The effects of Quasi-biennial fluctuations on Iran s winter precipitation

Precipitation is the result of complex atmospheric and oceanic phenomena and it has a special importance due to its vital role among other climatic events and is more complex than other climatic phenomena. Iran is located in the world's arid and semi-arid belt and receives more than half of its annual rainfall in the winter. teleconnection is the alternating and continuous anomalies of atmospheric patterns and particularly pressure on the planetary scales with relative prolonged return period. So it can be considered one of the key elements on climatic forecasts. Quasi-Biennial Oscillation (QBO), recognized in 1961,is one of the main oscillations on the planetary scale in the stratosphere layer with a mean return period of 26 months and is also one of the main components of short-term fluctuations in the climate. QBO can vary the surface weather with effect on polar atmospheric patterns. It can also affect the amount of ozone depletion on high geographical latitudes, in addition to the effect on solar cycle and connection with other teleconnections such as ENSO resulting in the climate change of the earth.

The total monthly precipitation from 100 synoptic stations from Iranian Meteorological Organization and QBO index data of National Oceanic and Atmospheric Administration(NOAA) during 1988-2017were used as the basic input data of the study. First, rainfalls from December to February were considered as winter precipitation and the corresponding QBO index was used. Then, precipitation data were standardized in order to make comparable data with QBO. The normalized data of winter rainfall as dependent variable and QBO index as independent variable were entered into the STATSTICA software. Pearson correlation coefficients were performed between them in each station. No simultaneous effect may be observed due to long distance between QBO generation areas and Iran, therefore, a three months lack also was tested, and hence, the correlation coefficients between autumn QBO and winter precipitation of the stations were calculated. In order to validate of correlation coefficients, precipitation difference of the stations in the positive phases of QBO compared with negative phases. In the next step, the correlation coefficients outputs entered into the GIS environment and mentioned maps were drawn using IDW approach. Finally, the trend chart of the winter precipitation anomaly was prepared to study the impacts of autumn and winter QBO on it.

The results showed that there is a significant inverse relationship between the positive phases of autumn QBO with winter rainfall in most stations, especially in the central and southern parts of Iran. The increase in the intensity of the positive phases in autumn QBO causes subnormal winter rainfall in the central and southern parts of the country. On the other hand, the occurrence of negative autumn QBO causes a minor increase in the rainfall rates of most parts of Iran. However, the occurrence of rainy winter in the case of autumn negative phases is not conclusive. But, the winter time extreme negative phases of QBO index lead to a significant decrease in the precipitation rates of most areas of the country especially southern and western parts, while the positive phases lead to minor decrease (increase) in the precipitation rates of the southern (northern)parts of Iran. The separate study of positive and negative phases of QBO in autumn and winter seasons revealed a contradictory effect on winter rainfalls in different parts of Iran, and in short, the effect of QBO on winter precipitation is lower in northern parts. The final results showed that the occurrence of rainy winter has been linked to the mild phases of QBO index in the autumn and winter. It can be concluded that the QBO teleconnection is one of the main factors controlling winter precipitation in different parts of the country, especially in the southern, central and western sectors. The main and tangible role of QBO is reducing the amount of precipitation. Meanwhile, there is no significant relationship between QBO index and winter precipitation of the northern parts of Iran, especially the northwest and Caspian Sea coasts.

In general, the correlation between the autumn and winter QBO index and winter precipitation is negative in the western half and positive in the eastern and southern parts of the country. So, by moving from the extreme negative values of QBO to the positive ones, a relative partial reduction is observed in the precipitation rate of northwest regions, western sides of Alborz Mountains and the western parts respectively, and there is an increase in other parts especially in the Caspian Sea coasts and the southern Iran. The autumn phases of QBO index are slightly more related to winter precipitation. Unlike the previous condition, there is a negative correlation between the positive phases of QBO and winter precipitation rates and this is more significant in most regions especially in central and southern parts of Iran. The highest correlation coefficient (-0.65 to -0.82) was observed on the shores of the Persian Gulf and the positive phases of this index happened to decrease the precipitation rates in the central and southern regions of Iran. There is no significant relationship between the negative phases of autumn QBO and winter precipitation rates. On the other hand, positive phases of winter QBO reduced precipitation rate in south and southwest regions and partially increased rainfalls in the northwest of Iran. However, the amount of rainfall decrement is more significant in the southern regions. Also, the occurrence of negative phases in the winter reduced the precipitation rate in most parts especially in the western parts, the coast of Persian Gulf and some parts of the eastern Iran. The occurrence of rainy winter has always been linked to the gentle phases of QBO index and it has not been associated with strong positive and negative phases. It seems that the extreme positive phases in autumn and the extreme negative phases in winter time have more significant effects on the reduction of rainfall in most parts of Iran especially in southern regions.