Decade changes in monthly mean of precipitation in recent five decades over the Caspian coast of Iran territory

Precipitation is one of the most important climatic elements which has been changed over the last few decades. Changing in climatic elements, especially precipitation, could dramaticaly influancs natural, social and economic aspects of the environment. Change in precipitation turn up in different ways.The purpose of current study is to examine the changes in the decade of the monthly mean precipitation of the cold season over the Caspian coast of Iran territory in the last half century.

For this purpose, the daily database of precipitation which resulted from the interpolation of 385 stations, under Meteorological Organization and the Ministry of Energy's supervision have been used for the period of 2016-1966 (51 years) with a spatial resolution of 3 × 3 km.

Our finding showed that the precipitation gradiant transfered from the west to east and the areas with low precipitation increased in compare to the areas with high precipitation. This decreasing general trends emerged in a decadal- sequential oscelatory pattern.
As the Caspian coast precipitation atributed to thermal high pressure systems and Meditraniean cyclones, the changes in values of precipitation and the area, are considered as a result of these factors which are investigated in many researches previously.

In addition to significant environmental impacts, precipitation variations in the decade have an undeniable effect on the quantity and quality of water resources.The results show that in the last 11 years (2006 to 2016), the average monthly precipitation and its location from the southwest of the Caspian Sea to the Alborz and eastern altitudes have been shifted and the extent of ​​the low amount precipitation is increasing.Changes in precipitation during the cold months of the year in the Caspian Sea region have been attributed to changes in thermal high pressure, as well as to the variations in the frequency and direction of the Mediterranean Cyclones, as shown in previous studies.Since precipitation variations are very complex and requires comprehensive studies, it is suggested that in order to plan and make more precise decisions, it is recommended to identify the changes of the Synoptic systems in this area.