On a remarkable case of dynamical and physical interaction between middle and low latitude weather systems over Iran

Abstract

This paper contains the results of a detailed study of a wet spell over and near north Iran during the second half of July 1956, which contributed to the severe floods in that country during that month. The daily sea level, 850, 700, 500 and 300-mb level charts for Asia, Eastern Europe and the adjoining areas for the period of the wet spell, the mean 700 and 500-mb charts for the month of July 1956 as a whole and the mean sea level and 500-mb charts for the period of the wet spell only, along with the relevant thermodynamic diagrams have been studied. On the basis of this study, the broad conclusion is reached that the rainfall in Iran during the wet spell even over regions as far north as the Caspian Sea was fundamentally due to the extension of the Indian southwest monsoon into those regions during a month when the monsoon was abnormally active over the whole of India. This development was associated with an unusually pronounced Tibetan High shifting far to the west of its usual position. This abnormal situation resulted in the monsoon easterlies in the lower troposphere being drawn into Iran and the troughs in the middle and upper troposphere which were basically part of closed cyclonic systems in the higher middle latitudes, being blocked and thereby remaining quasi-stationary just to the west of the Caspian Sea. The wave patterns in the upper tropospheric middle latitude westerlies under these conditions caused high level divergence over Iran which provided the main dynamical mechanism for lifting the monsoon air over that country. In addition, on the two days of peak-activity during the wet spell, colder air from the middle latitudes seemed to have undercut the warm humid monsoon air on the southern shores of the Caspian Sea and provided a secondary physical mechanism for lifting the monsoon air on those days. Thus we had in July 1956 the remarkable spectacle of monsoon rainfall over and near North Iran caused by dynamical and physical interaction between middle and low latitude weather systems over that country.