Tom Lachlan-Cope

Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent

Based on a new analysis of passive microwave satellite data, we demonstrate that the annual mean extent of Antarctic sea ice has increased at a statistically significant rate of 0.97% dec−1 since the late 1970s. The largest increase has been in autumn when there has been a dipole of significant positive and negative trends in the Ross and Amundsen‐Bellingshausen Seas respectively. The autumn increase in the Ross Sea sector is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea. Model experiments suggest that the trend towards stronger cyclonic circulation is mainly a result of stratospheric ozone depletion, which has strengthened autumn wind speeds around the continent, deepening the Amundsen Sea Low through flow separation around the high coastal orography. However, statistics derived from a climate model control run suggest that the observed sea ice increase might still be within the range of natural climate variability.

The SCAR READER Project: Toward a High-Quality Database of Mean Antarctic Meteorological Observations

A new dataset of monthly and annual mean near-surface climate data (temperature, surface and mean sea level pressure, and wind speed) for the Antarctic region has been created using historical observations [Scientific Committee on Antarctic Research (SCAR) Reference Antarctic Data for Environmental Research (READER)]. Where possible, 6-hourly surface synoptic and automatic weather station observations were used to compute the means. The ability to quality control the data at the level of individual observations has produced a more accurate series of monthly means than was available previously. At the time of writing, the mean data are available on the Internet (http://www.antarctica.ac.uk/met/programs-hosted.html). Data for 43 surface-staffed stations and 61 automatic weather stations are included in the database. Here, mean temperature, pressure, and wind speed data for 19 occupied stations with long records are provided.

Significant warming of the Antarctic winter troposphere

We report an undocumented major warming of the Antarctic winter troposphere that is larger than any previously identified regional tropospheric warming on Earth. This result has come to light through an analysis of recently digitized and rigorously quality controlled Antarctic radiosonde observations. The data show that regional midtropospheric temperatures have increased at a statistically significant rate of 0.5° to 0.7°Celsius per decade over the past 30 years. Analysis of the time series of radiosonde temperatures indicates that the data are temporally homogeneous. The available data do not allow us to unambiguously assign a cause to the tropospheric warming at this stage.

Recent temperature trends in the Antarctic

It is important to understand how temperatures across the Antarctic have changed in recent decades because of the huge amount of fresh water locked into the ice sheet and the impact that temperature changes may have on the ice volume. Doran et al. claim that there has been a net cooling of the entire continent between 1966 and 2000, particularly during summer and autumn. We argue that this result has arisen because of an inappropriate extrapolation of station data across large, data-sparse areas of the Antarctic.

The role of the non-axisymmetric antarctic orography in forcing the observed pattern of variability of the Antarctic climate

The pattern of inter-annual variability in the atmospheric circulation around Antarctica has a maximum over the Amundsen-Bellinghausen Sea (ABS), which is particularly strong during the winter (June, July and August). By using an atmosphere-only general circulation model the causes of this maximum have been investigated. In particular we study the effect of the non-axisymmetric nature of the local surface forcing (sea surface temperatures, sea ice and orography) by imposing axi-symmetric forcing fields at high southern latitudes. The results of these experiments show that the non-axisymmetric nature of the Antarctic orography is sufficient to explain the variability maximum in the ABS.

A positive trend in western Antarctic Peninsula precipitation over the last 50 years reflecting regional and Antarctic-wide atmospheric circulation changes

Abstract In situ observations of precipitation days (days when snow or rain was reported in routine synoptic observations) from Faraday/Vernadsky station on the western side of the Antarctic Peninsula, and fields from the 40 year European Centre for Medium-Range Weather Forecasts re-analysis (ERA-40) project are used to investigate precipitation and atmospheric circulation changes around the Antarctic Peninsula. It is shown that the number of precipitation days is a good proxy for mean sea-level pressure (MSLP) over the Amundsen–Bellingshausen Sea. The annual total of precipitation days at the station has been increasing at a statistically significant rate of +12.4 days decade–1 since the early 1950s, with the greatest increase taking place during the summer and autumn. This is the time of year when the Southern Annular Mode (SAM) has experienced its greatest shift to a positive phase, with MSLP values decreasing in the Antarctic coastal zone. The lower pressures in the circumpolar trough have resulted in greater ascent and increased precipitation at Faraday/Vernadsky.

Analysis of synoptic‐scale low pressure systems within the Antarctic Peninsula sector of the circumpolar trough

Satellite imagery of the Antarctic Peninsula, Bellingshausen Sea, and Weddell Sea region, together with operational meteorological analyses, are used to investigate the development, tracks, and structure of synoptic-scale weather systems within the Antarctic Peninsula sector of the circumpolar trough, which rings the Southern Hemisphere between the latitudes of 60° and 70°S. A number of previous studies have shown that more cyclogenesis events take place within this zone than in any other region of the Southern Hemisphere, although the mechanisms behind these developments have not been investigated previously. This study confirms the large number of cyclogenesis events taking place within the circumpolar trough and, in the year examined, the mean latitude of cyclogenesis within the sector was 64°S. During the year, 504 lows were observed, with 281 developing in the area and 223 moving in. The diameters of the systems observed ranged from 300 to 3500 km, with the smallest vortices being mesocyclones that later grew into synoptic-scale lows. The mean distance travelled by the depressions was only 1377 km (standard deviation 1205 km), reflecting the fact that the systems observed ranged from small quasi-stationary lows to large, very mobile depressions. The mean meridional movement of the lows during their existence was very small. The greatest number of cyclogenesis events were observed over the Bellingshausen Sea, where more than 0.48 events 10 000 km−2 year−1 were found, with a secondary maximum in the lee of the Antarctic Peninsula. The mean lifetime of the systems observed was 22 h, with over half the vortices existing for less than 24 h. A maximum of cyclolysis was found just to the west of the Antarctic Peninsula, with a secondary maximum in the South Atlantic. Of the systems that formed within the area, 36% were developments within pre-existing large areas of low pressure, which were similar to the ‘merry-go-round’ formations of mesocyclones. A further 39% of developments consisted of waves on major frontal bands. The remaining developments comprised lee lows and some cyclogenesis events in synoptically isolated regions.

THE SYNOPTIC ORIGINS OF PRECIPITATION OVER THE ANTARCTIC PENINSULA

The synoptic origins of precipitation on the western side of the Antarctic Peninsula over the one year period March 1992 to February 1993 are investigated using meteorological observations, satellite imagery and analyses produced by the UK Meteorological Office. Precipitation at Rothera Station was found to occur at 30% of the synoptic reporting time with 80% of precipitation reports being associated with cyclonic disturbances. Although three quarters of all precipitation reports were for snow, the proximity of Rothera to the zone of maximum cyclonic activity meant that incursions of mild air produced rain in all seasons. During the year 95% of all precipitation was classed as slight. Variability of precipitation on the intraseasonal timescale was highly dependent on the synoptic-scale circulation. The most common synoptic situation for precipitation was a frontal cyclone over the Bellingshausen Sea which accounted for 38% of all precipitation events and 62% of the moderate and heavy precipitation reports. Of the extra-tropical cyclones that gave precipitation 49% were found to have developed south of 60°S. None of the precipitation at Rothera was attributable to mesocyclones. Snow stake measurements from Rothera were a poor indicator of precipitation as a result of blowing snow.

Atmospheric water vapor over Antarctica derived from Special Sensor Microwave/Temperature 2 data

In polar regions, satellite microwave radiometry has not been successful in measuring the total water vapor (TWV) in the atmosphere. The difficulties faced in these regions arise from the very low water vapor burden of the atmosphere and the large and highly variable emissivities of ice surfaces in the microwave frequency range. By exploiting the advantages of the Special Sensor Microwave/Temperature 2 (SSM/T2), a method is developed to retrieve TWV over Antarctica from satellite data. This method shows very low sensitivities to the change of surface emissivity and to the presence of water clouds. However, ice clouds may have considerable effects. Results of radiative transfer model simulation show that they may cause one to underestimate TWV using the proposed method and that the amount of underestimation is proportional to the ice water path of the ice cloud. Validations using radiosonde measurements and numerical model analyzes suggest that SSM/T2 retrievals have a high accuracy (maximum error <10%) as long as TWV is <4.0 kg m−2. Above this value, retrievals show a systematic overestimation. Presumably, this is a result of the seasonal difference between the validation and the training radiosonde data sets. TWV retrievals of 1 years SSM/T2 data show clearly the seasonal variation of water vapor over Antarctica. Throughout the year the mean TWV over West Antarctica is nearly twice as high as that over East Antarctica; the temporal fluctuation of TWV over West Antarctica is also significantly stronger than over East Antarctica. This suggests that precipitation and water vapor transport in West Antarctica are more active than in East Antarctica. Using the same years TWV data, we estimated the mean residence time of atmospheric water vapor over the Antarctica to be merely 3–4 days. This, however, is much shorter than the global mean of 9–10 days.

Biodiversity of air-borne microorganisms at Halley station Antarctica

A study of air-borne microbial biodiversity over an isolated scientific research station on an ice-shelf in continental Antarctica was undertaken to establish the potential source of microbial colonists. The study aimed to assess: (1) whether microorganisms were likely to have a local (research station) or distant (marine or terrestrial) origin, (2) the effect of changes in sea ice extent on microbial biodiversity and (3) the potential human impact on the environment. Air samples were taken above Halley Research Station during the austral summer and austral winter over a 2-week period. Overall, a low microbial biodiversity was detected, which included many sequence replicates. No significant patterns were detected in the aerial biodiversity between the austral summer and the austral winter. In common with other environmental studies, particularly in the polar regions, many of the sequences obtained were from as yet uncultivated organisms. Very few marine sequences were detected irrespective of the distance to open water, and around one-third of sequences detected were similar to those identified in human studies, though both of these might reflect prevailing wind conditions. The detected aerial microorganisms were markedly different from those obtained in earlier studies over the Antarctic Peninsula in the maritime Antarctic.