The effect of mid-latitude atmospheric circulation changes on the position of the Mediterranean trough and the occurrence of super heavy precipitation in Iran

Spatial variations of pressure patterns under the influence of large-scale atmospheric circulation create different conditions in terms of the intensity of synoptic systems affecting on extreme climatic phenomena. Regarding large-scale patterns, not only the changes of extreme events are considered, but also the changes and developments of synoptic systems on a regional scale are studied. Weather extremes have a great significance for society, as they pose a threat to human life and can result in enormous economic damage and disruption. The physical understanding and timely prediction of extreme weather events are of enormous importance to society regarding associated impacts. Extreme precipitation events (EPEs) is one of the most frequent natural hazards that affects the domain, eventually leading to floods. Synoptic systems in mid-latitudes such as blocking, ridge and troughs cause spatial changes in westerly waves. Spatial variations of atmospheric waves occur when, which normally move from west to east, change their main direction to the north or south. The occurrence of precipitation in the Middle East is related to the changes in waves and the position of the Mediterranean trough. Extreme precipitation events in the Middle East often result from tropical-extratropical interaction, whereby midlatitude forcing and poleward transport of tropical moisture are of central importance. The Mediterranean Sea region is considered to be one of the most important and major areas for circulation in the Northern Hemisphere, so that most of Iran's rain cyclones are formed over the Mediterranean Sea. In other words, Iran's rainfall in the cold period of the year is influenced by the Mediterranean circulation patterns. Evidence shows that the large-scale atmospheric circulation over the North Atlantic plays an important role in the Mediterranean circulation changes. We investigated the effect of mid-latitude atmospheric circulation changes on the position of the Mediterranean trough and the occurrence of super heavy precipitation in Iran. In this research, two category of upper atmosphere data were used. The first is daliy precipitation data for 350 stations that were made available by the Iranian Meteorological Organization (IRIMO). The second dataset is the geopotential height, relative vorticity, potential vorticity, U, V-component of wind and Relative humidity, extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF-ERA5) reanalysis with grids at 0.25×0.25 resolution. In this regard, 150 extreme and widespread rains were identified, then using the sinuosity index, the changes of the most frequent contours (550, 565 and 575 decimeters) during the precipitations were identified over the North Atlantic and Mediterranean. The aim of this study was to find the relationship between the spatial variations of the upper level waves over the North Atlantic and the Mediterranean (-80° W to 70° E, 10° N to 60° N). According to sinuosity index, the minimum sinuosity value is about 1 indicating zonal flow. Any sinuosity value greater than 1 indicates an increase in atmospheric waves. The spatial changes of the Mediterranean trough are due to the meridian changes of currents over the North Atlantic, Europe. The blocking system over the Atlantic was responsible for both the amplification of the downstream trough, as well as the formation and expansion of the trough towards the Eastern Mediterranean. The results show that extremely heavy and widespread rains occur under the influence of the formation of the Mediterranean trough and the expansion of winds from the European side and the strengthening of the North Atlantic trough. The results showed that the position and changes of the Mediterranean trough are associated with the circulation pattern of the North Atlantic. Multiple Atlantic trough are associated with each case of Mediterranean trough downstream. The formation of the Atlantic trough causes an increase in the range of atmospheric waves and the formation of a ridge over Europe and the development of the Mediterranean trough. Under these conditions the Mediterranean Sea experiences 8 longitudinal and latitudinal changes compared to its average. Therefore, the spatial variations of the Mediterranean trough are due to the meridian changes of the European flows and the expansion of the high north side (tropical side) downstream of the western wind. Simultaneously with the formation of blocking over Europe in its eastern part, the dynamic ridge causes the transfer of cold winds over the Mediterranean, which strengthens and expands the Mediterranean trough to the east. These conditions, with the strengthening of the conditions of ascent and eastward movement of the Mediterranean trough, the Arabian Subtropical Anticyclone (ASA) also expands eastward. During the heavy rains of Iran, by strengthening the conditions of ascent and eastward expansion to the Mediterranean trough, the ASA also expands to the east. Therefore, with the location of the ASA over the northern Indian Ocean, the Arabian Sea, and the Oman Sea, more moisture is provided to the atmosphere, which is accompanied by the intensification of extremely intense and widespread precipitation. In general, the position of the Mediterranean trough during heavy rains is such that the western and southwestern regions of the country receive most of the precipitation. These conditions are more severe due to the orographic factor of ascent caused by the Zagros mountains in this region.