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Abstract
We present the first Antarctic-wide analysis of extreme near-surface air temperatures based on data collected up to the end of 2019 as part of the synoptic meteorological observing programs. We consider temperatures at 17 stations on the Antarctic continent and nearby sub-Antarctic islands. We examine the frequency distributions of temperatures and the highest and lowest individual temperatures observed. The variability and trends in the number of extreme temperatures were examined via the mean daily temperatures computed from the 0000, 0600, 1200, and 1800 UTC observations, with the thresholds for extreme warm and cold days taken as the 5th and 95th percentiles. The five stations examined from the Antarctic Peninsula region all experienced a statistically significant increase (p < 0.01) in the number of extreme high temperatures in the late-twentieth-century part of their records, although the number of extremes decreased in subsequent years. For the period after 1979 we investigate the synoptic background to the extreme events using ECMWF interim reanalysis (ERA-Interim) fields. The majority of record high temperatures were recorded after the passage of air masses over high orography, with the air being warmed by the foehn effect. At some stations in coastal East Antarctica the highest temperatures were recorded after air with a high potential temperature descended from the Antarctic plateau, resulting in an air mass 5°–7°C warmer than the maritime air. Record low temperatures at the Antarctic Peninsula stations were observed during winters with positive sea ice anomalies over the Bellingshausen and Weddell Seas.
Abstract
We present the first Antarctic-wide analysis of extreme near-surface air temperatures based on data collected up to the end of 2019 as part of the synoptic meteorological observing programs. We consider temperatures at 17 stations on the Antarctic continent and nearby sub-Antarctic islands. We examine the frequency distributions of temperatures and the highest and lowest individual temperatures observed. The variability and trends in the number of extreme temperatures were examined via the mean daily temperatures computed from the 0000, 0600, 1200, and 1800 UTC observations, with the thresholds for extreme warm and cold days taken as the 5th and 95th percentiles. The five stations examined from the Antarctic Peninsula region all experienced a statistically significant increase (p < 0.01) in the number of extreme high temperatures in the late-twentieth-century part of their records, although the number of extremes decreased in subsequent years. For the period after 1979 we investigate the synoptic background to the extreme events using ECMWF interim reanalysis (ERA-Interim) fields. The majority of record high temperatures were recorded after the passage of air masses over high orography, with the air being warmed by the foehn effect. At some stations in coastal East Antarctica the highest temperatures were recorded after air with a high potential temperature descended from the Antarctic plateau, resulting in an air mass 5°–7°C warmer than the maritime air. Record low temperatures at the Antarctic Peninsula stations were observed during winters with positive sea ice anomalies over the Bellingshausen and Weddell Seas.
Abstract
A detailed analysis of the evolution and structure of a mesoscale vortex and associated cloud comma that developed at the eastern edge of the Weddell Sea, Antarctica, during the early part of January 1986 is presented. The system remained quasi-stationary for over three days close to the British research station Halley (75°36′S, 26°42′W) and gave severe weather with gale-force winds and prolonged snow.
The formation and development of the system were investigated using conventional surface and upper-air meteorological observations taken at Halley, analyses from the U.K. Meteorological Office 15-level model, and satellite imagery and sounder data from the TIROS-N-NOAAseries of polar orbiting satellites. The thermal structure of the vortex was examined using atmospheric profiles derived from radiance measurements from the TIROS Operational Vertical Sounder. Details of the wind field were examined using cloud motion vectors derived from a sequence of Advanced Very High Resolution Radiometer images.
The vortex developed inland of the Brunt Ice Shelf in a strong baroclinic zone separating warm air, which had been advected polewards down the eastern Weddell Sea, and cold air descending from the Antarctic Plateau. The system intensified when cold, continental air associated with an upper-level short-wave trough was advected into the vortex. A frontal cloud band developed when slantwise ascent of warm air took place at the leading edge of the cold-air outbreak. Most of the precipitation associated with the low occurred on this cloud band.
The small sea surface-atmosphere temperature differences gave only limited heat fluxes and there was no indication of deep convection associated with the system. The vortex was driven by baroclinic forcing and had some features in common with the baroclinic type of polar lows that occur in the Northern Hemisphere.
Abstract
A detailed analysis of the evolution and structure of a mesoscale vortex and associated cloud comma that developed at the eastern edge of the Weddell Sea, Antarctica, during the early part of January 1986 is presented. The system remained quasi-stationary for over three days close to the British research station Halley (75°36′S, 26°42′W) and gave severe weather with gale-force winds and prolonged snow.
The formation and development of the system were investigated using conventional surface and upper-air meteorological observations taken at Halley, analyses from the U.K. Meteorological Office 15-level model, and satellite imagery and sounder data from the TIROS-N-NOAAseries of polar orbiting satellites. The thermal structure of the vortex was examined using atmospheric profiles derived from radiance measurements from the TIROS Operational Vertical Sounder. Details of the wind field were examined using cloud motion vectors derived from a sequence of Advanced Very High Resolution Radiometer images.
The vortex developed inland of the Brunt Ice Shelf in a strong baroclinic zone separating warm air, which had been advected polewards down the eastern Weddell Sea, and cold air descending from the Antarctic Plateau. The system intensified when cold, continental air associated with an upper-level short-wave trough was advected into the vortex. A frontal cloud band developed when slantwise ascent of warm air took place at the leading edge of the cold-air outbreak. Most of the precipitation associated with the low occurred on this cloud band.
The small sea surface-atmosphere temperature differences gave only limited heat fluxes and there was no indication of deep convection associated with the system. The vortex was driven by baroclinic forcing and had some features in common with the baroclinic type of polar lows that occur in the Northern Hemisphere.
Abstract
Increasingly, output from numerical weather prediction (NWP) models is being used for real-time weather forecasts for the Antarctic and for Antarctic-related climate diagnostics studies. Evidence is presented that indicates that in broad terms, the NWP output from the major global models is providing useful representations of synoptic-scale systems over high southern latitude areas. For example, root-mean-square (rms) errors in the European Centre for Medium-Range Weather Forecasts (ECMWF) model predictions of the 500-hPa height field indicate a day's gain in predictability since the mid-1990s: average rms errors in ECMWF +72 h 500-hPa height field prognoses for the calendar year 2000 were close to 50 m, compared to similar errors in the +48 h prognoses in 1995. Similar relative improvements may be noted for all time steps out to +144 h. Moreover, it is determined that, of the models considered here, the ECMWF model is clearly the most successful model at 500-hPa-height prediction for high southern latitudes, with the United Kingdom Met Office (UKMO) and National Centers for Environmental Prediction Aviation (AVN) models the next most accurate, and with the Australian Bureau of Meteorology's Global Assimilation Prediction (GASP) and Japanese Meteorological Agency (JMA) models lagging in accuracy. However, improvements in the temporal and spatial resolution of observational data that are available to the analysis and assimilation cycles of the NWP models, and improvements in the horizontal resolutions of the models, are required before the use of NWP output at high southern latitudes is as effective as in more northern areas of the world. Limited area modeling is seen as having potential for complementing the global models by resolving the finer-scale orography and topography of the Antarctic.
Abstract
Increasingly, output from numerical weather prediction (NWP) models is being used for real-time weather forecasts for the Antarctic and for Antarctic-related climate diagnostics studies. Evidence is presented that indicates that in broad terms, the NWP output from the major global models is providing useful representations of synoptic-scale systems over high southern latitude areas. For example, root-mean-square (rms) errors in the European Centre for Medium-Range Weather Forecasts (ECMWF) model predictions of the 500-hPa height field indicate a day's gain in predictability since the mid-1990s: average rms errors in ECMWF +72 h 500-hPa height field prognoses for the calendar year 2000 were close to 50 m, compared to similar errors in the +48 h prognoses in 1995. Similar relative improvements may be noted for all time steps out to +144 h. Moreover, it is determined that, of the models considered here, the ECMWF model is clearly the most successful model at 500-hPa-height prediction for high southern latitudes, with the United Kingdom Met Office (UKMO) and National Centers for Environmental Prediction Aviation (AVN) models the next most accurate, and with the Australian Bureau of Meteorology's Global Assimilation Prediction (GASP) and Japanese Meteorological Agency (JMA) models lagging in accuracy. However, improvements in the temporal and spatial resolution of observational data that are available to the analysis and assimilation cycles of the NWP models, and improvements in the horizontal resolutions of the models, are required before the use of NWP output at high southern latitudes is as effective as in more northern areas of the world. Limited area modeling is seen as having potential for complementing the global models by resolving the finer-scale orography and topography of the Antarctic.
Abstract
This paper examines the annual cycle and trends in Antarctic sea ice extent (SIE) for 18 models used in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that were run with historical forcing for the 1850s to 2005. Many of the models have an annual SIE cycle that differs markedly from that observed over the last 30 years. The majority of models have too small of an SIE at the minimum in February, while several of the models have less than two-thirds of the observed SIE at the September maximum. In contrast to the satellite data, which exhibit a slight increase in SIE, the mean SIE of the models over 1979–2005 shows a decrease in each month, with the greatest multimodel mean percentage monthly decline of 13.6% decade−1 in February and the greatest absolute loss of ice of −0.40 × 106 km2 decade−1 in September. The models have very large differences in SIE over 1860–2005. Most of the control runs have statistically significant trends in SIE over their full time span, and all of the models have a negative trend in SIE since the mid-nineteenth century. The negative SIE trends in most of the model runs over 1979–2005 are a continuation of an earlier decline, suggesting that the processes responsible for the observed increase over the last 30 years are not being simulated correctly.
Abstract
This paper examines the annual cycle and trends in Antarctic sea ice extent (SIE) for 18 models used in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that were run with historical forcing for the 1850s to 2005. Many of the models have an annual SIE cycle that differs markedly from that observed over the last 30 years. The majority of models have too small of an SIE at the minimum in February, while several of the models have less than two-thirds of the observed SIE at the September maximum. In contrast to the satellite data, which exhibit a slight increase in SIE, the mean SIE of the models over 1979–2005 shows a decrease in each month, with the greatest multimodel mean percentage monthly decline of 13.6% decade−1 in February and the greatest absolute loss of ice of −0.40 × 106 km2 decade−1 in September. The models have very large differences in SIE over 1860–2005. Most of the control runs have statistically significant trends in SIE over their full time span, and all of the models have a negative trend in SIE since the mid-nineteenth century. The negative SIE trends in most of the model runs over 1979–2005 are a continuation of an earlier decline, suggesting that the processes responsible for the observed increase over the last 30 years are not being simulated correctly.
Abstract
In contrast to earlier studies, the authors describe the climatological deep low pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen–Bellingshausen Seas low (ABSL), in terms of its relative (rather than actual) central pressure by removing the background area-averaged mean sea level pressure (MSLP). Doing so removes much of the influence of large-scale variability across the ABSL sector region (e.g., due to the southern annular mode), allowing a clearer understanding of ABSL variability and its effect on the regional climate of West Antarctica. Using ECMWF Interim Re-Analysis (ERA-Interim) fields, the annual cycle of the relative central pressure of the ABSL for the period from 1979 to 2011 shows a minimum (maximum) during winter (summer), differing considerably from the earlier studies based on actual central pressure, which suggests a semiannual oscillation. The annual cycle of the longitudinal position of the ABSL is insensitive to the background pressure, and shows it shifting westward from ∼250° to ∼220°E between summer and winter, in agreement with earlier studies. The authors demonstrate that ABSL variability, and in particular its longitudinal position, play an important role in controlling the surface climate of West Antarctica and the surrounding ocean by quantifying its influence on key meteorological parameters. Examination of the ABSL annual cycle in 17 CMIP5 climate models run with historical forcing shows that the majority of them have definite biases, especially in terms of longitudinal position, and a correspondingly poor representation of West Antarctic climate.
Abstract
In contrast to earlier studies, the authors describe the climatological deep low pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen–Bellingshausen Seas low (ABSL), in terms of its relative (rather than actual) central pressure by removing the background area-averaged mean sea level pressure (MSLP). Doing so removes much of the influence of large-scale variability across the ABSL sector region (e.g., due to the southern annular mode), allowing a clearer understanding of ABSL variability and its effect on the regional climate of West Antarctica. Using ECMWF Interim Re-Analysis (ERA-Interim) fields, the annual cycle of the relative central pressure of the ABSL for the period from 1979 to 2011 shows a minimum (maximum) during winter (summer), differing considerably from the earlier studies based on actual central pressure, which suggests a semiannual oscillation. The annual cycle of the longitudinal position of the ABSL is insensitive to the background pressure, and shows it shifting westward from ∼250° to ∼220°E between summer and winter, in agreement with earlier studies. The authors demonstrate that ABSL variability, and in particular its longitudinal position, play an important role in controlling the surface climate of West Antarctica and the surrounding ocean by quantifying its influence on key meteorological parameters. Examination of the ABSL annual cycle in 17 CMIP5 climate models run with historical forcing shows that the majority of them have definite biases, especially in terms of longitudinal position, and a correspondingly poor representation of West Antarctic climate.
Abstract
The effects of solar activity on the stratospheric waveguides and downward reflection of planetary waves during NH early to midwinter are examined. Under high solar (HS) conditions, enhanced westerly winds in the subtropical upper stratosphere and the associated changes in the zonal wind curvature led to an altered waveguide geometry across the winter period in the upper stratosphere. In particular, the condition for barotropic instability was more frequently met at 1 hPa near the polar-night jet centered at about 55°N. In early winter, the corresponding change in wave forcing was characterized by a vertical dipole pattern of the Eliassen–Palm (E–P) flux divergent anomalies in the high-latitude upper stratosphere accompanied by poleward E–P flux anomalies. These wave forcing anomalies corresponded with negative vertical shear of zonal mean winds and the formation of a vertical reflecting surface. Enhanced downward E–P flux anomalies appeared below the negative shear zone; they coincided with more frequent occurrence of negative daily heat fluxes and were associated with eastward acceleration and downward group velocity. These downward-reflected wave anomalies had a detectable effect on the vertical structure of planetary waves during November–January. The associated changes in tropospheric geopotential height contributed to a more positive phase of the North Atlantic Oscillation in January and February. These results suggest that downward reflection may act as a “top down” pathway by which the effects of solar ultraviolet (UV) radiation in the upper stratosphere can be transmitted to the troposphere.
Abstract
The effects of solar activity on the stratospheric waveguides and downward reflection of planetary waves during NH early to midwinter are examined. Under high solar (HS) conditions, enhanced westerly winds in the subtropical upper stratosphere and the associated changes in the zonal wind curvature led to an altered waveguide geometry across the winter period in the upper stratosphere. In particular, the condition for barotropic instability was more frequently met at 1 hPa near the polar-night jet centered at about 55°N. In early winter, the corresponding change in wave forcing was characterized by a vertical dipole pattern of the Eliassen–Palm (E–P) flux divergent anomalies in the high-latitude upper stratosphere accompanied by poleward E–P flux anomalies. These wave forcing anomalies corresponded with negative vertical shear of zonal mean winds and the formation of a vertical reflecting surface. Enhanced downward E–P flux anomalies appeared below the negative shear zone; they coincided with more frequent occurrence of negative daily heat fluxes and were associated with eastward acceleration and downward group velocity. These downward-reflected wave anomalies had a detectable effect on the vertical structure of planetary waves during November–January. The associated changes in tropospheric geopotential height contributed to a more positive phase of the North Atlantic Oscillation in January and February. These results suggest that downward reflection may act as a “top down” pathway by which the effects of solar ultraviolet (UV) radiation in the upper stratosphere can be transmitted to the troposphere.
The First International Symposium on Operational Weather Forecasting in Antarctica was held in Hobart, Australia, from 31 August to 3 September 1998. There were 40 attendees at the meeting from Australia, Belgium, Brazil, China, France, Italy, Russia, and the United Kingdom. In recent years there has been considerable growth in the requirement for weather forecasts for the Antarctic because of the increases in complex scientific research activities and the rapid growth of tourism to the continent. At many of the research stations there are now sophisticated forecasting operations that make use of the data available from drifting buoys and automatic weather stations, the output from numerical weather prediction systems, and high resolution satellite imagery. The models have considerable success at predicting the synoptic-scale depressions that occur over the ocean and in the coastal region. However, the many mesoscale systems that occur, which are very important for forecasting local conditions, are not well represented in the model fields and their movement is mainly predicted via the satellite data. In the future it is anticipated that high resolution, limited-area models will be run for selected parts of the continent. The symposium showed that great advances had been made during recent years in forecasting for the Antarctic as a result of our better understanding of atmospheric processes at high latitudes, along with the availability of high resolution satellite imagery and the output of numerical models. Outstanding problems include the difficulty of getting all of the observations to the main analysis centers outside the Antarctic in a timely fashion, the lack of upper air data from the Antarctic Peninsula and the interior of the continent, and the poor representation of the Antarctic orography and high latitude processes in numerical models. An outcome of the symposium will be a weather forecasting handbook dealing with the entire continent.
The First International Symposium on Operational Weather Forecasting in Antarctica was held in Hobart, Australia, from 31 August to 3 September 1998. There were 40 attendees at the meeting from Australia, Belgium, Brazil, China, France, Italy, Russia, and the United Kingdom. In recent years there has been considerable growth in the requirement for weather forecasts for the Antarctic because of the increases in complex scientific research activities and the rapid growth of tourism to the continent. At many of the research stations there are now sophisticated forecasting operations that make use of the data available from drifting buoys and automatic weather stations, the output from numerical weather prediction systems, and high resolution satellite imagery. The models have considerable success at predicting the synoptic-scale depressions that occur over the ocean and in the coastal region. However, the many mesoscale systems that occur, which are very important for forecasting local conditions, are not well represented in the model fields and their movement is mainly predicted via the satellite data. In the future it is anticipated that high resolution, limited-area models will be run for selected parts of the continent. The symposium showed that great advances had been made during recent years in forecasting for the Antarctic as a result of our better understanding of atmospheric processes at high latitudes, along with the availability of high resolution satellite imagery and the output of numerical models. Outstanding problems include the difficulty of getting all of the observations to the main analysis centers outside the Antarctic in a timely fashion, the lack of upper air data from the Antarctic Peninsula and the interior of the continent, and the poor representation of the Antarctic orography and high latitude processes in numerical models. An outcome of the symposium will be a weather forecasting handbook dealing with the entire continent.
Abstract
Convective available potential energy (CAPE) is an important index for storm forecasting. Recent versions (v15.2 and v16) of the Global Forecast System (GFS) predict lower values of CAPE during summertime in the continental U.S. than analysis and observation. We conducted an evaluation of the GFS in simulating summertime CAPE using an example from the Unified Forecast System Case Study collection to investigate the factors that lead to the low CAPE bias in GFS. Specifically, we investigated the surface energy budget, soil properties, and near-surface and upper-level meteorological fields. Results show that the GFS simulates smaller surface latent heat flux and larger surface sensible heat flux than the observations. This can be attributed to the slightly drier-than-observed soil moisture in the GFS which comes from an offline global land data assimilation system. The lower simulated CAPE in GFS v16 is related to the early drop of surface net radiation with excessive boundary layer cloud after midday compared with GFS v15.2. A moisture-budget analysis indicates that errors in the large-scale advection of water vapor does not contribute to the dry bias in the GFS at low levels. Common Community Physics Package single-column model (SCM) experiments suggest that with realistic initial vertical profiles, SCM simulations generate a larger CAPE than runs with GFS IC. SCM runs with an active LSM tend to produce smaller CAPE than that with prescribed surface fluxes. Note that the findings are only applicable to this case study. Including more warm-season cases would enhance the generalizability of our findings.
Abstract
Convective available potential energy (CAPE) is an important index for storm forecasting. Recent versions (v15.2 and v16) of the Global Forecast System (GFS) predict lower values of CAPE during summertime in the continental U.S. than analysis and observation. We conducted an evaluation of the GFS in simulating summertime CAPE using an example from the Unified Forecast System Case Study collection to investigate the factors that lead to the low CAPE bias in GFS. Specifically, we investigated the surface energy budget, soil properties, and near-surface and upper-level meteorological fields. Results show that the GFS simulates smaller surface latent heat flux and larger surface sensible heat flux than the observations. This can be attributed to the slightly drier-than-observed soil moisture in the GFS which comes from an offline global land data assimilation system. The lower simulated CAPE in GFS v16 is related to the early drop of surface net radiation with excessive boundary layer cloud after midday compared with GFS v15.2. A moisture-budget analysis indicates that errors in the large-scale advection of water vapor does not contribute to the dry bias in the GFS at low levels. Common Community Physics Package single-column model (SCM) experiments suggest that with realistic initial vertical profiles, SCM simulations generate a larger CAPE than runs with GFS IC. SCM runs with an active LSM tend to produce smaller CAPE than that with prescribed surface fluxes. Note that the findings are only applicable to this case study. Including more warm-season cases would enhance the generalizability of our findings.
Abstract
Commencing in 1956, observations made at Halley Research Station in Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 to 50 km in distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical changepoint algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.
Abstract
Commencing in 1956, observations made at Halley Research Station in Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 to 50 km in distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical changepoint algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.
