Acoustic sounding of the atmospheric boundary layer at Halley, Antarctica

first_imgThe records obtained from a monostatic acoustic sounder run at Halley, Antarctica, have been analysed with the use of data from instruments on a 32 m mast and from radiosonde ascents. Echoes representing ground-based layers, waves, and shallow gravity currents are discussed. The spiky ground-based echo is related to a westerly surface wind, whilst a layered wavy flow is related to surface easterlies. Such relationships are consistent with the sloped inversion wind regime at Halley.last_img

Antarctic krill aggregation characteristics from acoustic observations in the Southwest Atlantic Ocean

first_imgThe distributional features and physical characteristics of 4830 krill (Euphausia superba Dana) aggregations detected acoustically in the Southwest Atlantic between 26 January and 21 February 1981 are described. Results are compared with aggregations detected in the Indian Ocean. Aggregations in the Atlantic were larger, closer to the surface and to each other than in the Indian Ocean. Similar patterns in the distribution of aggregation spacing along survey transects were found in the two areas, although the pattern of spacings in the Atlantic indicates differences in the scale of aggregation. Serial interdependence of aggregation variables was minimal in the Atlantic, with aggregation thickness, length and spacing showing weak inter-relationships. Weak functional association, between water depth and aggregation thickness was evident. Investigation of variability in aggregation structure in relation to prevailing environmental conditions gave equivocal results and no clear association between any aggregation variable and prevailing hydrography was observed. The implications of these results for future studies on krill aggregation are discussed in relation to a conceptual framework which was developed from the present results and aimed at linking krill aggregation characteristics to environmental features.last_img read more

The effects of nitric oxide cooling and the photodissociation of molecular oxygen on the thermosphere/ionosphere system over the Argentine Islands

first_imgIn the past the global, fully coupled, time-dependent mathematical model of the Earth’s thermo-sphere/ionosphere/plasmasphere (CTIP) has been unable to reproduce accurately observed values of the maximum plasma frequency, foF2, at extreme geophysical locations such as the Argentine Islands during the summer solstice where the ionosphere remains in sunlight throughout the day. This is probably because the seasonal dependence of thermospheric cooling by 5.3 μm nitric oxide has been neglected and the photodissociation of O2 and heating rate calculations have been over-simplified. Now we have included an up-to-date calculation of the solar EUV and UV thermospheric heating rate, coupled with a new calculation of a diurnally varying O2 photodissociation rate, in the model. Seasonally dependent 5.3 μm nitric oxide cooling is also included. With these important improvements, it is found that model values of foF2 are in substantially better agreement with observation. The height of the F2-peak is reduced throughout the day, but remains within acceptable limits of values derived from observation, except at around 0600 h LT. We also carry out two studies of the sensitivity of the upper atmosphere to changes in the magnitude of nitric oxide cooling and photodissociation rates. We find that hmF2 increases with increased heating, whilst foF2 falls. The converse is true for an increase in the cooling rate. Similarly increasing the photodissociation rate increases both hmF2 and foF2. These changes are explained in terms of changes in the neutral temperature, composition and neutral wind.last_img read more

Detailed analysis of a substorm event on 6 and 7 June 1989. 2. Stepwise auroral bulge evolution during expansion phase

first_img[1] We have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm, which was observed by the UV imager aboard the Akebono satellite. It was found that there were three distinct stages in the evolution. Stage 1 was characterized by rapid poleward and azimuthal ( predominantly westward) expansions in a short time (about 2 min). Stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions. There was a certain period for transition between stage 1 and stage 2, and it was characterized by a very slow poleward and rapid eastward expansions. Stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions. The reactivation started around the initial onset meridian and then spread both eastward and westward. At the azimuthal front, the expansion first occurred at the lowest latitudes, spread poleward to around the highest latitudes of stage 1, and then spread further poleward after a brief interval. Hence, the local expansion also had three distinct stages similar to the global one. The ground-based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer (PSBL) region a few minutes before the onset. The further poleward expansion during the local third stage started with a significant intensification of the poleward-most auroral activity. During the local third stage, the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part. The proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage. Based on these observations, we discuss the evolution in the magnetosphere during the expansion phase.last_img read more

Springtime transition in upper mesospheric temperature in the northern hemisphere

first_imgAirglow emission observations by the wind imaging interferometer (WINDII) on the Upper Atmosphere Research Satellite and three optical ground-based stations previously revealed a “springtimetransition” in atomic oxygen. The transition is characterized by a rapid 2-day rise in the night-time oxygen nightglow emission rate by a factor of 2 to 3 followed by a subsequent decrease by a factor of 10 in the same period of time indicating a depletion of atomic oxygen that persists for days. The current study examines signatures in the upper mesosphere temperature field (70– height range), derived from the WINDII Rayleigh scattering observations, which may be associated with this springtime depletion of the atomic oxygen. Comparisons with ground-based OH airglow rotational temperatures, Na lidar and Rayleigh scattering lidar temperatures, and meteor radar temperatures at middle and high latitudes in the NorthernHemisphere are presented and discussed. Data from the northernspringtimes in 1992 and 1993 are reported upon in detail. It was found that all datasets used in the study agree well with each other taking account of the day/night time mean differences. A rapid temperature enhancement was observed at spring equinox at northern midlatitudes followed by a period of mean temperature colder than the one observed prior to the enhancement event, a pattern similar to that associated with the “springtime transition” observed in the oxygen emissions. The enhancement was also revealed in the average annual temperatures at , obtained by combining observations from 1992 to 1996, and in more recent temperature data from 1998 and 1999 at mid- and high northern latitudes. The results suggest that the temperature enhancement is associated with the last stratospheric warming event, observed at the end of March and early April.last_img read more

Monitoring Drake Passage with elephant seals: frontal structures and snapshots of transport

first_imgCTD-Satellite Relay Data Loggers (CTD-SRDLs), each recording conductivity, temperature and pressure, were attached to Southern elephant seals (Mirounga leonina) on the island of South Georgia. During the animals’ migration the CTD-SRDLs recorded and transmitted hydrographic profiles at a rate of approximately 2 profiles day−1 to an average depth of about 547m, representing transect-type sections with a spatial resolution of 16-47 km along the migratory routes of the seals. Two sections are used to clearly identify the locations of the Antarctic Circumpolar Current fronts across Drake Passage, providing in situ data complementary to satellite and other techniques. An empirical relationship between upper ocean temperature and baroclinic mass transport is used to determine the transport through Drake Passage at the times of the sections, and these transports are compared with estimates derived by other techniques. An absolute geostrophic velocity section across Drake Passage is calculated using CTD-SRDL data and data of absolute geostrophic surface velocities from altimetry. The mean total baroclinic transports in June 2004 and April 2005 are estimated to be 124×10^6 ± 14×10^6 m^3/s and 112×10^6 ± 14×10^6 m^3/s respectively.last_img read more

On the statistical significance of surface air temperature trends in the Eurasian Arctic region

first_imgThis study investigates the statistical significance of the trends of station temperature time series from the European Climate Assessment & Data archive poleward of 60°N. The trends are identified by different methods and their significance is assessed by three different null models of climate noise. All stations show a warming trend but only 17 out of the 109 considered stations have trends which cannot be explained as arising from intrinsic climate fluctuations when tested against any of the three null models. Out of those 17, only one station exhibits a warming trend which is significant against all three null models. The stations with significant warming trends are located mainly in Scandinavia and Iceland.last_img read more

A new Antarctic Peninsula glacier basin inventory and observed area changes since the 1940s

first_imgGlaciers on the Antarctic Peninsula have recently shown changes in extent, velocity andthickness, yet there is little quantification of change in the mass balance of individual glaciers orthe processes controlling changes in extent. Here a high-resolution digital elevation model and asemi-automated drainage basin delineation method have been used to define glacier systems between63°S–70°S on the mainland and surrounding islands, resulting in an inventory of 1590 glacier basins.Of these, 860 are marine-terminating glaciers whose ice fronts can be defined at specific epochs since the1940s. These ice front positions were digitized up to 2010 and the areas for all individual glacier basinswere calculated.Glaciological characteristics, such as geometry, slope and altitudes, were attributed to eachglacier, thus providing a new resource for glacier morphological analyses. Our results indicate that 90% ofthe 860 glaciers have reduced in area since the earliest recorded date. A north–south gradient of increasingice loss is clear, as is distinct behaviour on the east and west coasts. The area lost varies considerably betweenglacier types, with correlations apparent with glacier shape, slope and frontal-type. Temporal trendsindicate a uniform retreat since the 1970s, with a period of small re-advance in the late 1990s.last_img read more

An avionics platform for multi-instrument survey navigation

first_imgThe British Antarctic Survey regularly conducts airborne surveys with Twin Otter aircraft equipped with a variety of instruments. Each instrument captures its specific navigation requirements in a dedicated cockpit display that is unique and incompatible with that of other instruments. This creates unwanted logistical problems and training requirements, and necessitates extra air safety certification. In this paper we describe a new avionics display that is sufficiently flexible to capture the requirements of all of our instruments, as well as all of the preferences of our pilots. This Airborne Survey Navigation Device (ASCEND) dynamically routes aircraft within the constraints of the survey and features flexible and intuitive planning and navigation interfaces. ASCEND has been tested and compared to the instrument specific displays and is preferred, both for its ease of use and also for the effective accuracy of the pilot following a survey line.last_img read more

Habitat use and spatial fidelity of male South American sea lions during the nonbreeding period

first_imgConditions experienced during the nonbreeding period have profound long-term effects on individual fitness and survival. Therefore, knowledge of habitat use during the nonbreeding period can provide insights into processes that regulate populations. At the Falkland Islands, the habitat use of South American sea lions (Otaria flavescens) during the nonbreeding period is of particular interest because the population is yet to recover from a catastrophic decline between the mid-1930s and 1965, and nonbreeding movements are poorly understood. Here, we assessed the habitat use of adult male (n = 13) and juvenile male (n = 6) South American sea lions at the Falkland Islands using satellite tags and stable isotope analysis of vibrissae. Male South American sea lions behaved like central place foragers. Foraging trips were restricted to the Patagonian Shelf and were typically short in distance and duration (127 ± 66 km and 4.1 ± 2.0 days, respectively). Individual male foraging trips were also typically characterized by a high degree of foraging site fidelity. However, the isotopic niche of adult males was smaller than juvenile males, which suggested that adult males were more consistent in their use of foraging habitats and prey over time. Our findings differ from male South American sea lions in Chile and Argentina, which undertake extended movements during the nonbreeding period. Hence, throughout their breeding range, male South American sea lions have diverse movement patterns during the nonbreeding period that intuitively reflects differences in the predictability or accessibility of preferred prey. Our findings challenge the long-standing notion that South American sea lions undertake a winter migration away from the Falkland Islands. Therefore, impediments to South American sea lion population recovery likely originate locally and conservation measures at a national level are likely to be effective in addressing the decline and the failure of the population to recover.last_img read more