عنوان مقاله [English]
Climate change could be studied by detection of amount and tempo-spatial pattern of climatic elements. Precipitation is one of important climatic elements that could be studied. Precipitation is one of the most variant climatic elements that could have negative consequences on environment, society, economic and even culture of human society. Accordingly the changes and looking for change and variability of precipitation take the attention of climate experts. This is because of the importance of this sort of studies on climate forecasting and water resources management as well as environmental, economical and agricultural planning.
Precipitation as one of fundamental elements in climate has increased by 0.1% in middle and high latitude especially during 20th century. Meanwhile in subtropical lands (Northern latitude of 10-30 degree) decreasing in precipitation by 0.3% has been reported, also there are increasing by 0.2%-0.3% in tropical regions (Mosmann et al. 2004). In addition there are negative trends of precipitation in Mediterranean basin since 1950s. Because of this spatial differ in precipitation changes and in order to precise exploring the changes, a dozen of studies have been accomplished.
In this paper long term and oscillatory behavior of precipitation in the Northwest of Iran has been evaluated. The area under investigation is about 12654.4 square kilometers and contains four province including west and east Azarbayjan, Ardabil and Zanjan. It occupied about 7.7% of all country. The average elevation of northwest of Iran is about 1830 meters above sea level. The highest area is about 4500 meters above sea level.
In order to do this research, daily precipitation data of 260 stations during 1966-2005 have been analyzed. The length of recorded data have been used in this study wasn’t equal, so it had created a map for every single day using the stations that was available. This stage has been accomplished by Kriging interpolation method. Accordingly 15052 daily precipitation maps by 100*100 pixel resolution were created. To create the 40 annual maps, these maps were used. So the investigated area was covered by 5374 pixels. Finally the matrix of data by dimensions of 5374*41 has been established. The analyses of two approaches were accomplished on this matrix. They are as follow:
1- The trends of annual precipitation have been analyzed using linear non – parametric regression model.
2-annual cycles of precipitation have been estimated using spectral analyses. The cycle that has largest difference with null continuum, is chosen as the significant cycle.
Finally, all result of above two stages is presented in related maps.
Annual precipitation of northwest of Iran is about 360.9 mm. precipitation mean is spatially different from 200 mm (northeast of area in Dashte-Moghan) to 800 mm (in southwest of area. Accordingly spatial coefficient of variations is about 24.1%. The area with low precipitation is most extending than the area with high precipitation. For example about 58.8% of northwest of Iran has achieved the precipitation lower than its spatial average.
The annual precipitation of northwest of Iran (except of small parts in southwest and north) during the studied period has experienced decreasing trend. The amount of decrease in about 84.5% of area is about 3 mm per year. This kind of trends has been occurred especially in eastern and southern halves. Across west to east and toward the mountains the amounts of trend is decreased. About 16.8% of area is covered by Non –significant trends that contain Dashte-Moghan and western mountains.
The region under increasing trends is about 8% of the area in which the trends is about 5-9 mm per year.
In the 58%-71% of the area, precipitation has experienced the values more than long term average during the first three decades. Meanwhile during the forth decade the area with precipitation lower than long term average has increased by 72%.
By using spectral analyses technique on precipitation data, significant oscillations in 2-3, 3-5, 5-11 and 11 years have been discovered. Major area experienced 5-11 years oscillations. As it has reported by many experts these oscillations periods are due to macro-scales circulations between atmosphere and hydrosphere e.g. quasi binomial oscillation is related to ENSO phenomenon.
Based on what discovered in this paper it has been cleared that long-term mean of precipitation in 84.5% of northwest of Iran has been decreased. The increased trends values of precipitation are happened in southwest. The decreasing in precipitation is due to last decade in which the area with low precipitation has increased by 72%.Spectral analyses showed significant oscillations in 2-3, 3-5, 5-11 and 11 years. Major area experienced 5-11 years oscillations. As it has reported by many experts, these oscillations periods are due to macro-scales circulations between atmosphere and hydrosphere e.g. quasi binomial oscillation is related to ENSO phenomenon.