Half-precessional dynamics of monsoon rainfall in East Africa

A recent reaserch article in the journal Nature show that monsoon rainfall in East Africa varied at half-precessional (~11,500-year) intervals, because the southeasterly and northeasterly Indian Ocean monsoons, which together create the bimodal seasonal distribution of equatorial rainfall, were strengthened in alternation when the inter-hemispheric insolation gradient was maximized. Dry conditions prevailed when neither monsoon was particularly strong, and minima in local March or September insolation weakened the rain season that followed.

http://www.nature.com/nature/journal/v462/n7273/full/nature08520.html