[1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

/pdf/gravity-wave-dynamics-and-effects-in-the-middle-atmosphere-4huexaq7y8.pdf

Gravity wave dynamics and effects in the middle atmosphere

The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.

/pdf/a-decade-of-the-super-dual-auroral-radar-network-superdarn-1v7hx0kq9o.pdf

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

Lead Authors: Marco Bindi (Italy), Sally Brown (UK), Ines Camilloni (Argentina), Arona Diedhiou (Ivory Coast/Senegal), Riyanti Djalante (Japan/Indonesia), Kristie L. Ebi (USA), Francois Engelbrecht (South Africa), Joel Guiot (France), Yasuaki Hijioka (Japan), Shagun Mehrotra (USA/India), Antony Payne (UK), Sonia I. Seneviratne (Switzerland), Adelle Thomas (Bahamas), Rachel Warren (UK), Guangsheng Zhou (China)

Impacts of 1.5°C Global Warming on Natural and Human Systems

Since the 1950s, research stations on the Antarctic Peninsula have recorded some of the largest increases in near-surface air temperature in the Southern Hemisphere. This warming has contributed to the regional retreat of glaciers, disintegration of floating ice shelves and a 'greening' through the expansion in range of various flora. Several interlinked processes have been suggested as contributing to the warming, including stratospheric ozone depletion, local sea-ice loss, an increase in westerly winds, and changes in the strength and location of low-high-latitude atmospheric teleconnections. Here we use a stacked temperature record to show an absence of regional warming since the late 1990s. The annual mean temperature has decreased at a statistically significant rate, with the most rapid cooling during the Austral summer. Temperatures have decreased as a consequence of a greater frequency of cold, east-to-southeasterly winds, resulting from more cyclonic conditions in the northern Weddell Sea associated with a strengthening mid-latitude jet. These circulation changes have also increased the advection of sea ice towards the east coast of the peninsula, amplifying their effects. Our findings cover only 1% of the Antarctic continent and emphasize that decadal temperature changes in this region are not primarily associated with the drivers of global temperature change but, rather, reflect the extreme natural internal variability of the regional atmospheric circulation.

/pdf/absence-of-21st-century-warming-on-antarctic-peninsula-2whbq1mx1w.pdf

Absence of 21st century warming on Antarctic Peninsula consistent with natural variability

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014EA000089

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

[1] Statistically measurable responses of atmospheric circulation to solar wind dynamic pressure are found in the Northern Hemisphere (NH) zonal-mean zonal wind and temperature, and on the Northern Annular Mode (NAM) in winter and spring. When December to January solar wind dynamic pressure (PswDJ) is high, the circulation response is marked by a stronger polar vortex and weaker sub-tropical jet in the upper to middle stratosphere. As the winter progresses, the Arctic becomes colder and the jet anomalies shift poleward and downward. In spring, the polar stratosphere becomes anomalously warmer. At solar maxima, significant positive correlations are found between PswDJ and the middle to late winter NAM all the way from the surface to 20 hPa, implying a strengthened polar vortex, reduced Brewer–Dobson circulation and enhanced stratosphere-troposphere coupling. The combined effect of high solar UV irradiance and high solar wind dynamic pressure in the NH middle to late winter is enhanced westerlies in the extratropics and weaker westerlies in the subtropics, indicating that more planetary waves are refracted toward the equator. At solar minima, there is no correlation in the NH winter but negative correlations between PswDJ and the NAM are found only in the stratosphere during spring. These results suggest possible multiple solar inputs that may cause refraction/redistribution of upward wave propagation and result in projecting the solar wind signals onto the NAM. The route by which the effects of solar wind forcing might propagate to the lower atmosphere is yet to be understood.

/pdf/possible-solar-wind-effect-on-the-northern-annular-mode-and-2z3dvknta3.pdf

Possible solar wind effect on the northern annular mode and northern hemispheric circulation during winter and spring

The digital ionospheric sounder, or dynasonde, located at the British Antarctic Survey's research station at Halley, Antarctica, has recently been operated as an imaging Doppler interferometer (IDI). The implementation of this mode of operation does not require any changes to the system hardware but involves a specially written sounding configuration and newly designed data analysis software. Using four independent antennas and two receivers, the signals returned from mesospheric altitudes are range-gated and processed using Doppler sorting and spatial interferometry. A three-dimensional (3-D) image of the scattering locations can then be built up for a region stretching to over 30 degrees from the zenith. The sky map locations and Doppler velocities of the individual scattering points are used to fit a 3-D velocity vector representing the motion of the neutral wind in the mesosphere. In this paper the radar hardware, sounding configuration, and data processing are described, and the relative merits of the IDI technique as applied at Halley are discussed. The characteristics of the scattering points are outlined, and initial neutral wind measurements are presented.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97RS01825

First mesospheric observations using an imaging Doppler interferometer adaptation of the dynasonde at Halley, Antarctica

Case studies of the equatorward movement of the main F region trough are presented, using data recorded by a digital ionospheric sounder located at Halley Bay (76°S, 27°W; Λ = 618, L = 42) Three events were chosen to give a range of magnetic conditions and local times Comparison of the results, projected into the equatorial plane, with theoretical predictions of the simple ‘teardrop’ model of the plasmapause, shows remarkably good agreement for the two occasions when the level of magnetic activity remained approximately constant over the observing period Consequently, estimates of the strength of the dawn-dusk convection electric field in the magnetosphere have been made However, magnetic activity changed during the course of the other event and the comparison gave poor results The main F region trough results are also compared with the predictions of contemporary empirical equations which express the invariant latitude of the trough minimum as a function of local time and magnetic index, Kp, based on global satellite surveys It is shown that none of these equations adequately accounts for the observed equatorward motion of the trough for these three cases In particular, they contain insufficient allowance for the observed strong dependence of equatorward velocity upon magnetic activity Some suggestions upon possible improvements to the empirical models are made which, if implemented, would be of great value for improving the reliability of predictions for high frequency, transauroral radio wave communications

Radio studies of the main F region trough in Antarctica

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at Rothera (67.5degreesS, 68.0degreesW), Antarctica in the austral summer of 2002-2003.The Rothera PMC are much weaker, less frequent, and not as high as the PMC observed at the South Pole. The mean PMC altitude is 83.74 +/- 0.25 km, which is approximately 1.3 km lower than the South Pole clouds. A comparison of numerous cloud observations indicates that southern hemisphere PMC are about 1 km higher than northern clouds at similar latitudes. Lidar measurements also show that the mesopause region temperatures at Rothera in late January are warmer than at the South Pole, while the Fe layer at Rothera has higher density and a lower peak altitude compared to the summertime Fe layer at the South Pole. These Fe density and temperature observations are qualitatively consistent with the PMC observations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL018638

Lidar observations of polar mesospheric clouds at Rothera, Antarctica (67.5°S, 68.0°W)

We present an analysis of a long-term archive of
horizontal wind data derived from meteor wind
observations from a SuperDARN radar at Halley,
Antarctica (76S, 27W). Systematic differences between
the 12-hour component in the meridional wind and the
climatological mean are observed showing evidence of a
quasi-biennial modulation of the high-latitude semidiurnal
tide in the upper mesosphere. The amplitude of the observed
tides is enhanced when the equatorial stratospheric quasibiennial
oscillation above 10 hPa is westerly. This
enhancement is greatest in the summertime tidal
amplitudes when the zonal wavenumber one (S = 1) nonmigrating
component dominates the semidiurnal wind field,
and is coincident with an enhancement of the summertime
planetary wave activity in the upper mesosphere. These
observations strongly support the hypothesis that the S = 1
component of the semidiurnal tide observed at high latitudes
is due to a non-linear interaction between the migrating S = 2
semidiurnal tide and quasi-stationary S = 1 planetary waves.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GL031282

Martin J. Jarvis

Papers

Possible solar wind effect on the northern annular mode and northern hemispheric circulation during winter and spring

First mesospheric observations using an imaging Doppler interferometer adaptation of the dynasonde at Halley, Antarctica

Radio studies of the main F region trough in Antarctica

Lidar observations of polar mesospheric clouds at Rothera, Antarctica (67.5°S, 68.0°W)

Quasi‐biennial modulation of the semidiurnal tide in the upper mesosphere above Halley, Antarctica