Variability of coastal Antarctic climate derived from fast-ice observations.
Measurements of land-fast sea-ice have been carried out from Mawson and Davis Stations intermittently since the mid to late 1950s. The routine measurements (see measurements section) were formalized through an Australian Antarctic Science project entitled ‘Variability of the coastal Antarctic climate derived from fast-ice observations’. That project has directed the intermittent sampling and processing of fast-ice data at Australian Antarctic Stations since the 1950s and at this stage extends to 2021/22.
Fast ice measurements are tasked to the sea ice research group that is part of the Climate Variability and Change group within the Antarctic Climate and Ecosystems Cooperative Research Centre, and the Climate Processes and Change program of the Australian Antarctic Division.
By definition, fast ice is a continuous sheet of sea ice that is fixed to the land and does not move. During austral winter, land-fast ice surrounds most of the Antarctic continent. Vertical growth of fast-ice is determined largely by thermodynamic processes.
Long-term observations of fast-ice provides valuable indicators of change in the Antarctic ocean-ice-atmosphere system. The fast-ice measurements from Mawson and Davis are included in the Coasts and Oceans theme of the State of the Environment [SoE] report.
Fast-ice data can be analysed for indicators of change, and then, for example, related to atmospheric events. For this reason, we are not only interested in the snow and ice thickness, but also in the dates of ice formation, and ice break-out, including ice removal due to autumn storms or blizzards. Shifts in the date of ice formation, maximum ice thickness, etc., may be indicators of larger-scale changes.
ANARE data of fast-ice observations have been used in a number of published scientific studies such as Mellor , Allison , Heil et al. , Heil and Allison , Heil , and Lei et al. .
This State of the Environment record is relatively easy to obtain with the assistance of volunteers from the ANARE expeditioners that over-winter at Davis and Mawson.
The observations of fast-ice primarily are weekly measurements of ice thickness at a set of fixed locations. Data on the level of seawater in the drill-hole (freeboard), and the thickness of snow on top of the sea ice, are also recorded simultaneously at each sampling location.
In addition, information regarding the dates of ice-formation and ice-breakout, as well as observations of fast-ice extent, are compiled to build up complete records of the changes in fast ice conditions both seasonally and interannually.
It is important that fast-ice observations at Davis and Mawson are carried out at the same locations every year. This enables analysis of changes over time, and thereby adds value to the long-term data set. At Casey the fast ice near the station is not stable, and only a simpler set of observations are kept.
When the fast ice consolidates, the first task is to reestablish sampling locations using a GPS. Each site is marked with (bamboo) cane.
Regular measurements are taken from each sampling location, but not redrilled in previously used holes. A full season of measurements is usually within an area of about 2 m2 . Working cleanly in that area is important, taking care to minimize disturbance to the snow.
Typically, the ice cover forms in early to mid-March at Davis,
and in late March to early April at Mawson. These fast ice observations can be made only
when the ice is safe for travel (Table). However, during the onset of ice formation,
daily growth rates are largest, so it is important to obtain early and regular measurements
of the growth of young ice.
|Mode of transport||Minimum bearing capacity||ANARE approved|
|Person on skis||10 cm||20 cm|
|Person on foot||15 cm||20 cm|
|Single person on quad||20 cm||40 cm|
|Single person on skidoo||20 cm||40 cm|
In the past, over-ice travel to the sampling locations has been approved only once
the ice was stable and above 10 cm thick. Now we request early in-situ observations of ice thickness
at the sites within station limits (i.e. sites A and B at Mawson; Ice1, Ice2 and Ice7 at Davis).
Note that in March and April the ice thickness will reach 10 cm within 1-2 days of consolidation,
and it will reach 20 cm within about 5 days.
Once the ice is safe for travel, weekly measurements are made at each of the standard sites.
If bad weather prevents measurements on a specied date, then the measurements are taken one or two days
later, and the weekly measurements resumed without adjusting for the delay.
1.Snow thickness measurements
The measurement is read from a stainless steel ruler inserted into an undisturbed snow surface,
and pushed down hard 2 or 3 times to make sure that the ruler reaches the surface of the sea ice.
With increasing snow depth icy crusts may form more commonly within the snow. It might become difficult
to read the snow thickness from undisturbed snow. In those circumstances, a snow pit is dug,
making sure that all removed snow is piled in a small area, preferably one that has been disturbed
by a previous measurement. The measurement is read from the stainless steel ruler at the ice surface
against the top edge of the undisturbed snow. Readings of snow thickness are to the nearest 0.1 cm (1 mm).
2. Ice thickness measurement
First, the snow is removed from a small area using the shovel. Then the ice is drilled
with a narrow drill head (5 cm diameter stainless steel auger). The measurement tape with a brass rod
and recovery line are lowered down through the hole. Once the brass rod is below the lower ice edge,
the tape is pulled up against the bottom layer of the ice gently so as not destroy any fragile crystals
at the ocean-ice interface. The reading is taken from the tape at the upper ice surface to an accuracy
of 0.1 cm (1 mm) and noted in the field log.
3. Freeboard measurement
Before recovering the measurement tape, the freeboard of the fast ice is also measured.
To do this, the measuring tape is kept gently pulled upwards against the bottom of the ice,
and the depth to which the water reaches within the hole is read from the tape. If the water level
is below the surface of the ice, then the freeboard measurement is less than that for the ice thickness.
If the hole is flooded, the freeboard measurement will be larger of the two.
The freeboard measurement is noted in the field log to the nearest 0.1 cm (1 mm).
1. Sea ice logbook
All measurements are carefully recorded in situ directly after the measurement.
For this a field note book is used, but not the full Sea Ice Log. The primary record
is a hand-written Sea Ice Log maintained in a standard ANARE log book (with carbon-copy).
During each year, this log and the electronic log are maintained by a single nominated sea-ice observer
at each station. Field observations (made by the sea-ice observer or an assistant)
are recorded in a field note book and transcribed into the Sea ice log.
2. Electronic web log for fast ice data
A web-based electronic log is operational for Davis, Mawson and Casey stations.
In the current format a web-log entry is created for each measurement day.
It covers all sites of the respective station. The web-logs are entered and submitted
by the main ice observer within a couple of days of the physical ice measurement. In addition,
at the end of each month an electronic copy of the station’s monthly weather summary
issued by the station’s meteorological staff is emailed to the sea ice research group.