Somalia is mostly dry and semi-arid with the exception of few areas of greenery in the northern mountains or the riverine agricultural fields in the south. The main long rainy season (gu) from April to June is followed by a dry season (xagaa) from July to September, which is in turn followed by a shorter rainy season (deyr) from October to November, and the main dry season (jilaal) from December to March completes the seasonal cycle. In parts of northwestern Somalia, the xagaa dry season is replaced by the karan rainy season.
The climate of Somalia, and indeed that of East Africa, is linked to the condition of the sea surface temperature of the Indian and Pacific Oceans and the Walker Circulation. The Walker Circulation is an air flow over tropical regions in the lower part of the atmosphere. This air flow occurs in the lower part of the atmosphere (troposphere) and parcels of air follow a closed circulation in latitudinal and vertical directions. The Walker Circulation is primarily controlled by the difference in surface air pressure and temperature over the western and eastern tropical Pacific Ocean. When the Walker Circulation becomes weak over the Indian and Pacific oceans, warm air rises in the western Indian Ocean causing increased rainfall over Somalia (see figure A below). This happens during an El Niño and a positive Indian Ocean Dipole (the Indian Ocean equivalent to the Pacific El Niño-Southern Oscillation). These changes in the atmosphere are linked to increased warming of tropical sea surface temperatures (B).
Like other tropical semi-arid areas, the seasons (and livelihoods) in Somalia revolve around rainfall. The harvest period depends on the type of crop, but there are generally two main harvest seasons in Somalia. For the gu, farmers plant their crops in early April and harvest them around July/August, and for the deyr, farmers plant their crops in early October and harvest them in January/February. Prices of crops follow a seasonal trend: they decrease in July/August as the gu harvest replenishes stocks, increase between September and December as market stocks are used up, and decrease again in January/February with the deyr harvest.