Читать книгу The Terrestrial Macroinvertebrates of the Sub-Antarctic Iles Kerguelen and Ile de la Possession - Maurice Hulle - Страница 16

As with all remote sub-Antarctic islands, both the Îles Kerguelen and Île de la Possession have an oceanic cold climate, strongly influenced by the South Indian Ocean. They are located at the same latitude as France and therefore have an equivalent photoperiodic regime, with long days in summer and short days in winter. The mean air temperature for 1951–2013 at sea level was 4.8°C at Îles Kerguelen and 5.5°C at Île de la Possession (Figure 1.6). Seasonal variations are low, between 2°C and 8°C at Îles Kerguelen and 3°C and 8°C at Île de la Possession. Summer temperatures may temporarily reach or exceed 15°C. At the Îles Kerguelen, average temperatures increased by 1.9°C between 1964 and 1981 and then stabilized to around 5.2°C. Concomitantly, the number of freezing days has decreased from 130 days per year in the 1960s to 100 days per year now. At Île de la Possession, the number of freezing days is less than 60 per year.

Air temperature decreases by approximately 1°C per 100 m increase in altitude (Pointe Suzanne and Port-Jeanne d’Arc, Kerguelen; M. Hullé unpublished data). This steep decay means that the thermal thresholds of development of many insects may be reached rapidly with altitude. The mean temperature is below 3°C from 200 m a.s.l., and insect life is consequently mainly concentrated on low-lying coastal areas (Figures 1.4 and 1.5).

The average rainfall of Île de la Possession is high with 2,390 mm per year (Figure 1.6). On the Îles Kerguelen, there is a rainfall gradient from west to east. The west coast receives about 2,000 mm per year, and Port-aux-Français, located to the east, receives about 700 mm per year. At Port-aux-Français, rainfall oscillates between years of drought, for example, 1965 with 350 mm, and wet years, for example, 1987 with 1,150 mm.

Winds are predominantly from the northwest, west and southwest. The average wind speed is 35 km/h on the Îles Kerguelen and 38 km/h on Île de la Possession. Strong winds are frequent. They are mostly from the northwest (Figure 1.7). The strongest gusts can exceed 200 km/h. They reach an average speed of 131 km/h on the Îles Kerguelen and 141 km/h on Île de la Possession. The number of days with winds above 100 km/h is 68 per year on the Îles Kerguelen.

Figure 1.6. Climate at Port-aux-Français (Îles Kerguelen) (left) and Base Alfred-Faure (Île de la Possession) (right): ① Annual mean temperature, ② Monthly temperature (mean in gray, maximum in red and minimum in blue), ③ Annual precipitation (data from Météo France)

Figure 1.7. Direction of the strongest winds (data from Météo-France)