7.3.1.1                          Data sources and services provided

The station on Signy Island has an Inmarsat link to Cambridge and can use this to download observations from the GTS and forecast products from the UK Meteorological Office. However, as a forecaster is not based on the islands it is not normal for this information to be collected. At Orcadas Station there is a surface meteorological observing programme.

7.3.1.2                          Important weather phenomena and forecasting techniques used at the location

General overview

The climate in the South Orkney archipelago is strongly affected by the presence or absence of ice from the Weddell Sea. Sea ice moves in clockwise direction around the Weddell Sea and flows north past the South Orkneys. During a heavy ice year the ice edge can be well to the north of the islands during the winter and this tends to move the position of the circumpolar trough to the north of the Island and can make the winter be dominated by cold air masses moving up from the Weddell Sea. Normally the islands are dominated by weather systems moving eastwards in the circumpolar trough making the islands one of the cloudiest places in the world.

Surface Winds and the pressure field

The mean–monthly station–level and MSLP values for Orcadas are shown in Tables 7.3.1.4.1 and 7.3.1.4.2 (in Appendix 2). The stations in the South Orkneys are situated in sheltered areas and the wind observed at the stations is dominated by local orographical effects. Foehn winds are sometime experienced at Signy in northerlies flowing over Coronation Island.

The pressure field is dominated by the large number of active depressions that cross the area and is forecast using NWP model output.

Upper wind, temperature and humidity

No radiosonde ascents are available for the islands so upper fields are predicted using the NWP model output.

Clouds

The South Orkneys are in one of the cloudiest places in the world, although this does depend on the position of the sea ice edge. When the sea ice edge is to the north of the archipelago the cloud cover tends to be lower.

A feature of the islands is the orographic clouds that form over Coronation Island particularly in winter.

Prediction of clouds in carried out using satellite imagery.

Visibility

No specific information on forecasting has been obtained.

Surface contrast including white–out

Surface contrast is important for travel on the islands and is predicted based on an estimate of the expected cloud cover.

Horizontal definition

Not relevant on the islands as there are no flying activities.

Precipitation

The mean–monthly precipitation for Orcadas is shown in Table 7.3.1.4.3 (in Appendix 2). Precipitation is a frequent occurrence on the islands due to the many depressions that cross the area. It can be predicted using model rain/snowfall fields if available or from the model MSLP fields. In addition, satellite imagery can provide useful information on small–synoptic or mesoscale systems that may bring precipitation.

Temperatures and chill factor

The mean–monthly temperatures for Orcadas and Signy are shown in Tables 7.3.1.4.4 and 7.3.1.4.5, respectively (in Appendix 2). Temperatures are also greatly effected by the position of the sea ice edge. In summer, the average temperature is often above zero and the air temperature is often similar to those found on the west coast of the Antarctic Peninsula. However, during winter when the sea ice edge moves north from the Weddell Sea the temperatures drop, –40°C becoming quite common as the circumpolar trough moves north and the islands come under the influence of cold air from the Weddell Sea.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

These are not reported on the islands.

Sea ice

Sea ice is highly variable around the islands and both heavy and light ice years are experienced, which can cause problems for ships attempting to re–supply the stations.

Wind waves and swell

No specific information on forecasting has been obtained.