تأثیر بی‌هنجاری‌های ماهانة الگوهای فشاری بر شرایط بارشی پاییزه در سواحل شمالی ایران

Based on the climate divisions, Iran is mostly located in the arid and semi-arid regions. Changes in amount of precipitation have a great effect on water resources, agriculture and the economy of regions. Atmospheric circulation pattern has a determinative role in the temporal and spatial distribution of precipitation and some other meteorological parameters. Thus, abnormalities of these patterns compared to the long-term mean in dry and wet spells is one way that can determine the effective atmosphere patterns in changes in precipitation during these spells. A number of studies have described the relationship between the pressure patterns and the precipitation anomalies over monthly and seasonal time scales. Kutiel et al. (2001) found that the relationship between regional sea-level pressure (SLP) patterns and the dry or wet monthly precipitation conditions over Turkey was significant in winter and non-existent in summer. Pressure patterns associated with dry conditions usually show positive SLP departures, and vice versa. There is a strong relationship between the pressure patterns associated with wet conditions. Similar atmospheric variations and relationships have been found for precipitation over the eastern Mediterranean Basin and Greece, especially during winter (Maheras et al. 1999). They investigated the influence of the large-scale winter mid-tropospheric circulation on the Greece's precipitation. They concluded that the spatial distribution of winter precipitation over Greece was related to the eastern North Atlantic-European mid-tropospheric circulation fields.
    A considerable part of precipitation in the northern coasts of Iran falls in autumn season. Investigating the relation between the mean monthly anomalies of pressure patterns and the extreme low precipitation conditions in autumn in the northern coastline of Iran (Mazandaran, Gilan and Golestan provinces) is the main purpose of this study.
    Rainfall data comprised monthly totals for the period from 1977 to 2011, in seven stations across the northern coasts of Iran. It consisted of monthly precipitation totals (mm) for October to December in a 35-year period from the meteorological synoptic stations. Also, the grid point data (Sea level pressure and 500hPa) were extracted from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset. A grid data with 2.5°×2.5° resolution was selected for the area between 0°E -90°E and 15-80°N for three months (October-December) over a 30-year period (1981-2010).
    Therefore, using monthly precipitation data of seven synoptic stations during 35 years (from 1977 to 2011) for October, November and December, dry and wet spells were determined based on precipitation departures from the standard deviation so that, if a difference between the long-term mean precipitation and the standard deviation of precipitation in that month was less (or more) than the occurred precipitation, it was selected as dry (or wet) spell. Then monthly and seasonal pressure patterns and their anomalies were studied for those months of autumn that had minimum or maximum of precipitation during the 35 years.
    Also, seasonal and monthly mean sea level pressure and 500-geaopotential height anomalies for the two regions defined in the text  were extracted during the period 1981-2010. Then, by normalizing the values of pressure, 500-geaopotential height and rainfall during the months of October, November, December, and the season of autumn of these quantities and relationships between them were studied.
The results of study are as follow:
1-  In low (high) precipitation  periods, the geopotential height is dominated by a ridge (trough) and positive (negative) anomaly in north of Iran.
2-  Another reason for low precipitation in north of Iran is the development and establishment of a low pressure in this region that is usually accompanied at the same time with development of a high pressure in Siberia. In high precipitation, the situation is reversed.
3-  Attenuation (development) of the zonal component of the wind in Caspian Sea has an effective role in increasing (decreasing) the precipitation of this area.
4-  Pressure patterns over the two regions  have an important role in producing dry and wet spells in the northern coasts of Iran
5-  Developing and strengthening of a high-pressure system over Europe and low-pressure systems in the Northeastern region of the Caspian Sea (Area 1) are associated with wet conditions along the southern coast of the Caspian Sea and vice versa.                               
    Considering the precipitation mechanism in the northern coast of Iran is of great complexity and that the numerical weather prediction models are not accurate enough to predict rainfall in this region, especially in the medium and long term forecasts, the anticipated large-scale quantities such as pressure are expected to enhance the ability to predict rainfall in the northern coast of Iran.