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The region in and around the eastern mountains in southwest Asia has been suffering from drought for the last three years. Climate data from this region is frequently unavailable on an operational near-real time basis and historical data is incomplete in many cases, which makes it difficult to compute a regional drought index. To complicate matters, the "normal" climate of the region varies considerably from mountains to plains and the region is generally classified as a desert.
In order to assess the magnitude of the dryness in the southwest Asia eastern mountains region, a precipitation index was created from the precipitation data for ten stations that currently report operationally and that have reasonably complete historical records. The impact of missing data was minimized by using the following methodology. Long-term monthly averages for the twelve months, January - December, for each station were computed over the 1951-2000 period. These 50-year average monthly precipitation values were used as a reference base. The percent of average precipitation for each month and each year in the record for each station was computed using the station's respective monthly base values. These monthly percentages were averaged for the ten stations to produce a seasonal precipitation (i.e., "drought") index for the region.
As seen in the graph below left, the region has experienced below-average precipitation for much of the last three years. The annual precipitation index (graph below right) shows how the dryness compares to the 20th century record. The last three years were very dry by this index, having ranks of fifth driest for 1999, third driest for 2000, and seventh driest for 2001 (out of 102 years).
There is a pronounced seasonality to the precipitation regime, with winters generally being wetter than summers (see graph below left). Here the winter wet season is defined as December through April. The dryness during the last three winter seasons has been even more pronounced (see graph below right), with 1998-99 ranking as the eleventh driest, 1999-2000 as the driest, and 2000-01 as the second driest (out of 101 winters).
During the past few winters, anomalously strong upper level high pressure ridging (associated with sinking air and a lack of precipitation-producing weather systems) centered over the Middle East has severely limited precipitation amounts in the southwest Asia eastern mountains region, resulting in increasing water deficits that have contributed to the severity of the current drought. This circulation regime is also evident in the average annual pattern for the past three years.
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