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Steven Colwell and John Turner

Abstract

An assessment is made of the availability of Antarctic synoptic observations on the World Meteorological Organization (WMO) Global Telecommunication System (GTS) during the trial periods (5–9 July 1993 and 1–15 February 1994) and winter and summer special observing periods (SOPs) (July 1994 and January 1995) of the Antarctic First Regional Observing Study of the Troposphere project. The data collected at two nodes of the GTS—Melbourne, Australia, and Bracknell, United Kingdom—are considered. Data received at Melbourne were passed on to the Australian Bureau of Meteorology in Hobart and those received at Bracknell were passed similarly on to Cambridge. The trial periods showed that there were large differences in the number of surface observations received at the two nodes. Although Hobart always received more upper-air data than Cambridge, the reverse was true with automatic weather station (AWS) data. The experience from the SOPs indicates that there are now almost 50% more AWS observations on the GTS than surface observations from the staffed stations.

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Gareth J. Marshall and John Turner

Abstract

Using data obtained during January 1995—the third of three special observing periods associated with the Antarctic First Regional Observing Study of the Troposphere project—over a sector of the Southern Ocean (SO), this study investigates the capabilities of European Remote Sensing satellite (ERS) scatterometer winds to portray accurately synoptic-scale weather systems and comments upon their potential contribution to the forecasting process in this region.

A sample population of cyclones was defined using satellite imagery and analyzed charts. The scatterometer successfully “captured” more than 60% of these systems that were existent over the open ocean. For manual analyses, the wind vectors proved extremely good for locating the positions of fronts, apparent as a marked turning in the wind direction, which coincided closely with frontal bands observed in contemporaneous satellite imagery. In most cases the wind vectors were also able to locate cyclone centers: their superior spatial resolution as compared with numerical analysis schemes revealed significant positional errors in the latter. This study demonstrates that typically each cyclone was captured twice by a scatterometer swath: such multitemporal data can provide information on the development of a system through changes in the strength of its associated winds.

Those 40% of systems that were not captured generally had a duration of less than a day and in that time were never encompassed by the scatterometer swath, a limiting factor in the instrument’s effectiveness, as noted by other studies. However, this study reveals that the most significant problem in high southern latitudes appears to be missing data resulting from the use of the operationally mutually exclusive synthetic aperture radar instrument over coastal Antarctica. Additional limitations of scatterometer data for observing synoptic-scale systems are shown to be the maximum and minimum restrictions on the range of wind speeds that can be successfully derived and the granularity problems that are still existent in some ERS data. Nonetheless, scatterometer data have the potential to provide extremely important information for the forecasting process over the data-sparse SO, with the near-surface winds able to give an accurate reflection of the degree of activity of a weather system.

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Gareth J. Marshall, Andrew Orr, and John Turner

Abstract

The scientific literature portrays a temporally invariant spatial relationship between the phase of the southern annular mode (SAM) and the sign of surface air temperature (SAT) anomalies across Antarctica. However, here the authors describe a predominant switch from a negative to positive SAM–temperature relationship (STR) across East Antarctica in austral summer/autumn during the first decade of the twenty-first century, when the SAM was generally weakly positive. Of the nine years that had a positive regional STR from 1957 to 2010, seven occurred during the last decade. This reversal appears to be a response to anomalous high pressure over East Antarctica, resulting from variability in the phase and amplitude of the local component of the zonal wavenumber 3 pressure pattern. In years when a reversed (positive) regional STR exists the anomalous circulation is such that there is greater energy flux into the region, while enhanced katabatic drainage across the continental interior disrupts the surface temperature inversion leading to warmer SATs inland, too. The average summer/autumn SAT increase across East Antarctica for years with reversed versus standard STR is ~1°C. Anthropogenically forced models fail to reproduce the trend toward the anomalous high pressure pattern so it is likely that the STR switch is due to natural internal climate variability. That such broadscale STR reversals can take place on decadal time scales needs to be considered when detecting and attributing recent Antarctic climate change and when utilizing isotope data from the East Antarctic ice core record to provide a proxy SAM index prior to the instrumental record.

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Jessica K. Turner and John R. Gyakum

Abstract

Surface observations, soundings, and a thermodynamic budget are used to investigate the formation process of 93 arctic airmass events. The events involve very cold surface temperatures—an average of −42.8°C at Norman Wells, a centrally located station in the formation region—and cooling in the 1000–500-hPa layer. A multistage process for their formation in northwestern Canada is proposed. This process is contrary to the classical conceptualization of extremely shallow, surface formations.

In the first stage of formation, snow falls into a layer of unsaturated air in the lee of the Rocky Mountains, causing sublimational cooling and moistening the subcloud layer. Simultaneously, the midtroposphere is cooled by cloud-top radiation emissions. In the second stage, snowfall abates, the air column dries, and clear-sky surface radiational cooling predominates, augmented by the high emissivity of fresh snow cover. The surface temperature falls very rapidly, up to a maximum of 18°C day−1 in one event. In the final stage, after near-surface temperatures fall below the frost point, ice crystals and, nearer the surface, ice fog form. At the end of formation, there is cold-air damming, with a cold pool and anticyclone in the lee of the Rockies, lower pressure in the Gulf of Alaska, and an intense baroclinic zone oriented northwest to southeast along the mountains.

There have been secular changes in the characteristics of the arctic air masses over the period 1948–2008. The surface temperature during the events has become warmer, and the air masses are deeper and moister. The 1000-hPa diabatic cooling during events, which includes latent heat and radiative processes, has decreased by 2.2°C day−1.

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Anna E. Jones, Tanya Bowden, and John Turner

Abstract

A regular occurrence during the 1990s has been the excursion of the edge of the springtime Antarctic ozone hole over the southernmost region of the South American continent. Given the essential role of atmospheric ozone in absorbing incoming solar ultraviolet radiation, the populations in this area are thus exposed to much higher ultraviolet-B irradiance than is normal for this time of year. The authors report here on a simple technique that might be used to forecast these low ozone episodes, based upon data readily available on the World Meteorological Organization Global Telecommunications System. Using this technique, total ozone during October 1991 at Punta Arenas, Chile, is predicted with a root-mean-square error of 34.4 DU (12.8%) and a mean error of 14.8 DU (5.5%).

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Hua Lu, Thomas J. Bracegirdle, Tony Phillips, and John Turner

Abstract

The Eliassen–Palm (E-P) flux divergences derived from ERA-40 and ERA-Interim show significant differences during northern winter. The discrepancies are marked by vertically alternating positive and negative anomalies at high latitudes and are manifested via a difference in the climatology. The magnitude of the discrepancies can be greater than the interannual variability in certain regions. These wave forcing discrepancies are only partially linked to differences in the residual circulation but they are evidently related to the static stability in the affected regions. Thus, the main cause of the discrepancies is most likely an imbalance of radiative heating.

Two significant sudden changes are detected in the differences between the eddy heat fluxes derived from the two reanalyses. One of the changes may be linked to the bias corrections applied to the infrared radiances from the NOAA-12 High-Resolution Infrared Radiation Sounder in ERA-40, which is known to be contaminated by volcanic aerosol from the 1991 eruption of Mt. Pinatubo. The other change may be due in part to the use of uncorrected radiances from the NOAA-15 Advanced Microwave Sounding Units by ERA-Interim since 1998. These sudden changes have the potential to alter the wave forcing trends in the affected reanalysis, suggesting that extreme care is needed when one comes to extract trends from the highly derived wave forcing quantities.

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John Turner, David H. Bromwich, and Gary M. Carter
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Jessica K. Turner, John Gyakum, and Shawn M. Milrad

Abstract

Northwestern Canada is a genesis region of arctic air masses often considered to be formed primarily through radiative processes. However, the details of their life cycle are poorly understood. This paper examines the formation, maintenance, and dissipation of an intense and long-lived arctic air mass, using a thermodynamic budget analysis.

The airmass formation is characterized by a deep-layer, multistage process that begins with snow falling into a nascent air mass. Radiative cooling from cloud tops begins the process. After the snow abates and clear skies are observed, the surface temperature drops rapidly, aided by the high emissivity of fresh snow cover, falling 17°C in two days, creating an intense but shallow temperature inversion. Once the surface temperature falls below the frost point, ice crystals form. Afterward, although the surface temperature remains constant, the height of the inversion rises, as radiative cooling at the top of the ice fog layer decreases temperatures.

During the maintenance phase, a cold-air damming structure is present with an anticyclone in the lee of the Canadian Rockies, low pressure in the Gulf of Alaska, and an intense baroclinic zone parallel to the mountains, separating warmer maritime air from colder continental air. The air mass persists for 12 days, undergoing several cycles of deep-layer weakening and intensification.

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Sheeba Nettukandy Chenoli, John Turner, and Azizan Abu Samah

Abstract

In situ observations, satellite imagery, numerical weather prediction, and reanalysis fields are used to investigate the synoptic and mesoscale environment of a strong wind event (SWE) at McMurdo Station/Ross Island region on the Ross Ice Shelf, Antarctica, on 10 October 2003. The SWE occurred during the passage of a sequence of three mesoscale low pressure systems from the central Ross Ice Shelf to the southwest Ross Sea. A potential vorticity (PV) analysis showed that the lows drew air of continental origin down the glacial valleys of the Transantarctic Mountains and onto the ice shelf as a katabatic drainage flow. However, the analysis indicated that the air mass associated with the SWE was of recurved maritime origin drawn in by the second mesoscale low (L2). This air mass approached McMurdo Station from the south where interactions with the orography played a critical role. In the early stages of the event, when the wind speed was less than 10 m s−1, the air was deflected around the topographical features, such as Minna Bluff and Black and White Islands. As the pressure gradient increased, winds of more than 10 m s−1 crossed the orography and developed mountain waves along the lee slopes. When the Froude number became larger than 1, large-amplitude vertically propagating mountain waves developed over the McMurdo Station/Ross Island area, increasing the wind to 16 m s−1. The reanalysis fields did not resolve the mesoscale lows; however, the Antarctic Mesoscale Prediction System (AMPS) model was able to simulate important characteristics of the SWE such as the mesoscale low pressure system, flow around the topographical barrier, and the mountain wave.

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Zhaomin Wang, John Turner, Yang Wu, and Chengyan Liu

Abstract

Between 2014 and 2016 the annual mean total extent of Antarctic sea ice decreased by a record, unprecedented amount of 1.6 × 106 km2, the largest in a record starting in the late 1970s. The mechanisms behind such a rapid decrease remain unknown. Using the outputs of a high-resolution, global ocean–sea ice model we show that the change was predominantly a result of record atmospheric low pressure systems over sectors of the Southern Ocean in 2016, with the associated winds inducing strong sea ice drift. Regions of large positive and negative sea ice extent anomaly were generated by both thermal and dynamic effects of the wind anomalies. Although the strong wind forcing also generated the warmest ocean surface state from April to December 2016, we show that enhanced northward sea ice drift and hence increased melting at lower latitudes driven by strong winds made the dominant contribution to the large decrease in total Antarctic sea ice extent between 2014 and 2016.

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