Steve Colwell

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.

Variability of precipitation over the coastal western Antarctic Peninsula from synoptic observations

Observations of precipitation events at Faraday and Rothera Stations are analyzed to investigate the spatial and temporal variability of precipitation along the western coastal (Pacific) side of the Antarctic Peninsula. The record of observations made at Faraday since 1956 show a statistically significant increase in the number of winter-season precipitation events. During this season, there are now, on the average, almost 50% more reports of precipitation than during the 1950s. On a year-to-year basis the number of precipitation events is not correlated with the mean surface temperature on the western side of the Antarctic Peninsula but is dependent on synoptic weather system activity. The annual total number of precipitation events at Rothera is also increasing, but because the length of the record is relatively short, this is not statistically significant. The semiannual cycle in the latitudinal location and depth and position of the circumpolar trough is reproduced in the record of precipitation events at both Faraday and Rothera. It is argued that the systematic increase in the number of precipitation events at Faraday since the 1950s is associated with changes in the depression tracks across the Bellingshausen Sea, with an increase in the number of depressions approaching from outside the Antarctic rather than from the west.

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.

A role for newly forming sea ice in springtime polar tropospheric ozone loss? Observational evidence from Halley station, Antarctica

Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology, as well as standard meteorological parameters, are also measured at the CASLab. Combining these data allows us to probe the transport pathway of air masses during ozone depletion events (ODEs). ODEs were observed at Halley on several occasions during Antarctic spring 2003. On some occasions, extremely rapid loss of ozone was observed (loss of 16 ppbv in 1 min on one occasion), which was associated with regional-scale transport. For each such event during 2003, the air mass originated in the southern Weddell Sea, an area of vigorous sea-ice production. On other occasions the development of the event and its recovery were strongly associated with the build-up and decline of a stable boundary layer. In these cases, air masses had had recent contact with a nearby open water lead where sea-ice production is known to occur. The data presented here are entirely consistent with the idea that halogens responsible for ozone loss are derived during new sea-ice formation from an associated surface such as brine slush or frost flowers.

Has the Antarctic Vortex Split before 2002

In late September 2002, the Antarctic ozone hole was seen to split into two parts, resulting in large increases in ozone at some stations and the potential for significant modification of chlorofluorocarbon (CFC)-induced ozone loss. The phenomenon was dynamical (a split vortex), causing large increases in stratospheric temperature above stations normally within the vortex. Temperatures at Halley, Antarctica, at 30 hPa increased by over 60 K, and temperatures at South Pole at 100 hPa increased by over 25 K. It is important to know if this has happened before, since if it happens in the future, it would significantly alter the total hemispheric ozone loss due to chlorine from CFCs, particularly if it happens in August or September. Temperatures in winter and spring measured at Halley or the South Pole since 1957 and 1961, respectively, show no other comparable increases until the final warming in late spring, except for two dates in the 1980s at Halley when meteorological analyses show no vortex split. There are very few periods of measurements missing at both Halley and the South Pole, and analyses in those few periods show no vortex split. Measurements in August and September at sites normally near the edge of the vortex show very few suspicious dates, and analyses of those few suspicious dates again show no vortex split. It is concluded that the vortex has probably not split before the final warming since Antarctic records began in the late 1950s, and almost certainly not in August or September.

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.

Anomalous atmospheric circulation over the Weddell Sea, Antarctica during the Austral summer of 2001/02 resulting in extreme sea ice conditions

During the Austral summer of 2001/02 exceptionally heavy sea ice conditions were experienced over the eastern Weddell Sea. Satellite microwave imagery showed that large negative (positive) ice anomalies were present from October 2001 to January 2002 over the north-western Weddell Sea (off the coast of Dronning Maud Land). These were a result of anomalously high (low) atmospheric pressure over the South Atlantic (southern Weddell Sea and Bellingshausen Sea), which gave strong north to north-westerly cyclonic flow over the northern and eastern Weddell Sea. This resulted in convergence of sea ice into the southern Weddell Sea and inhibited ice advection along the coast of Dronning Maud land. The atmospheric anomalies around the Weddell Sea were part of an Antarctic-wide amplification of the mean wavenumber 3 pattern resulting in more intrusions of mid-latitude air into the interior of the continent, giving rise to near-record warm temperatures at several locations.

Seasonal variations of gravity wave activity in the lower stratosphere over an Antarctic Peninsula station

An 8 year series of 965 high-resolution radiosonde soundings over Rothera (67 degrees S, 68 degrees W) on the Antarctic Peninsula are used to study gravity wave characteristics in the lower stratosphere. The gravity wave energy is shown to have a seasonal variation with peaks at the equinoxes; the largest peak is around the spring equinox. During the winter months and extending into the spring, there is both an enhancement in the downward propagating wave activity and a reduction in the amount of critical-level filtering of upward propagating mountain waves. The horizontal propagation directions of the gravity waves were determined using hodographs. It was found that there is a predisposition toward northward and westward propagating waves above Rothera. This is in agreement with previous observations of gravity wave momentum flux in the wintertime mesosphere over Rothera. These results are consistent with a scenario whereby the stratospheric gravity wavefield above Rothera is determined by a combination of wind flow over topography-generating waves from below, and sources such as the edge of the polar stratospheric vortex-generating waves from above, especially during winter and spring.

An assessment of the Polar Weather Research and Forecasting (WRF) model representation of near‐surface meteorological variables over West Antarctica

Despite the recent significant climatic changes observed over West Antarctica, which include large warming in central West Antarctica and accelerated ice loss, adequate validation of regional simulations of meteorological variables are rare for this region. To address this gap, results from a recent version of the Polar Weather Research and Forecasting model (Polar WRF) covering West Antarctica at a high horizontal resolution of 5 km were validated against near-surface meteorological observations. The model employed physics options that included the Mellor-Yamada-Nakanishi-Niino (MYNN) boundary layer scheme, the WRF Single Moment 5-Class cloud microphysics scheme, the new version of the Rapid Radiative Transfer Model for both shortwave and longwave radiation, and the Noah land surface model. Our evaluation finds this model to be a useful tool for realistically capturing the near-surface meteorological conditions. It showed high skill in simulating surface pressure (correlation ≥0.97), good skill for wind speed with better correlation at inland sites (0.7-0.8) compared to coastal sites (0.3-0.6), generally good representation of strong wind events, and good skill for temperature in winter (correlation ≥0.8). The main shortcomings of this configuration of Polar WRF are an occasional failure to properly represent transient cyclones and their influence on coastal winds, an amplified diurnal temperature cycle in summer, and a general tendency to underestimate the wind speed at inland sites in summer. Additional sensitivity studies were performed to quantify the impact of the choice of boundary layer scheme and surface boundary conditions. It is shown that the model is most sensitive to the choice of boundary layer scheme, with the representation of the temperature diurnal cycle in summer significantly improved by selecting the Mellor-Yamada-Janjic boundary layer scheme. By contrast, the model results showed little sensitivity to whether the horizontal resolution was 5 or 15 km.