چكيده لاتين :
Introduction
Long-term annual extreme climate events, especially temperature and precipitation, have generally been used as indicators for the assessment of climate change. One of the most changeable climatic variables in time and space is precipitation. There are great number of research papers focusing on extreme precipitation on global, regional and national scales have been written. The results of these studies of precipitation and extremes indicate that there are changes in intensity, amount, duration, timing, rate of precipitation and change on trend of extreme precipitation events (IPCC, 2007) and these precipitation irregularities and extremes are impact of climate change. Change on trend of extreme precipitation events have been caused change on severity, permanence, temporal distribution and rate of precipitation in many regions and can be cause drought and floods or other hazards. Generally, Study of extreme precipitations and their frequency is very important in order to understand why they happen and if there are some signs or particular periodicities. Moreover this kind of researches are fundamental in order, first of all, to save human beings, but also to avert or to limit damages done by these extreme events (Boccolari and Malmusi, 2013).
Studies of variability in precipitation can be classified into two categories: long-term changes (trends) and short term (oscillation) (Asakereh & Razmi, 2012). Many studies show that there are a change on trend of extreme precipitation events in Iran, but since the change in this events are associated with indexes and factors that influencing the precipitation of Iran, so this study focus to reveal the extreme precipitation variability in the West and North West of Iran and the relationship of theirs with the effective index of precipitation in Iran.
Study Area
Iran is located in the west of Asia, and in the arid and semi-arid belts. The study province is West and Northwester of Iran, that’s includes six state of Kermanshah, Kurdistan, Hamadan, Zanjan, East and West Azerbaijan (from 44.03 to 52.49 E and 33.37 to 39.46 N). These areas are located near the Iranian border with Iraq, Turkey, Armenia and Azerbaijan in western and Northwestern Iran.
Material and Methods
The aim of this study is analysis of variability of extreme precipitation events on West and North West of Iran and theirs relationship with teleconnection patterns include ENSO, MEI, NAO and AO and also pressure centers includes Mediterranean center (MLP), Sudan low (SLP), Siberian high pressure (SHP) and Black Sea low pressure (BSLP) center and finally with Sunspots.
The extreme precipitation indices that we used are Maximum 1-day precipitation (Rx1day), Maximum consecutive 5-day precipitation (Rx5day), Simple precipitation intensity index ( SDII ), Number of days with precipitation equal to or greater than 10 and 20 (R10mm abd R20mm), Maximum number of consecutive dry and wet days (CDD and CWD ), Total precipitation when daily amounts are greater than 95th and 99th percentile of wet days( R95 p and R99 p ), and Total precipitation in wet days ( PRCPTOT ).
Our used data were limited to daily precipitation data from only 8 of Iranian synoptic stations, includes Tabriz, Oroomieh, Khoy, Saghez, Sanandaj, Hamedan, Kermanshah and Zanjan, that have reliable data and covering period 1961-2010. For time series studies and trend analysis in climatological variables, various statistical methods have been used. In this study the non-parametric Mann–Kendall test, which is frequently used to evaluate statistically significant trends in climatological variables time series was used.
Studies exploring short-term oscillations in precipitation are based on a different group of methods, including autocorrelation functions, harmonics analysis, spectral analysis and wavelet analysis. One of the most common methods used in climate studies is spectral, which was employed in this study. The short term oscillation of macro scale factors influencing variability in precipitation for the middle Zagros includes El-Nino Southern Oscillation (ENSO), Multivariate ENSO Index (MEI), North Atlantic Oscillation (NAO) and Arctic oscillation (AO), pressure variability from the MLP, SLP, SHP and BSLP pressure center, and potentially sunspots, in relationship with extreme precipitation in the north west of Iran have been analyzed.
Results and Discussion
About the main factors that affecting the precipitation of West and North West of Iran, Negative phases of the ENSO index has increased and this factor can reduce of winter precipitation in study area. In AO and NAO, trend in positive phases is increased and by this condition can be caused reduce in annual precipitation. Z-statistic in pressure centers indicates in a way the pressure of those centers over the past 50 years has increased. Change in pressure of these centers probably can affect the pattern of precipitation in the study area and cause change of precipitation rate in study area.
The Z statistic of the 10 extreme precipitation indices shows the trends in all the extreme precipitation indices in more stations have been decreased. Rx1day in Tabriz, Khoy, Kermanshah and Znjan were negative trend and in others are positive. By using spectral analyses technique on Rx1day, significant oscillations 2-3 and 3-5 years have been discovered. This oscillation is related to ENSO, MEI, AO and NAO and also is related to MLP, SLP, SHP and BSLP oscillation. Rx5day only in Hamedan and Sanandaj are positive and the significant oscillations 2-3 is related to ENSO, MEI, AO and NAO, MLP, SLP and BSLP oscillation. SDII trend in Oroomieh, Kermanshah and Zanjan were negative.
The 2-3 years oscillation in Hamedan is related to Sudan low oscillation and 3-5 years in oroomieh is related to ENSO. R10mm in all stations, except in Sanandaj, were negative trend and the 2-3 years oscillation in this index in Hamedan is related to NAO and the 3-5 years oscillation in Kermanshah and Zanjan is related to MLP and also in oroomieh is related to ENSO. As R10mm index, except in Sanandaj, the R20mm and CWD indices the trends in all stations were decreased. R20mm in Kermanshah and oroomieh stations 2-3 years oscillation consequently is related to MLP and ENSO. The CDD trend in Hamedan, Zanjan and Sanandaj were negative and in others station were positive. The 2-3 years oscillation in Zanjan and Hamedan is related to NAO, ENSO or MEI oscillation. The 12 years oscillation of this index in Khoy and Oroomieh is related to 12 years oscillation in Sunspots. The R95p trend in Sanandaj and Saghez were positive and in the others were negative. This trend indicates indicate that number of heavy rainfall in more stations have been decreased. Finally the PRCPTOT in all station, except in Sanandaj, were negative that’s show the trend of precipitation in study area have been decreased. The 2-3 years oscillation in this index in Oroomieh, Hamedan, Kermanshah and Khoy consequently are related to ENSO, NAO and MLP and also in Zanjan is related to ENSO and MLP and in Tabriz is related to NAO, AO, ENSO, MLP and BSLP.
Conclusion
Typically the trend of precipitation in the study area was negative that’s show trend of precipitation has been decreased. Only the precipitation trend in Sanandaj station was positive. The trend of teleconnection patterns in a way that justifies the decrease in precipitation and also this condition applies to pressure centers.
Spectral analyses showed significant oscillations in 2-3, 3-5, 5-8 and 12 years. Major area experienced 2-2 and 3-5 years oscillations. These oscillations periods are due to macro-scales circulations; mainly include the oscillation in ENSO, NAO and AO and also Mediterranean Low Pressure.
