Marion Island lies approximately 200km north of the Antarctic Convergence in the middle of three water masses, where cold Antarctic waters sink below the slightly warmer sub-Antarctic waters. The mean monthly air temperature of the island closely tracks that of the ocean, which ranges from 3.6°C in August to 7.9°C in February. Sunshine duration averages 3.6 hours a day, or 29% of the possible, and the climate could be summarized like all other sub-Antarctic islands as cold, wet and windy.

The island lies in the middle of the proverbially tempestuous "Roaring Forties" and subjects itself to the "Westerlies" - the winds that blow predominantly in these latitudes. Circumpolar winds are constantly interrupted by cyclones with cold fronts that can produce rain or snow, and anti-cyclones that are accompanied by dry spells and fair weather. Nonetheless, the continual presence of cyclones (100-130 per year) near the island make for a high level of cloudiness; bright sunshine days are a rare occurrence.

While the island is never warm, the average temperature only varies by about 4°C per annum; it also does not experience the bitter cold of continental tundras. This feature and the absence of strong diurnal temperature ranges are a consequence of the moderating influence of the ocean on the island's climate. However, the temperatures felt are often substantially lower due to the wind-chill factor (the combined effect of strong wind and low temperature).

The climate on the western side of the island, which is more overcast and exposed to strong winds, rain and wild seas can be substantially different from that on the east where the meteorological station is situated. Despite these seemingly constant conditions the weather can fluctuate considerably over the course of one day.

Temperature

The mean annual temperature is approximately 5°C, with the warmest months from January to March having an average of 7.5°C and the coldest months from June to September having an average of 3.6°C. An absolute maximum temperature 23.8°C and an absolute minimum temperature of -6.8°C have been recorded (2001), while the difference between extremes is close to 30°C. Above-mentioned temperatures were recorded at the meteorological station which is on the sheltered east coast at 22m above sea level. Sea surface temperatures are warmest around March with an average maximum of 10°C, and coldest in September with an average minimum of 3°C.

Precipitation

Rain occurs throughout the year with an average of 317 days per year, but most frequently during the winter months averaging 28 days a month. The average yearly rainfall of 2 500mm can vary on different parts of the Island. One could expect higher figures on the northwestern side from where weather systems approach and lower averages on the higher peaks where precipitation mainly comes in the form of ice and snow.

Snow and frost may occur in any month of the year, but average annual snow days at Marion base are approximately 80. On higher altitudes snow cover remains for most of the year, but there can be periods during summer when even the highest peaks are snow free. Permanent ice is found only on the ice plateau where soil may freeze in winter and stay frozen for up to a week, which usually disrupts the water supply to the base.

Hail (ice pellets/graupel) and snow or sleet occur mostly during winter, and fog is frequent during late summer but an average of 2-3 foggy days per month may be expected. Thunderstorms and lightning also occur, although rarely, about 4 times a year.

Wind

Northwesterly winds are the predominant and strongest winds with regular gale force strength (exceeding 55km/h), occurring approximately on 107 days of the year. Gusts of 160km/h frequently occur, and gusts of over 200km/h have been recorded. Calm conditions are rare and the wind direction can also predictably effect the climate. Frequent westerlies are associated with wet cloudy conditions, whereas southerly winds bring cold, clear conditions. Infrequent northerly and easterly winds are responsible for warm and sunny conditions on the island.

Tides and waves

Tidal sea level changes are small, with amplitude of 71cm. Swells can, however, vary enormously in accordance with prevailing storm systems. Most wave effects are generally westerly during winter but some occur from the east. During such easterly storms, swells with tremendous force welling up to 12m break over the eastern cliffs and are most noticeable in the pronounced effect that they have on the infrastructure at Marion base. However, westerly storms may be more violent than those from the east.

Meteorological station and personnel

A weather station was established on Marion following annexation in 1947 and it has been in operation ever since.

This weather station is considered to be one of the most important stations in the world. It is one of few weather stations in the Southern Indian Ocean. It helps to provide information of weather systems over the oceans surrounding South Africa - areas from where very little information is available.

According to the needs of the World Meteorological Organisation the South African weather service manage two fully manned island weather stations; Gough and Marion Islands. We have automatic weather stations on Tristan da Cunha, SANAE (Antarctica), Southern Thule and Zavadovski Islands, and 30-40 drifting weather buoys are deployed annually.

SA Agulhas Weather Office

Marion Island Weather Station

Meteorologists do hourly surface observations which are augmented by an automatic weather station which records data every five minutes. Elements recorded are temperature, rainfall, sunshine, radiation, wind speed, wind direction, barometric pressure, humidity and sea surface temperatures. Visual observations include cloud identification and weather conditions. Upper air soundings are done twice a day to determine temperature, pressure, humidity, winds speed and wind direction up to 30 000 metres. A LUT on the island receives data from drifting weather buoys within a radius of up to 3000 km. The buoy data is transmitted to Pretoria and used in weather forecasting. This happens 4-6 times a day - depending on satellite passes. Meteorological observations are coded and transmitted to the central forecasting office in Pretoria immediately after the observation. From Pretoria it is made internationally available on the Global Telecommunications System. Extremes and averages are transmitted to Pretoria on a monthly basis.

Weather forecasting

Climate recearch