2002 Annual

Thursday, 17 January 2002: 10:45 AM
Ongoing drought in southwest Asia: The role of large-scale climate variability and the tropical oceans
Mathew A. Barlow, International /Research Institute, Columbia University, Palisades, NY; and H. Cullen and B. Lyon
A widespread, ongoing drought in southwest Asia has contributed to the severe famine affecting over 50 million people from Syria and the Caucasus to Afghanistan and Pakistan. The drought, coupled with protracted civil unrest, has resulted in crop failure, livestock loss, and mass migration, particularly in Afghanistan. Rainfall over the drought region since 1998 is less than 55% of the long-term average; in Iran some areas have reported no measurable rainfall for 30 consecutive months.

The evolution and spatial extent of this drought is linked to coherent, large-scale anomalies in precipitation, atmospheric circulation, and sea surface temperatures (SSTs).

In particular, there is a negative relationship between precipitation in the eastern Indian Ocean and precipitation in Southwest Asia during boreal winter. The strength, spatial scale, and timing of the Indian Ocean rainfall and upper level wind anomalies are consistent with the production of Rossby waves that propagate into the southern flank of the westerly jet over Southwest Asia. The associated upper level anticylconic forcing and interaction with the jet structure appears to negatively influence synoptic storm systems, which typically provide the bulk of precipitation in the drought region. Aspects of the Indian Ocean rainfall anomaly are consistent with a modified Madden-Julian Oscillation.

Although not previously noted as part of the classic El Nino - Southern Oscillation (ENSO) signal, the extension of the positive tropical rainfall anomalies into the Indian ocean (and associated changes over SW Asia) are shown to be closely related to a subset of cold phase ENSO events characterized by an enhanced western Pacific warm pool. Pacific decadal variability and upward trends in Indo-Pacific ocean temperatures also affect the warm pool SSTs and appear to contribute to the recent prolonged cold phase ENSO event in the extension of Pacific rainfall anomalies into the Indian ocean and associated suppression of precipitation over southwest Asia.

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