Identifying patterns of Synoptic Anomalies Resulting in Pervasive Droughts and Wet periods in Iran

Drought has imposed a huge damage to country’s economy especially in recent decades. Pasture yield losses, reduction of agricultural and drinking water, reduction of groundwater and surface water resources, outbreak of plant and animal pests and diseases and an increase in migration are among the negative effects of drought. The aim of this research is understanding atmospheric circulation patterns with modern methods and based on further data; therefore in case of synoptic patterns identification associated with drought and wet years, these patterns can be used to forecast country’s wet and dry periods with a very high accuracy.

To identify and extract widespread Iran’s droughts and wet years and in order to achieve the objectives of the research, two databases were required: Surface environment data and upper atmosphere data. Surface environment data was used from rainfall data of 43 synoptic stations in a 33-year period (1976-2009) which was received from the Iran Meteorological Organization. High atmospheric variables which were used in this research include: Geopotential height, sea level pressure, zonal wind, meridional wind, air temperature and specific humidity. Data and maps of all these variables were collected from www.esrl.noaa.gov/psd website as monthly data. Data related to Geopotential height variables; zonal wind, meridional wind and the air temperature at 17 levels and specific humidity in 8 level data are available. A suitable index was selected in order to analyze inclusive droughts and wet years of Iran, so droughts and wet years could be separated according to it. In this research Standardized Precipitation Index (SPI) was selected given its advantages compared to other drought indexes. Synoptic analysis method of patterns related to Iran’s monthly inclusive droughts and wet years, were divided into three categories based on a principle location: local droughts (or wet years): Droughts (or wet years) which declared about 25% and less of studied stations in Iran’s droughts (or wet years). Semi-inclusive droughts (or wet years): Droughts (or wet years) which declared about 25 to 75 percent of studied stations in Iran’s droughts (or wet years). Inclusive droughts (or wet years): Droughts (or wet years) which declared about 75% and more of studied stations in Iran’s droughts (or wet years).

The results showed that in the inclusive droughts, positive temperature anomalies and in inclusive wet years, negative temperature anomalies can be observed in Iran. Composite maps of humidity anomalies also nicely demonstrate that in the times of drought across Iran, negative anomalies have been found and in the wet years contrast status of droughts have happened and the positive anomaly got over Iran. In the months that inclusive drought happened in Iran, it can be seen that European and Asian high pressure combined together and put the whole of Europe and Asia under domination which under these conditions, Iran experienced drought months as during this time cyclonic circulation formed on the Arabian Sea which made humidity of the Arabian Sea not to transferred to Iran. The second affecting anomalies on this phenomenon, is the Mediterranean Sea; in these months a major anti-cyclonic circulation dominated on Europe. This major anti-cyclonic circulation affected Mediterranean Sea and even North Africa. In these circumstances Mediterranean cyclone formation was disrupted and so no humidity transferred to Iran. When European high pressure move to the North Atlantic Ocean and provide space for the establishment of a polar low pressure on Europe; in these circumstances regional low pressure is formed on Mediterranean Sea which causes more humidity entering Iran which results will be wet months; as well as Anti-cyclonic circulation ruling on of the Arabian Sea that crosses over the Red Sea is causing humidity injection in Iran.

In the months that inclusive drought happened in Iran, it can be seen that European and Asian high pressure combined together and put the whole of Europe and Asia under domination which under these conditions, Iran experienced drought months as during this time cyclonic circulation formed on the Arabian Sea which made humidity of the Arabian Sea not to transferred to Iran. The second affecting anomalies on this phenomenon, is the Mediterranean Sea; in these months a major anti-cyclonic circulation dominated on Europe. This major anti-cyclonic circulation affected Mediterranean Sea and even North Africa. In these circumstances Mediterranean cyclone formation was disrupted and so no humidity transferred to Iran. When European high pressure move to the North Atlantic Ocean and provide space for the establishment of a polar low pressure on Europe; in these circumstances regional low pressure is formed on Mediterranean Sea which causes more humidity entering Iran which results will be wet months; as well as Anti-cyclonic circulation ruling on of the Arabian Sea that crosses over the Red Sea is causing humidity injection in Iran.