The 2011 Thailand flood: climate causes and return periods

The 2011 Thailand floods were caused by a combination of a strong southeast Asian summer monsoon that brought high rainfall across the country between May and October, and the remnants of four tropical storms that brought high rainfall to northern Thailand between June and October. The tropical storms contributed one third of the excess monsoonal rainfall north of 16°N. During the monsoon season the Southern Oscillation Index (SOI) was moderately positive and, as a result, is likely to have contributed to the high monsoonal rains. The SOI enhancement of Thai monsoonal rains probably arises because the ascending branch of the Walker Circulation over the Pacific Ocean and southeast Asia shifts westward towards Thailand during positive SOI conditions.