The climate of Socotra Island (Yemen): A first-time assessment of the timing of the monsoon wind reversal and its influence on precipitation and vegetation patterns

摘要

The climate of Socotra, influenced by the Indian Ocean Monsoon, is poorly known, hampering understanding of its paleoclimate and (endemic) biodiversity. Mean annual rainfall and temperature, measured in a network of meteorological stations from 2002 to 06, were 216 mm and 28.9 °C. Combined with cloud cover information from satellite images, this data provides clear ideas on inter- and intra-annual variability. Precipitation derived from the northeast (NE) winter monsoon influences especially the NE plateaus and windward side of the Haggeher Mountains because of orographic effects. The southwest (SW) summer monsoon concentrates at the southern half of the island and generally produces less rainfall. During the SW summer monsoon, clouds cover the highlands and plateaus south of the Haggeher Mountains, creating fog. Preliminary measurements suggest that at higher altitudes, fog-derived moisture may constitute up to two-thirds of total moisture, amounting up to 800 mm. The predominant SW aspect of the enigmatic dragonblood tree underlines the importance of fog. Long-term weather observations by Socotri put these short-term meteorological observations into a longer perspective. Socotri informants also described the drought years when livestock populations crashed, after which windows of opportunities for the regeneration of dragonblood and other grazing-sensitive trees may have occurred.