The National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) monthly mean reanalysis dataset has been used to analyze spatial variations of summertime subtropical anticyclones over the Asia–Africa region. The geopotential height and zonal wind components of 1000, 500, 200, and 100 hPa in a 30‐year period (1971–2000) have been used to determine the spatial and temporal variations of the anticyclone centres, their monthly frequency and latitudinal axis variations during April–October. The results revealed that there is a clear difference in the location of the summer anticyclone centres in lower, middle and upper levels of the troposphere. In the lower levels, the Azores subtropical anticyclone is located at the east of North Atlantic. In the middle levels, the frequencies of anticyclone centre are concentrated over the northwest of Africa, Arabian Peninsula and Iranian Plateau. In the upper troposphere, the geographical location of the anticyclone centres and their frequencies in the summer season exhibit a scattered pattern from south of China up to western Iran at 200 hPa, and a bimodal pattern over the Tibetan and the Iranian Plateaus at 100 hPa. In fact, in the entire study domain, the Iranian Plateau is a preferable location of the middle and upper troposphere anticyclones. The highest observed latitude of the subtropical anticyclone at 100, 200 and 500 hPa levels have been seen over north of Tibetan plateau, a large area from east to west of Asia and Iran during August, July–August and July, respectively. The maximum monthly variation in the latitude of the ridgeline is seen at 500, 200, and 100 hPa from June to July which goes even up to 10 degrees at some longitudes. Copyright © 2009 Royal Meteorological Society

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Using daily rainfall data from 183 stations, influences of the Madden-Julian Oscillation (MJO) on precipitation over southern Iran are examined. When composites of daily rainfall anomalies are constructed by introducing eight phases in the time evolution, wet conditions are observed in MJO phases 1-2 and 7-8. During these phases, atmospheric circulation anomalies favor moisture advection from the western Indian Ocean to southern Iran. In contrast, dry conditions during MJO phases 3 to 6 are associated with advection of drier air from the north.

During December 10-13, 1995 a destructive flood occurred over the south part of Iran resulting from the intensive rainfall causing the social and economical damages over that area. This study intends to reveal the main moisture source flux which feeds in the rain fall weather system. The precipitation amount over the study area is four times greater than normal mean in December. The relationships between local atmospheric circulation and moisture flux divergence is investigated for this case of rainfall over south of Iran. The data used in these study includes rainfall data, from data processing center of IRIMO (Iran meteorological organization) and reanalysis daily data at every six hours interval for the same period and eight levels of NCEP-NCAR, with resolution 2.5 degrees in 0-80 E and 10-60 N domain. The high pressure circulation over the North East of Arabian peninsula at surface to 700hPa coupled with trough over the East of Mediterranean and west of the Red sea, transfer sufficient moisture in low levels of the troposphere from west of Indian ocean (Oman and Arabian seas), Aden gulf and Red sea to the center and North of Arabia and south of Iran. The average of perceptible water content in all troposphere level (1000-300 hPa) that comes from the above mentioned region to the South West and south of Iran is about (6-8) 10^-3 kg/m²s^-1. The most divergence of moisture flux in the lower level is from the west of the Indian Ocean while the source of moisture in middle and higher levels are from Aden gulf and the Red sea.

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<p class="western" align="justify">Because of low precipitation and its severe fluctuations in Iran, understanding the dynamics of large scale climate modes and probability of annual and intraseasonal precipitation variation is essential for water management. This study investigates the characteristics of the combined effects of Madden-Julian Oscillation (MJO) and North Atlantic Oscillation (NAO) on precipitation over Iran. Daily precipitation and atmospheric data (relative humidity and vertical velocity) were analysed over Iran during wet season (October to May) for the period 1961 to 2018. The results indicated that: 1) Distinct difference can be observed in spatial distribution of the probability of daily precipitation above upper tercile for MJO phases, phase 1 and 2 wetter while 4 and 5 are drier. The relative humidity is higher in phases (1-3 and 7-8) and lower in phases (4-6). The vertical velocity shows upward (downword) motion in phases 1-2 and 7-8 (3-6). 2) Response of rainy season precipitation over Iran to MJO is more affected by the large-scale atmospheric variation associated with negative NAO as compared to positive NAO. In the negative NAO, the MJO increase (decrease) the probability of upper tercile precipitation 1.2 (0.7) times in phases 2-3 (4-6) and significant tests show a significantly large response for west and North west of Iran. In contrast of positive NAO, the relative humidity and vertical velocity is more affected by negative NAO state. The more (less) humidity and upward (downward) motions increase (decrease) precipitation in phases 2-3 (4-6).</p>

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