Local Summer Research Articles

Interaction of the quasi‐biennial oscillation and stratopause semiannual oscillation

Analysis of rawinsonde and rocketsonde data at Ascension Island (7.6°S, 14.4°W) and Kwajalein (8.7°N, 167°E) in 1962–1991 suggests that the quasi‐biennial oscillation (QBO) in the middle stratosphere is synchronized with the seasonal cycle and that descending westerly phases of the stratopause semiannual oscillation (SAO) are strongly influenced by the underlying QBO. The effect of the seasonal cycle on the QBO in the middle stratosphere is revealed in two, perhaps unrelated, observations: first, a tendency for deseasonalized QBO westerly maxima to occur in local winter (or to avoid local summer); second, a smooth, uninterrupted connection between descending SAO westerly shear zones and the formation of a new QBO westerly shear zone aloft. The timing of deseasonalized QBO westerly maxima in the middle stratosphere allows a simple composite of 2‐ and 3‐year cycles to be constructed from the data, illustrating the effect of the QBO on descending westerly phases of the stratopause SAO.

Nighttime thermospheric meridional winds at Cachoeira Paulista (23°S, 45°W): Evidence for effects of the equatorial midnight pressure bulge

We have studied the local time and seasonal variations of thermospheric nighttime meridional winds at the low‐latitude station, Cachoeira Paulista, Brazil, corresponding to quiet geomagnetic conditions of high solar activity (Fl0.7 > 150 units). The meridional winds are derived from F layer peak height (hmax) data using a modified form of the “servo” model wherein plasma transport due to electric field is taken into account. It is found that during the solstices the meridional winds exhibit the transequatorial neutral airflow from the summer to the winter hemisphere, i.e., the nighttime winds are primarily equatorward (poleward) during the December (June) solstice. It is shown that this neutral wind flow pattern is effectively modulated by winds related to equatorial midnight temperature maximum (MTM) and the associated midnight pressure bulge, especially in the December solstice (local summer). The seasonal visibility of the effect of MTM on low/equatorial latitude meridional winds seems to be different in the east Brazil (45°W) and Indian (75°E) sectors.

Quasi 2-day oscillation of the ionosphere: A statistical study

Abstract A large database of hourly ionosonde measurements, encompassing more than 20 stations during 1957–1991, is examined for evidence of the quasi-two-day (QTD) oscillation. A QTD (frequencies between 0.50±0.08 day−1) oscillation in ƒ o F 2 at the 95% confidence level is found to exist with the following characteristics: 1. 1. Amplitude variations primarily in the range ±0.4–1.0 MHz; 2. 2. Occurrence frequency (i.e. in a given month, at a given station, the percentage of years in which a QTD event was detected) maximizing at about 30–60% during local summer in both hemispheres; 3. 3. At mid-latidues, zonal structure corresponding either to westward propagation with zonal wavenumber s = 1, or a stationary oscillation with indeterminable zonal wavenumber; at lower latitudes (ca. 20°) a much broader spectrum of wavenumbes (s = −2 to s = +4) is evident; 4. 4. No variation in occurrence frequency with solar cycle. At the same confidence level a QTD oscillation in hmF2 occurs much less frequently than ƒ o F 2 , with typical amplitudes of order ±8–16 km.

The ozone hole over Punta Arenas, Chile

Three fundamental questions are addressed for the location of Punta Arenas (53.0°S, 70.9°W), Chile: Does the Antarctic ozone hole extend over the city? If so, at what height is most of the ozone depleted? And by how much does the UVB radiation increase under ozone hole conditions? It is shown that Punta Arenas is affected by the Antarctic ozone hole, even though it is a considerable distance away from the pole. In comparison with the average global trend of −0.25% per year, the downward ozone trend at Punta Arenas is −0.5% per year (twice as large as the global average) and −1.2% per year (almost 5 times larger than the global average) using the monthly October averages. To obtain these figures, an ozone climatology of TOMS (Total Ozone Mapping Spectrometer) data from 1979 to 1992 has been used. Observations of the ozone hole over Punta Arenas are described during the austral spring periods of 1992, 1993, and 1994. Systematic observations with a Brewer spectrophotometer have been made in order to measure the vertical column of ozone and the simultaneous variations in UVB radiation. In addition, the vertical distribution of ozone has been investigated during ozone hole conditions, by using ozone ECC (Electrochemical Concentration Cell) sondes launched on balloons, in order to determine at which height the ozone is reduced most. Using the 220 DU (Dobson Units) threshold as reference, we identified the ozone hole over Punta Arenas on five occasions during the 3 year period of these observations, September 1993, October 1992, 1994; and November 1993 and 1994. The vertical distribution of ozone during ozone hole conditions shows maximum ozone destruction efficiency near 15–17 km, where ozone nearly vanishes completely, on several occasions. The largest amount of ozone depletion, however, occurs above this height, just under and at the main ozone peak, where it may reach about 50%. The UVB radiation in small spectral bands may increase dramatically during ozone hole events; at 295–297 nm the observed factors of increase for Punta Arenas were between 10 and 38. The maximum integrated/weighted intensities in October, during hole events increased over background values between 2 and 3 times, which represent values near the local summer maximum but not yet beyond levels normally observed at low‐latitude stations.

Daily and Seasonal Activity Patterns in Two Species of High Altitude Skink, Niveoscincus microlepidotus and N. metallicus, from Tasmania

Daily and Seasonal Activity Patterns in Two Species of High Altitude Skink, Niveoscincus microlepidotus and N. metallicus, from Tasmania

Simulations of the low‐latitude midnight temperature maximum

To investigate possible sources of the low‐latitude midnight temperature maximum (MTM), a series of simulations was made with the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The MTM is a tidal phenomenon and thus is ultimately due to the thermospheric tidal sources: in situ thermal excitation; ion‐neutralmomentum coupling; and the tidal waves arising in the lower atmosphere which penetrate the thermosphere. In the TIEGCM, the development of the MTM is due to the upward propagating semidiurnal tides. The model simulations suggest that the observed seasonal variation of the MTM, in which the MTM occurs earlier and more strongly in the local summer hemisphere, may be due to the interaction of the 2,2 and 2,3 modes: in summer, the two modes reinforce each other, and in winter they offset each other. This study further suggests that much of the variability observed in the MTM may be due to the variability in the upward propagating tides. The semidiurnal modal mix impinging on the lower thermosphere may vary on a day‐to‐day basis, depending on the background mean temperature and winds.

Abundances and distributions of picoplankton populations in the central equatorial Pacific from 12°N to 12°S, 140°W

Abundances and distributions of picoplankton were studied on two cross-equatorial transect cruises (12°N, 140°W-12°S, 135°W) during February–March (TT007) and August–September 1992 (TT011). Samples were collected in the upper 200 m from early-morning and late-afternoon hydrocasts at 15 stations on each cruise (60 depth profiles, 820 samples). Populations of heterotrophic bacteria, Prochlorococcus, Synechococcus and small autotrophic eukaryotes were enumerated by dual-beam flow cytometry. At the northern end of the transect (7–12°N), abundances and vertical distributions were similar to those reported for the oligotrophic North Pacific gyre, with Prochlorococcus and heterotrophic bacteria dominating in the upper euphotic zone, and Synechococcus and eukaryotes exhibiting peaks in cell abundance at depth. All populations were abundant throughout the euphotic zone close to the equator and at the southern end of the transect. Heterotrophic bacteria and Synechococcus were generally more abundant in late-afternoon hydrocasts. The diel enhancement followed the temporal pattern in beam c and suspended particulates, and was particularly strong in the equatorial upwelling region where it averaged 13.6% of the morning population for heterotrophic bacteria and 22.3% for Synechococcus. Conservative estimates of daily growth rates from these data are 0.25 and 0.40 day −1, respectively, for the two populations. Near-surface maxima in heterotrophic bacteria were symmetrical around the equator, centered around 5°S and 5°N. Prochlorococcus was most abundant during local summer conditions at the respective ends of the transect. A minimum occurred in association with a dense aggregation of buoyant diatoms at the convergent front of a tropical instability wave (2°N, TT011). The ratio of Prochlorococcus to total bacteria was generally in the range of 0.15-0.2 for the upper water column, but varied during TT011 from > 0.3 for the most northern stations to < 0.1 at the 2°N front. At higher latitudes, Synechococcus was more numerous during El Niño conditions (TT007) on both sides of the equator and at southern stations on both cruises. Autographic eukaryotes were more abundant during local winters at the ends of the transect and during the “cold tongue” conditions (TT011) at the equator. Picoplankton account for most of the chlorophyll biomass and primary production in the central equatorial Pacific. Nonetheless, their abundances and distributions are relatively stable and conservative while other populations, such as diatoms, respond more dramatically to environmental forcing.

A statistical analysis of wind in Terra Nova Bay, Antarctica, for the austral summers 1988 and 1989

A statistical analysis of wind frequencies has been done analyzing Sodar and Automatic Weather Stations (AWS) data recorded during two successive summer campaigns in Terra Nova Bay area, Antarctica. A preliminary analysis performed using AWS data for the same periods showed that no particular differences occurred between the two summers. This encouraged a comparison between sodar data at the two sites and the AWS as they would have been recorded at the same time. The local circulation is largely dominated by the katabatic winds originating by radiative cooling of near-surface air along the slopes of Reeves (270°) and Priestley (330°) glaciers. The surface wind frequencies showed a strong dependence on the station location confirming the preminent role of the orography on the air flow. Sodar data evidenced that a considerable variation of wind intensity and direction may occur in the upper levels and highlighted some aspects of the three-dimensional air motions. The contemporary analysis of the sodar profiles and AWS surface measurements allowed us to bring out some peculiarities of the local summer circulation related to the variation of the relative buoyancy of the two flows blowing from the Priestley and Reeves glaciers. During the summer, contrary to the winter, the airstream issuing from the Priestley does not cross the Northern Foothills and flows into the Nansen Ice Sheet at the confluence of Reeves and Priestley glaciers. Consequently, at the coastal site stations, the observed air flow is attributable to the Reeves glacier, which is more buoyant and capable, because of the greater air mass involved and of the sufficient speed to cross the Northern Foothills. Moreover, it has been shown how the Northern Foothills prevent barrier winds, running parallel to Transantarctic Mountains from reaching the coastal site where the Italian base is located. On the contrary a peak for barrier wind was observed when sodar was on Nansen Ice Sheet.

Paleo sea surface salinities in the low‐latitude Atlantic: The δ18O record of Globigerinoides ruber (white)

On the basis of the comparison of δ18O values of Globigerinoides ruber (white) (δ18OG. ruber) from modern sediments and measured sea surface temperatures (SST) and salinity (SSS), δ18OG. ruber values most clearly record summer SST and SSS of the uppermost 50 m of water in the low‐latitude Atlantic. A new transfer equation is presented for estimating local summer paleo SSS at 0–50 m depth using δ18OG. ruber, paleo SST, and the global δ18O ice effect of sea water, with the standard error reaching ±0.72 ‐ ±0.77‰ SSS. The equation was applied to a 35 kyr long δ18OG. ruber record and paleo SST estimates based on planktonic foraminiferal assemblages to reconstruct the paleo SSS changes along the east Atlantic margin off the northwest Sahara. The resulting local SSS show a general increase by 0.3–1.4‰ during the last glacial maximum (LGM) and large parts of glacial Termination I, suggesting a general increase in the upwelling of the highly saline North Atlantic Central Water and/or a reduced lateral advection of less saline Canary Current water. Two short salinity lows (up to 0.35‰ less than today) during and after the LGM are coeval with the Heinrich meltwater events 1 and 2 in the North Atlantic, indicating the breakdown of coastal upwelling and meltwater advection from the north. Both events are followed by an extreme SSS maximum (38.15‰) along with the restoration of the salinity conveyor belt. At 9.0–5.5 ka the salinity minimum (up to 0.65‰ less than today) reflects the continental runoff linked to the phase of strong North Saharan humidity during the early Holocene climatic optimum.

Solar cycle and seasonal variations in F-region vertical drifts over Kodaikanal, India

Abstract. Measurements of the changes in phase path of F-region reflections at normal incidence at Kodaikanal (77° 28'E, 10° 14'N, dip 3°N) from February 1991 to February 1993 are used to determine the variation of the equatorial evening F-region vertical drifts (V z) with season, solar and magnetic activity. It is found that on average, at Kodaikanal, the post-sunset peak in Vz(Vzp) is higher in equinox and local winter months than in local summer. The day-to-day variability in V zp is highest in summer and lowest in winter. This seasonal trend persists even on magnetically quiet days (Ap \\leq14). Vzp is found to increase with 10.7 cm solar flux in all three seasons but tends to saturate for large flux values (&gt;230 units) during local summer and winter months. Magnetic activity [represented by Ap as well as the time-weighted accumulations of a p and ap (τ)] does not seem to have any statistically significant effect on Vzp , except during equinoctial months of moderate solar activity, when Vzp decreases as magnetic activity increases.

Macroscale modeling and mesoscale observations of plasma density structures in the polar cap

The seasonal and UT variation of mesoscale structures (10 km – 100 m) in the central polar cap has been obtained from an analysis of 250‐MHz intensity scintillation observations made at Thule, Greenland. It has been established earlier [Su. Basu et al., 1990] that mesoscale structures causing scintillations of satellite signals may develop at the edges of macroscale structures (several hundred km) such as discrete polar cap plasma density enhancements or patches through the gradient drift instability process. As such, we examined the seasonal and UT variation of polar cap patches simulated by using the USU Time Dependent Ionospheric Model (TDIM) under conditions of southward Bz. A fairly remarkable similarity is found between the scintillation observations and the model predictions of patch occurrence. For instance, both the patch and scintillation occurrences are minimized during the winter solstice (northern hemisphere) between 0800–1200 UT while also having their largest seasonal intensity between 2000–2400 UT. Little UT dependence of patches and scintillations is seen at equinox with high intensity being observed throughout the day, while during local summer the intensity of macroscale patches and mesoscale irregularities are found to be a minimum at all UT. These results indicate that macroscale features in the polar cap are routinely associated with plasma instabilities giving rise to smaller scale structures and that the specific patch formation mechanism assumed in the simulation is consistent with the observations. This ability to bridge between macroscale modeling and mesoscale observations provides a natural framework for the modeling of mesoscale structures themselves. This mesoscale modeling, in turn, can be utilized in a variety of radar and communication systems applications in the polar region.

Short-Period(5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator.

Measurements of F region vertical drift, Vz made at Kodaikanal (dip 4°N) on 133 days during March 1991-February 1992 under non-equatorial spread-F(ESF) conditions, are analysed for the characteristics of short-period (5-33 min) variations in Vz in the evening hours (1630-0130 IST). The spectral content of the fluctuations in Vz is found to depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (σ2), increases during the period of postsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, while functuations in Vz in the entire band 5-33 min manifest in the postsunset hours, the longer-period (13-33 min) components dominate thereafter (2200-0000 IST). The amplitude of the fluctuations and its day-to-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of Vz functuations increases with peak height in the interval 1756-2004 IST corresponding to the epoch of the evening enhancement in Vz. The results support the view that the short-period variations in F region vertical drift near the dip equator during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.

A World-Wide Annual Variation in Sq.

The amplitude of the annual variation in the horizontal component of Sq is larger in the southern hemisphere than the northern hemisphere. This asymmetry may be represented as the superposition of a world-wide annual variation with its maximum at or near perihelion and summer-winter annual variations of equal amplitudes in the two hemispheres and maxima at the local summer solstice. The amplitude of the world-wide annual variation, as a fraction of the yearly mean value of Sq, is about 0.07 ± 0.02. The variation in ionospheric conductivity associated with the earth's orbital eccentricity is inadequate alone to account for this annual variation in Sq. If joule heating by the Sq currents is appreciable, its annual and semi-annual variations could account for the similar variations in the temperature of the thermosphere.

Epidemic of Salmonella enteritidis infection of childhood in the northern area of Nagasaki

Salmonella Enteritidis infections were a local epidemic in the northern area of Nagasaki, Japan, during August to September in 1992. Out patients (142) visited our hospital because of diarrhea and-or abdominal pain and 96 patients had stool cultures and 51 patients were diagnosed as Salmonella infection. Of the 51 patients it was found that the Salmonella serogroups were 09, 07, 08 and the number of patients were 42, 8, 1, respectively. Phage type 1 was identified in all of the S. Enteritidis infected 38 patients who were examined. About 90% of the patients were under 10 years old. Namely, this Salmonella Enteritidis epidemic in childhood. The peak epidemic period was consistent with the local summer festival and the etiology of infection was thought to be caused by polluted handmade ice cream. S. enteritidis was identified from the ice cream which was also phage type 1. The latent period was 87 hrs.

Circulation and representativeness of precipitation and air temperature in the southeast of the Quinghai-Xizang Plateau

Based on time series of 98 stations from the Qinghai-Xizang Plateau, the Indo-Gagnetic Plains, the River Gorge Country and Eastern China, the representativeness of the locally observed interannual variations of temperature and precipitation was analyzed by a statistical method. After an overview, which regards the main circulation patterns as well as the spatial distribution of temperature and precipitation in winter (January) and summer (July), the monthly distribution of the representativeness is discussed in a climato-genetic context. The monthly temperature representativeness is characterized by strong spatial and seasonal contrasts. The values range from less than 200 km in the basins and valley bottoms of NW Sichuan, caused by local summer circulation patterns, and more than 1600 km in the lower areas of E China, affected by the far reaching influence of the winter monsoonal surges of cold continental airmasses. With only a few exceptions mainly occurring in December, the precipitation representativeness does not exceed a 300 km limit. Due to the comparatively modest spatial disparities — except December when a steep rise, greater than 600 km occurs in the upper Tsangpo valley and adjacent areas — it can only be concluded, that a tendency of higher (lower) representativeness is predominantly related to advective (convective) regimes.

Study of calcium-K network evolution from Antarctica

To study the evolution of large convective cells known as supergranules, a solar telescope was set up at Maitri, Indian permanent station in Antarctica region, during the local summer months (December 1989 through March 1990). A continuous sequence of calcium K-line filtergrams for 106 hours spaced at intervals of about 10 min was obtained. The analysis of the data indicates that the most probable lifetime of the calcium-K network is about 22 hours. The lifetime depends upon the size of the cell and is larger for bigger cells. The data also show that cells (of a given size) associated with remnant magnetic field regions live longer than those in the field-free region. This may mean that the magnetic field plays an important role in the confinement of these structures.

Comparative studies of scatterers observed by MF radars in tbe southern hemisphere mesosphere

Using data from three radar sites (low, mid and high latitude) we study the aspect sensitivity of mesospheric scatterers probed by MF radars operating at or near a frequency of 2 MHz. The aspect sensitivity ( θ s ) is calculated by the spatial correlation technique. Calculations of θ s arc further extended to estimate the lcngth-to-depth ratio of the scatterers which indicate that the scattcrers arc quasi-specular. We find that high and mid latitude sites exhibit clear seasonal behaviour, with maxima occurring in the local summer while the low latitude site shows no significant seasonal behaviour.

Human impact on the Holocene forest line in the Central Alps

Palaeoecological investigations of a small mire in Otztal, Tyrol, Austria, situated about 50 m above the potential tree-line, indicates that human impact on the landscape started with burning of heath at approximately 5300 B.P. At about 4800 B.P. a weak increase in important apophytes may reflect the local presence of domestic animals. Between 4000 and 3500 B.P. a clear decline in pastoral activity occurred. From 3000 B.P. a strong increase in the representation of apophytes suggests local summer settlement, while in the interval 2600–2200 B.P. anthropogenic activity declined. After 2150 B.P. there was a marked increase in summer farm activity. Fresh information is presented on tree-line fluctuations during the Holocene: Pinus cembra forest ascended above the present potential tree-line by more than 50–100 m between 9000–8000 B.P., 6000–5500 B.P., and 3800–3000 B.P. A Betula maximum between 7000 and 5500 B.P. is probably due to succession in nearby avalanche tracks, as well as to a higher tree-line. Low humification and low loss-on-ignition values around 6000 B.P. may reflect the Frosnitz stadial (6900–6000 B.P.). The Rootmoos I stadial (5400 B.P.) and probably the early Sub-Atlantic stadial maximum (3000–2300) are also reflected in the physical properties of the peat profile.

Impact of Local Convective Cloud Systems on Summer Daytime Shelter Temperature

The potential impact of daytime local summer convective cloud systems on shelter air temperature is illustrated by numerical modeling and observations. Prolonged reductions in surface solar irradiance due to cloudiness result in a noticeable decrease in shelter temperature over drylands and a moderate temperature fall over wet surfaces. When cloudiness is abruptly diminished, shelter temperature increases rapidly. Numerical modeling of downdrafts associated with rainfall in a dry convective atmosphere indicates a pronounced drop in shelter temperature (as high as 12°C). The modeling results are consistent with observations. The significance of the results and their potential applications are outlined.

Sunrise effect on dayside Pc pulsations at the dip equator

A flux gate magnetometer with an amplitude resolution of 6 pT/LSB was installed at Huancayo, Peru (12°S, 750°W in geographic coordinates, 0.8°N, 335.6° in geomagnetic coordinates) in December 1985 and recorded field variations for 1 year with 3‐s sampling intervals. Horizontal field variations of Pc 3‐4 pulsations (periods from 150 to 30 s) were studied and it was found that the oscillations were observed predominantly in the H component (magnetic north‐south), though oscillations in the D component (magnetic east‐west) are superimposed temporarily at local sunrise. The D component signals, which last for ≈2 hours after local sunrise, bring about a large tilt of the polarization azimuth angles in the H‐D plane without a significant change in the ellipticity. The azimuth at sunrise lies in the NW‐SE quadrant during local summer (November to January), while it lies in the NE‐SW quadrant for local winter (March to September). On the basis of the observed seasonal change of the azimuth angles, we present a model of the ionospheric current system setup by the incident Pc 3‐4 pulsations in the dayside region of the dawn terminator at the dip‐equator.