عنوان مقاله [English]
An extreme event is an event that rarely occurs in other than normal conditions. Extreme events, stimulate changes in natural and human systems to more than the average climatic conditions (Peterson et al, 2008). Extreme days (veryhot and cold days) have impacts on human and natural systems such as agriculture, ecology, etc. A heat and cold wave is a hot and cold period which lasts from a few days to a few weeks and may be accompanied by high humidity for heat waves, andextreme wind for cold waves. The impacts of heat waves that have occurred in the world are as follows: heart pains, severe shortage of water, heat stroke, rising food prices, power outages, increased natural disasters, outbreaks of insects such as mosquitoes, fire, loss of life and living organisms. Also, frost, frost damages to crops and mortality are the results of the cold waves. By far, there are many studies about this climatic events in the world which have been done for example by Panmao and Xiaohua, 2003, and Peterson et al., 2013. Many studies have confirmed that heat and cold waves are increasing and decreasing respectively in recent years. In Oklahoma, the results of the effects of heat island after a heat wave in the year of 2008 showed that the heat-island at the core of the city is 5 degrees Celsius (during the day)and 2 degrees (during the night) warmer than the rural areas and due to the presence of humidity and heat waves, higher value of heat is generated (Basara et al, 2010). Studies about this extreme events weredone also by few meteorologist and climatologist researchers in Iran, for example Lashkari (2009) did the case study of cold wave which took place over Iran from December 11 to 16, 2003. He found that Siberian high pressure and low temperature are responsible for the extreme cold wave (Lashkari, 1387). By far, cold waves and heat waves have not been studied on coastal regions of the Caspian Sea. The objective of this research is to provide a general overview of heat and cold waves in this area.
This research has studied heat and cold wave events using minimum and maximum temperature which was received from Iran meteorological organization. After sorting the data and examining them for selecting threshold, cold waves with 0.02 percentile and heat waves with 0.98 percentile were evaluated during 1961-2010 in Southern coastal regions of the Caspian Sea over five stations namely Bandar- E -Anzali, Rasht, Ramsar, Babolsar and Gorgan. For better interpretation, wave periods were divided into three periods as follows: long-term waves (more than 6 days), medium term waves (3 to 6 days) and short term waves (less than three day).
For synoptic analysis, NCEP/NCAR reanaly data including sea surface pressure and geo-potential height at the level of 500 were received from the website with 2.5x2.5 resolution for an area covered from 20 to 70 N and 10 to 80E in this region. All the data were analyzed in GRADS software. Then, all the results were interpreted with respect to warm and cold waves.
Results and discussion
Overall, the results showed an increasing trend for minimum temperature and a decreasing trend for maximum temperature. Maximum frequency of cold wave have took place in the years of 1963, 1964, 1969, 1972, 1973, 1989, 1991 and2008.But, continuity of the waves is observable in the year of 1964 and 1972. Maximum frequency of heat wave have taken place in 1962, 1966, 1980, 1983, 2000, 2009 and 2010, while continuity of the waves have been recorded in the year of 1966 and 1971. Ramsar station has recorded maximum frequency of both cold and heat waves. Gorgan has recorded minimum frequency of heat waves, while Rasht and Anzali are specified by minimum frequency of cold waves. The synoptic analysis of NCEP/NCAR sea surface pressure andgeo-potential height at the level of 500 showed that high pressure inEastern Europe, Siberian anticyclone, high pressurein Alpine Mountain and low pressure in Ural Mountains caused the cold waves. Low pressure in Saudi Arabia, Pakistan, andthe Persian Gulf have created heat waves.
So far, no research has been done onheat and cold waves on coastal regions of the Caspian Sea with maritime climate. This research addressed these waves in 1961 to 2010 period, using weather stations and reanalyzed data at five stations with the most comprehensive weather data. Different results were obtained for stations which are linked to synoptic conditions. In addition to weather and NCEP/NCAR data, it is suggested that the use of ERA-interim and satellite data such as MODIS land surface temperature which have different resolution, can be a great help in filling the gaps between stations and improving the obtained results.