7.3.4                                Deception Island, South Shetland Islands  

7.3.4.1                          Orography and the local environment

Deception Island is located at the south–western end of the South Shetland group and is about 110 km (~60 nm) from the Antarctic Peninsula (see Figure 7.3.1 and Figure 7.2.1.1.1). The island is of volcanic origin being made up of a submerged volcanic crater. The rim of the crater has been breached and a channel 400 m wide allows ships to enter the crater, which acts as a large natural harbour called Port Foster (see Figure 7.3.4.1.1 and Figure 7.3.4.1.2). A good anchorage is available just inside the entrance to the harbour in Whaler’s Bay adjacent to the station Decepción (62º 59´ S, 60º 34´ W, 7 m AMSL). The hills surrounding the harbour rise up 570 m (~1,870 ft) in the east and are lowest towards the northwest. The volcano is still active, the last major eruption being in 1966 when British and Argentine research stations had to be evacuated.

7.3.4.2                          Operational requirements and activities relevant to the forecasting process

Scientific activities on Deception Island at present are limited to summer only work operating out of small huts. In the past, aircraft, both wheeled and float planes, have operated from Deception. The aircraft operated from a strip near the shore of Whalers Bay. Winged aircraft have landed on the strip in recent years but there has not been any operational use of Deception as a base for air operations since the 1966 eruption.

Deception Island is a favourite site for visiting tourist ships and every year many tourist parties are put ashore. The island offers a very good harbour and within Port Forster calm conditions for landing parties can be almost guaranteed.

7.3.4.3                          Data sources and services provided

No meteorological observations are available from Deception on a routine basis.

7.3.4.4                          Important weather phenomena and forecasting techniques used at the location

General overview

In the summer most of the island is bare loose packed volcanic ash while in the winter the ground is mostly snow–covered. The climatological conditions are similar to those found on King George Island. The island is subject to the weather systems that develop with the circumpolar trough.

Surface wind and the pressure field

The direction of the surface wind on Deception Island is predominately either westerly or northeasterly. The local hills around Port Foster will cause local variations in the wind direction and strength. As with other the South Shetland Islands the area is affected by the semi–annual oscillation and surface pressures are lowest (highest) in the spring/autumn (winter and summer) (see Table 7.3.4.4.1 (in Appendix 2)).

Upper wind, temperature and humidity

No radiosonde data are available for the region but upper–air conditions can be predicted using numerical weather prediction model output. These fields tend to be reasonably accurate, although errors can occur in the strength and position of the jets.

Clouds

Low stratiform clouds with heights around 300–600 m (~1,000–2,000 ft) are quite common at this location and are normally associated with synoptic scale weather systems approaching from the west. Orographic cloud is commonly observed.

Visibility: precipitation and fog

Fog is quite common and the visibility is less than 1,000 m (~3,300 ft) around 10% of the time. This low visibility is normally associated with frontal systems approaching from the north. Steam is sometimes formed by volcanic activity but rarely significantly affects the visibility.

Surface contrast including white–out

Surface contrast is usually good during the summer when the ground is clear of snow. During the winter there may be periods with poor contrast when the ground is snow covered and the cloud base low.

Horizontal definition

The presence of large areas of black volcanic ash means that the horizontal definition is normally good. It may deteriorate during periods of dense fog.

Precipitation

Precipitation is often in the form of rain during the summer, and although the amount can be very variable from year to year; monthly totals during the summer in access of 100 mm are common. During the winter the precipitation amounts are normally lower and usually falls in the form of snow, although rain can fall in any month.

Temperature and chill factor

The average annual temperature is around –4ºC with temperatures falling to a minimum of around –30ºC during the winter. The dark volcanic ash will tend to absorb radiation during the summer helping melt the winter snowfall quickly. Hot volcanic springs tend to keep a narrow band of the ocean sea ice-free but these springs do not have any effect on the air temperature. Table 7.3.3.4.2 (in Appendix 2) shows the mean temperature for Deception Station from 1944 to 1967.

Icing

Most of the clouds observed at Deception Island are composed of water droplets and this means that icing can be a problem, especially after sudden drops in temperature.

Turbulence

Turbulence can be formed by the airflow around the steep hills of Deception Island. The presence of turbulence can sometimes be indicated by orographic clouds forming over the island.

Hydraulic jumps

Hydraulic jumps have not been observed on Deception Island.

Sea ice

Whalers Bay, and probably the greater part of Port Foster, is ice covered for most of the winter. A narrow ice–free channel near the shore of Whalers Bay is kept ice–free by hot volcanic springs.

Wind waves and swell

Port Foster and Whalers Bay are very sheltered and wind waves or swell are not a problem.