb Circulation for the South American Sector

Abstract Principal modes of anomalous upper-tropospheric circulation and outgoing longwave radiation (OLR) are determined for the South American sector. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filtered so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting patterns show a consistent relationship between anomalous OLR and anomalous upper-tropospheric circulation features. The first and second interannual modes contain many atmospheric features related to extremes in the Southern Oscillation. These include 1) variations in the pattern of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equatorial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southern summer season. For positive (negative) amplitudes of this mode, anomalous upper-tropospheric weste...

A climatology of Southern Hemisphere extratropical cyclones

A climatology of extratropical cyclones determined by an objective automatic scheme applied to 15 years (1975–89) of once-daily Australian Bureau of Meteorology hemispheric analyses is presented. Contour maps of the positions of formation (cyclogenesis), dissipation (cyclolysis) together with other cyclone statistics are presented. The distribution of cyclones through the hemisphere was found to be dominated by a permanent high latitude core coincident with the circumpolar trough. During the winter and intermediate seasons, two mid latitude branches are evident in the cyclone density originating in the Tasman Sea and South American sectors, both spiraling poleward and merging with the circumpolar core in the Southern Oceans. Systems were observed to move in an castsouth-east direction, away from their location of formation, exhibiting peak speeds of migration in the mid latitudes. Little seasonality was evident in the densitiy distribution of cyclones through the Southern Oceans, but a considerable amount was found in their central pressure.

How the effects of winds and electric fields in F2-layer storms vary with latitude and longitude: A theoretical study

The effects of thermospheric winds and electric fields on the ionospheric F2-layer are controlled by the geometry of the magnetic field, and so vary with latitude and longitude. We adopt a simple model of the daytime F2-layer and study the effects at midlatitudes (25°–65° geographic) of three processes that accompany geomagnetic storms: (i) thermospheric changes due to auroral heating; (ii) equatorward winds that tend to cancel the quiet-day poleward winds; and (iii) the penetration of magnetospheric electric fields. At ± 65°, the effects of heating and electric fields are strongest in the longitudes towards which the geomagnetic dipole is tilted, i.e. the North American and the South Indian Ocean sectors. Because of the proximity of the geomagnetic equator to the East Asian and South American sectors, the reverse is true at ± 25°.

On the Functioning of the Southern Oscillation in the South American Sector. Part II. Upper-Air Circulation

Abstract The functioning of the Southern Oscillation (SO) in the South American sector is analyzed with particular emphasis on the upper-air circulation anomalies. Throughout the year, but especially during austral summer, the negative SO-phase (defined as anomalously low/high pressure at Tahiti/Darwin) is typically associated with a relatively warm tropical troposphere, while negative temperature anomalies prevail during the positive SO- phase. Outside the tropics, upper-air circulation anomalies are most pronounced during the winter. Thus, in boreal winter the negative SO phase is associated with reduced 500 and 200 mb heights at 20–30°N, but positive height departures southward of 20°N. The implied steepening of the meridional thickness gradient is consistent with anomalously intense upper-air westerlies over the Gulf of Mexico and Caribbean region. In austral winter during the negative SO phase, relatively cold tropospheric conditions prevail over the southern portion of South America. Case studies fo...

On the Functioning of the Southern Oscillation in the South American Sector. Part I: Surface Climate

Abstract The functioning of the Southern Oscillation (SO) over South America and the surrounding tropical Atlantic and eastern Pacific Oceans is studied through analyses of bimonthly patterns of correlation between an SO index and the fields of pressure, wind, temperature and rainfall; hydrometeorological anomalies being further ascertained from river discharge and lake level records and measurements of outgoing longwave radiation. Most pronounced is the anomalously low pressure over the eastern Pacific during the negative SO phase (defined by anomalously low/high pressure at Tahiti/Darwin). Consistently positive SO-pressure correlations over the eastern tropical Pacific extend eastward into the North Atlantic region and southern South America during the respective winter semester. During the austral winter the tendency for anomalously abundant rainfall in central Chile during the negative SO phase appears related to an anomalously weak and northward displaced South Pacific subtropical high, while relativ...

Stratigraphy and geochemistry of accreted fragments of the ancestral pacific floor in southern south America

The outer Pacific archipelago of southern Chile has allochthonous fore arc rocks that were accreted prior to the Late Jurassic along the ancestral Pacific margin of the southern South American sector of Gondwana. The fore arc rocks in the Archipielago madre de Dios (50°-50° 47′S) are divisible into three assemblages: the Tarlton Limestone, a sequence of massively bedded Upper Carboniferous to Lower Permian limestone; the Duque de York Complex, a sequence of flysch-like sedimentary rocks; and the Denaro Complex, an association of pillow basalt, bedded chert, detrital calcareous rocks, and shale. The Denaro Complex is interpreted as a sequence of the late Paleozoic ancestral Pacific floor. The Tarlton Limestone may represent a shallow water carbonate platform also built on a basaltic substrate located within the ancestral Pacific realm. The basalt units of the Denaro Complex have tholeiitic major element patterns similar to those of modern day “MOR” and “within plate” basalts. Chemical data for the Denaro Complex allow for the partial discrimination of biogenic, hydrothermal, hydrogenous, and detrital components within the sedimentary units, and facilitate comparison with modern day sediments as well as with other ophiolite sequences. Chemical data together with stratigraphic and structural field relations suggest that the evolution of the Denaro Complex and Tarlton Limestone involved seafloor spreading, hydrothermal activity, seamount volcanism, carbonate reef development, and convergence of the oceanic realm with the margin of Gondwana.

Ionospheric scintillation observations at Natal

We report ionospheric scintillation observations made at Natal, Brazil, during a period of 2½ years. The nighttime scintillations of satellite signals at 250 MHz commonly exceeded 20 dB peak‐to‐peak. Seasonally, it showed a typical behavior for the South American sector with a single peak centered around October–November months and the local evening hours of 2100–2300 LMT. This seasonal behavior was different from that of the African sector and very different from that of the Pacific sector. On a spacing of 278 m the eastward irregularity drift speed was measured by the method of similar fades. This, together with the determination of Fresnel frequency from the power spectrum analysis, was used to calculate the height of the irregularity patch. A value of 294 km was obtained. Multisatellite scintillation observations indicated that even though the initiation of an irregularity cloud commonly occurred earlier on signals transmitted by the eastern satellite, there were substantial cases when the reverse was observed. One case which was analyzed in detail suggested that the irregularity clouds formed independently along two radio paths separated by 520 km.

Atmospherics measurements in San Miguel, Argentina

Measurements of VLF atmospherics taken at San Miguel, Argentina, are presented. Results from one 13‐year measurement period (1956–1969) as well as one 2‐year interval (1971–1973) are used to document the morphology of the diurnal and annual variations in atmospherics activity at San Miguel. The atmospherics rates obtained during the second interval, which may be ordered according to arrival azimuth, show that most of the activity originates in thunderstorm cells located NE and NW of the observation station. Distance determinations from the dispersion of the atmospherics impulses, together with an estimate of the instrument's range, indicate that these activity cells lie primarily over the South American continent. Additional measurements of atmospherics activity taken in Berlin from 1971 to 1975 are also shown, from which a certain trend in the activity in the South American sector may be inferred. Both the Berlin and San Miguel records indicate a decline in atmospherics activity over the measurement period.

Universal time control of the south polarFlayer during the IGY

A reexamination of south polar F layer data from the International Geophysical Year has been made, and these results were compared with recent satellite measurements of low-energy electrons. The ionospheric data were plotted so that they would show more clearly the dominance of universal time in the control of polar F layer electron densities. This plot of the data also establishes the clear dependence of polar F layer electron densities on corrected geomagnetic latitude. Comparison between the latitudinal dependence of the F layer electron densities and that of the low-energy electron measurements shows close agreement. This agreement suggests that influxes of low-energy electrons into the ionosphere may be responsible for maintaining or enhancing the south polar F layer. Thus, if universal time control of the F layer is due to precipitation of low-energy electron fluxes, one could say that electron precipitation itself must be dependent on universal time. The latitudinal dependence of south polar F layer electron densities indicates that in winter the area of universal time control is restricted to corrected geomagnetic latitudes greater than 50°. Other workers have indicated that in summer universal time control existed at midlatitudes, but only in the South American sector. Present satellite measurements do not support an intrusion of these low-energy electrons down to midlatitudes. Thus, an unknown process confined to this area in summer appears to be operating.

Research Note: Chlamydia Antibodies in Antarctic Fauna

The Chlamydia group is known to parasitize a wide range of hosts from tropical and temperate countries and al'so some freeflying bird species from polar regions (2). Recent collections of serum and organs by the Australian National Antarctic Research Expeditions staff at Macquarie Island and Mawson (Fig. 1) have enabled us to detect the presence of Chlamydia in one species of seal and three species of penguins, and to confirm findings in another two penguin species. Sieburth (5) published results of a survey for ornithosis antibodies in penguins collected in the South American sector of the Antarctic. Among 10 birds of 4 species tested, he found one suspect reaction in a Gentoo penguin (Pygoscelis papua). This was the first indication of the possible presence of Chlamydia in the Sphenicidae. Later, Sladen (6) demonstrated low titers of complement-fixing psittacosis antibody in sera from Adelie penguins (Pygoscelis adeliae) and a Crabeater seal (Lobodon carcinophagus) collected in the Australian and New Zealand sectors of Antarctica. The presence of these agents in the Sphenicidae was confirmed by Cameron (1), who isolated Chlamydia from an Emperor penguin chick (Aptenodytes forsteri) collected at Mawson in 1965. We record here results of a survey on specimens from Adelie penguins and Weddell seals (Leptonychotes weddelli) coll'ected at Mawson by Dr. J. Hudson, and specimens from Rockhopper penguins (Eudyptes crestatus), Royal penguins (E. chrysolophus schlegeli), and Gentoo penguins collected on Macquarie Island by Mr. P. Shaughnessy.

The Zonal Flow of the Free Atmosphere Between 10N and 80S, in the South American Sector

Abstract Data from all available sources have been used to construct vertical cross sections of the seasonal mean flow of the atmosphere between 900 and 30 mb, over the western and southern part of South America. Monthly zonal wind profiles are presented for the subpolar region where the stratospheric circumpolar westerlies in late winter are strongest. For these latitudes, an extrapolation of the zonal wind up to the 10-mb level is attempted.