| Summary: | In the present investigation, we have focused on long-term precipitation variability over the northwestern of Iran in relation to climate profile of North Atlantic Oscillations (NAO). We have used some statistical methods such as Pearson correlation, Linear regression, variance, standard deviation and homogeneity test to find out the relationship between the North Atlantic Oscillation (NAO) index and precipitation-changes over the Urmia Lake`s basin. In the next phase some dynamic and thermodynamic anomalies were studied in different atmospheric levels, including geopotential height (in the height of 500 hPa). Also sea level pressure and wind speed (in the hight of 30 Hpa) were analyzed. The results illustrated that a reverse relationship could be observed between NAO phases and precipitation changes over Urmia Lake Basin during winter season (December, January and February). As the Pearson correlation and linear regression coefficients are -0.37 and -0.13 respectively, and the homogeneity test confirms this reverse relationship as well. Furthermore, about 69% of events show that negative NAO phase is accompanied with high precipitation in the region and vice versa. Analyzing the dominant atmospheric patterns in the wet period (winter 1986-1987) over the northwest of Iran revealed that geopotential height in 500 hPa has increased up to 20 meters, sea level pressure anomaly has decreased about 1 to 2 hPa, wind speed has increased 10 meters per second in 300 hPa which has resulted in a series of instabilities and turbulences. On the other hand, in the dry period (winter 1988-1989) we have experienced a 20 meter decrease in the geopotential height, sea level pressure has increased 4 hPa and wind speed has decreased about 4 meters per minute.
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