Conspicuous among observed changes in 20th Century climate has been the progressive warming of the warmest sector of the world oceans. This so-called "warm pool" is found over the tropical west Pacific and Indian Oceans where sea surface temperatures (SSTs) have increased 1°C since 1970. This steady warming, to a level unprecedented in the instrumental record, makes the Indian Ocean more competitive for being a key driver of extratropical climate change. The warming trend is also consistent with the ocean's response to increased greenhouse gas forcing, so it is very likely that warm pool SSTs will increase further during the 21st Century.

Modeling results confirm the appreciable impact of the warm pool SST changes on observed regional extratropical climate change. In particular, the drying trend experienced in the Mediterranean region during recent decades involves strong dynamical feedback that has been driven by the warm pool warming. There is also evidence that the severe and sustained drought inflicting the United States, the Mediterranean, and Southwest Asia during 1998-2002 was tropically forced, and that the unprecendented warmth of Indian Ocean SSTs was key.

It is reasonable to expect that regional climate change of the 21st century, and precipitation changes in particular, will depend on the trajectory of tropical SSTs. We discuss the sources for uncertainty in those SST changes. Also, using climate model simulations, the risk for future regional drought that is associated with particular scenarios of the tropical SST trajectory are discussed. We stress especially the further increase in drying that would emerge over the Mediterranean and Southwest Asian regions due to the likely continued warm pool warming.