عنوان مقاله [English]
The notion of climate change indicates a significant change in climate and environmental conditions over a long period of time (from a few decades to centuries). These changes can occur in mean radiation, temperature, precipitation, atmospheric patterns, wind, and other climate parameters. Increased global average temperature and occurrence of climate extremes such as floods, storms, hails, tropical storms, heat waves, sea level rise and melting of polar ice caps are the most important effect of climate change. The present study sought to analyze the effect of climate change and global warming on temperature trends in Iran atmospheric levels. One advantage of the present study is that it investigates temperature changes at sea surface and other atmospheric levels, whereas many recent researches just emphasize on sea level.
Materials and methods
The present study used data received from the European Center for Medium-range Weather Forecast (ECMWF) for a period of 60 years, from 1951 to 2010, with a network resolution of 1 × 1° Latitude and Longitude for sea level data (Slp) and 850, 700 and 500 hPa levels. After converting extracted data using statistical extension of Net-cdf for excel 2007, the temperature trend for sea levels of 850, 700 and 500 hPa were calculated. The correlation between temperature and its anomalies was measured using elevation levels of 850, 700 and 500 hPa and the temperature anomaly maps and synoptic pattern were developed on a regional scale, and finally their relationship with temperature trends were analyzed and interpreted.
Results and Discussion
Iran had an average temperature of 18.06 °C during the 60 year period (1951 to 2010). 1999, with an average temperature of 20 C°, was the hottest year during this time. From 1993 onwards (except for 1997 and 2007), the average temperature was more than the 60-year average (18.06 C°). By comparing 30-year periods (from 1951 to 1980 and from 1981 to 2010) with each other, we observed that sea level temperature increase in the second 30-year period was more than the first period temperature increase at other atmospheric levels. This increase is most possibly due to the effects of global warming. Temperature increase in the first and second periods were 0.24 and 0.63 °C, respectively. Because of closeness to sea level and under the influence of surface conditions, 850 hPa level shows maximum temperature increase compared to other atmospheric levels (after sea level). Also due to the impact of sea level during the first and second periods, this factor is highly correlated with the sea level atmospheric condition. Despite the fact that correlation values of 850, 700 and 500 hPa levels were significant in both first and second periods at 1% level, they have increased in second period at all atmospheric levels. In other words, there is a clear increasing trend in the second period and few decreasing changes are observed.
Regarding the patterns observed at sea level in the second period, two low-pressure closed cell trough which had been observed in the first period in India and Pakistan, merged in the second period. At 850 hPa, the subtropical high pressure located over Atlantic in the first period moved to East Africa in the second period and created a closed high pressure subtropical cell over Libya with an elevation of 1500 hPa. Compared to the first period, this high pressure cell has a higher altitude. At 700 hPa level, STHP ridge extended significantly in the second period, and in this period, central regions of Iran exhibit wide ranges of air sinking with a deep layer of warm air.
Over the 60 year-period, temperature of atmospheric levels in Iran have exhibited an increasing trend, which from 1993 onwards had a much steeper slope of increase. Compared to the first period (with almost normal periods of increasing and decreasing, and a slightly fluctuating rhythm), the second thirty-year period is expected to exhibit a constant and continuous increase. Additionally, warmer SLP at sea level and 850 hPa level, the northward expansion of the Hadley cell, and finally more intense subsidence of STHP toward lower atmospheric levels (above sea level and 850 hPa) exacerbate the effects of global warming on Iran atmosphere.