عنوان مقاله [English]
Atmospheric circulation patterns result in different weather conditions. Rainfall changes are controlled by seasonal and irregular variations of atmospheric circulation pattern. A variety of climate changes such as drought and flood can result in changes in the spatio-temporal precipitation, one of the important parameters of heterogeneity in moisture transmission and injection in the rainy systems of Iran. One component of the atmospheric circulation of moisture transmission is the high pressure centers and the most important point is their location on the water surface.
Saudi Arabian high pressure is one of the main elements of the general circulation on the lower level of the atmosphere which affects climatic characteristics of the region. Therefore, the main aim of recent study was to investigate Saudi Arabian High Pressure circulation patterns and its impacts on the moisture and precipitation by using cluster analysis.
Materials and Methods
In order to assess the role of Saudi Arabian high pressure patterns on the rainfall and moisture of Iran, we have derived hourly data viz., 00, 06, 12 and 18 global standard geopotential data in height of 850 hPa for 11 years during the period 2000 to 2010 for the area in 30° to 80° longitude and 5° to 30° latitude with a spatial resolution of 1° * 1°. The data are from the re-analysis of the ERA interim center of Europe medium-term forecasts (ECMWF).
By scripting in MATLAB software, we employed cluster analysis for correlation method and performed Clustering based on the threshold correlation coefficient of 0.5. We initially obtained the 160 groups from which the greater number of days whose the higher correlation were more than 0.5 have been placed in one group.
In the later steps to reduce the groups between correlations to less than coefficient threshold, we have reduced number of groups in each step using reiteration of operation by calculation and integration of the average of any of days of the each cluster groups to finally obtain the 5 large patterns.
In order to evaluate the resulting patterns all the days in each pattern were pulled out. Traced maps, the mean geopotential height, zonal and meridional component of wind were obtained through the scripting GrADS software. In addition, rain and moisture advection based on the total days of each pattern on a large scale were mapped and frequency of the monthly patterns were calculated as well.
Results and Discussion
In this study, 5 large patterns were detected such that 2 of them were rare patterns and 5 dominant patterns. The 4 patterns have been more abundant in the cold season and 1 pattern in the warm season. The second pattern of the cold season is the highest and most pervasive precipitation of Iran, and fourth pattern are in second order. The summer pattern has lowest rainfall pattern during the 11 years of the study.
The transfer and placement of center core of high pressure over the seas in the south of the country, especially over the Arabian Sea, are involved in southwest flows. It is clear that the moisture injection could be observed in the second and fourth pattern of high precipitation and high pressure core with the stretching on the sea. Moreover, the existence of a trough at 500 mb level over the country has additional impact on the recent condition.
The first and second patterns have less rainfall with the high pressure displacement to the western and northern parts. In the warm season of the year, the expansion of the Ganges low pressure at the lower level and integration and expansion of the subtropical high pressure and Saudi Arabian high pressure at the higher levels over the country result in sharply decrease in the amount of moisture and precipitation in the country. This causes consequently a reduction in the activity of this high pressure. While there are other atmospheric systems in the higher latitudes such as the blocking system and its link with Saudi Arabia high pressure behavior, this can lead to specific situations that caused different precipitation conditions in various parts.
The results showed that the spatio-temporal behavior of Saudi Arabian high pressure such as east-west, north-south displacement and even center core stretching has very significant role in the infusion of moisture and consequently the precipitation over Iran. The spatial distribution of precipitation in the country depends on the location, shape and area of circulation of high pressure. Generally, the high pressure has more impact at cold season with remarkable role in moisture and precipitation of Iran. However, in the warm seasons, the role of the high pressure is greatly reduced due to its integration with subtropical high pressure and transfer to the North over the country and increase in air stability of the region.