The Role of Subtropical High Pressure Jet Streams in Controlling Precipitation in Iran

Introduction Wind speed is not equal in all parts of the atmosphere. Subtropical jet stream says to cores speed of 30 meters per second at the equator near westerly winds that is effective control of the low and middle latitude weather. Jet streams can also control intensity, frequency, and duration of precipitation. Since vast areas of Iran have arid and semi-arid climates, factors that influence precipitation—e.g. velocity, location, and movement of subtropical jet streams—must be evaluated more carefully. Materials and Methods The data used in this research can be divided into two datasets. The first dataset includes precipitation data (sum monthly precipitation and precipitation frequency in day per monthly) for 180 synoptic stations in Iran. The statistical period is 1951-2005, but not all stations had data for the 55-year period. The second dataset includes data related to zonal wind velocity (u-wind) at different levels of the atmosphere with a resolution of 2.5° (extracted from www.cdc.noaa.gov), extending from the equator to 90°N and from 20°W to 100°E. The dataset contains 1000 to 10 hPa levels. Pearson correlation and multiple correlation were applied to examine the relationship of monthly precipitation with maximum velocity, latitude, and the north-south movement velocity of the core of the jet stream. First, a database was created based on the data from 180 stations and 660-months, and a 180*660 matrix was generated. First, to examine the effect of the position of the subtropical jet stream on precipitation in Iran, the isotach map of the jet stream was created at the 200 hPa level for all the months of the studied period. Then, to identify the cross-sectional expansion (profile) of this jet stream over Iran, longitude 52.5°E was mapped as the vertical axis. Finally, the relationship between precipitation in each of the stations (sum monthly precipitation and precipitation frequency in day per monthly) and jet stream characteristics (core velocity, latitude, and north-south movement velocity) was examined using Pearson correlation and multiple correlation. Findings Based on the results of evaluating the changes in the velocity of the jet stream, maximum velocity occurred from December to March (greater than 45 ms-1). Annual means of maximum jet stream velocity over Iran during the period 1951-2005 along with annual oscillation suggest a cyclical and incremental trend; however, oscillations have decreased since 1985. At some points (e.g. in 1957, 1972, and 1982), the jet stream has exceptionally been at southernmost latitudes. The isotach maps of zonal winds indicate a mean monthly velocity of 20 ms-1 at the 200 hPa level. These maps show that winds are strongest during the period January-April, but after May wind speed decreases and they shift to higher latitudes. Examining the effect of maximum velocity of the jet stream core on precipitation at synoptic stations shows that there is a significant correlation between precipitation at 37% of the stations and maximum jet stream velocity with r coefficients ranging from 0.41 to 0.50. Moreover, there was a significant correlation between precipitation at 34% of the stations and latitude of the jet stream with r coefficients ranging from -0.60 to -0.51. Examining the correlation between precipitation and north-south movement velocity of the jet stream core indicates lack of a significant correlation in 71% of the stations. Multiple correlation analysis shows that precipitation and jet stream characteristics are correlated for 36% of stations with r coefficients ranging from 0.51 to 0.60. Conclusion The results of the present research show constant annual changes in the location of the jet streams over Iran which is estimated to be about 1° toward northern latitudes in a 55-year period (1951-2005). However, monthly changes in the north-south movement velocity of the jet stream core do not follow a specific trend. Although other studies have argued that the northernmost location of jet streams which occurs in hot days of the year is over Tehran, the results of the present research show that the northernmost location of the jet stream is over the Southern Caspian. The results of examining the relationship between jet stream characteristics and precipitation show that the overall effect of jet steam characteristics on precipitation is greater than the effect of each individual component. Moreover, jet stream characteristics are more strongly correlated with precipitation frequency than sum monthly precipitation. The strongest correlations are observed for regions along the Zagros Mountains and the weakest correlations are observed for Northwest Iran, Caspian coastline, and Southeast Iran. The reason for weak correlations for Caspian coastline stations is the sea itself which controls precipitation in the region. But Northwest Iran has the northernmost latitudes and is less affected by subtropical jet streams. Moreover, this region is more quickly and more directly affected by westerly winds. Finally, the weakest correlation was observed for Southeast Iran and a possible reason is monsoon systems that control precipitation in the region and have a greater effect than changes in subtropical jet streams.