عنوان مقاله [English]
نویسندگان [English]چکیده [English]
A tropical cyclone (TC) is a synoptic-scale to mesoscale low-pressure system over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation. Tropical cyclones are massive storms formed in tropical waters, capable of producing violent winds, flooding, and heavy amounts of rainfall. They have significant impacts on the weather and climate of tropical countries. (Riehl, 1979). Thus, the trend of their frequency and intensity in the North Indian Ocean is very important for the maritime regions of South Asia(Singh etal,2000:11). Most of the severe cyclones of the North Indian Ocean are formed during November and May and strike the coasts of India and Bangladesh (Islam and Peterson, 2008).
In June 2007, the Middle East was taken by a rare tropical cyclone that skirted the coasts of Oman and made landfall in Iran. The tropical Cyclone Gonu was formed in the Arabian Sea and traveled northwest towards Oman, intensifying as it tracked across the open waters. Gonu was the strongest recorded tropical cyclone in the Arabian Sea on 3rd of June in a category of 5 hurricanes on the Saffir-Simpson Hurricane Scale (Paterson, 2007). It weakened as it approached the Arabian Peninsula, though it retained the equivalent of category 1 status while very closely passing Sur and Muscat in Oman, and then declined over the Gulf of Oman to the tropical storm status before making landfall in Iran.
The aim of this paper is to evaluate the climatic impacts of Gonu TC in southeast of Iran (Sistan & Baluchestan). In June 2007, during the Gonu activity from birth to destroying, some significant variations occurred in both the surface and lower and middle levels of the atmosphere of the region.
Materials and Methods
For the analysis of the Gonu TC mechanism and storm tracks ,the satellite images of MODIS sensor (Terra and Aqua satellites) and the Meteosat 7 satellite products from NRL(The Naval Research Laboratory) of Monterey Marine Meteorology Division have been used
(NRL, 2008). Also the daily climatic data of meteorological stations of the region were analyzed. The reanalysis dataset was created through the cooperative efforts of the United States National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) to produce relatively high-resolution global analyses of atmospheric fields over a long time period (Kalnay et al, 1996). For this study, we used these data from the onset to destruction of Gonu TC. (2-9 June, 2007).
For this event, surface conditions of the middle and upper atmosphere during the main event days (2, 9 June, 2007) are considered to determine the influences of the Gonu on the climate conditions of the region. The synoptic maps for anomalies of pressure, Geopotential heights, air temperature, specific humidity, vertical velocity (Omega), sea level pressure, precipitation and wind direction and velocity have been analyzed.
Results and Discussion
The results of the daily composite maps showed that with the increasing force of Gonu, the axis of subtropical high pressures in the lower levels of atmosphere migrated to the east, and in the mid levels they moved to the north. This replacement prepared the atmospheric condition for an intense convection and heavy precipitation. From the 7th to the 9th of June, precipitation in major weather stations of region was reported. The maximum amount of precipitation was in Nikshahr (144mm). Chabahar, Saravan, khash, Iranshahr and Zahedan have received some amounts of precipitation during the life of Gonu cyclone. The minimum pressure in the eye of cyclone is calculated to be about 898hp using the images of Metosat satellite and a dipole pattern was prevailed over the region that the max pole (+5hp) was located over Pakistan and the min pole with -4.5hp anomaly was prevailed over the southeast of Iran. The SLP in Chabahar station from 999.5hp in 6th of June increased to 1004hp in June 7th.The Geopotential height in 85hp on June 5th showed a -40m negative anomaly in the east of Oman Gulf and Makran coastline that indicated the northward migration of Subtropical High Pressure (STHP). On June 6th this anomaly increased to -60m. The surface air temperature decreased in most of the stations. In 500HP level the temperature anomaly of up to 8°c occurred and in 850hp, -8°c anomaly was calculated over the region. These anomalies caused severe condensations in 850hp and the release the latent energy in 500hp level. In Chabahar, the wind velocity increased up to double the normal average. The winds with with over 50 knots speed dominated over the northern parts of Oman Gulf and huge waves attack the coastline and the height of some waves was reported to be over 6 meters. The vertical velocity (Omega) maps in 5th of June showed maximum vertical motions over the east and southeast of Oman Gulf. In June 6th these conditions dominated over both the lower and the middle levels of atmosphere which indicated the severe convective activities. By the injection of the moist air from the sea surface to the region in June 6th, the humidity increased 30% in comparison with the normal periods and extended to north of Kerman and southern Khorasan provinces.
The results of daily composite maps showed that with the increasing force of Gonu, the axis of subtropical high pressures in the lower levels of the atmosphere migrated to the east, and in the mid levels they moved to the north. This replacement prepared the atmospheric condition for an intense convection and heavy precipitation. The maximum amount of precipitation was in Nikshahr (144mm).
The omega equation maps confirm the ascending motions over the atmosphere of the region during 4–7 June. Increasing of an intense vertical motion in 500hp level indicated a huge and deep convection over the region. The temperature negative anomalies in the surface (because of the intense cloudiness) and in the 850hp level (caused by vigorous condensation of severe humid air convection) are very important. Most of the stations have shown temperature anomalies of about -10°C and in the 850hp level and also about -8°C in comparison with the normal climatology, but in the 500hp level, because of the release of water vapor latent energy, positive temperature anomalies of about 10°C have occurred.
The variation of velocity and directions of the sustained wind, synchronized with the activity of Gonu over the region, caused a significant variation in winds in comparison with normal conditions.
Over the past decades, the frequency of tropical cyclones in the North Indian Ocean has been registered as significant increasing trends during the months of May and June which account for maximum number of intense cyclones. Based on this trend in the future, the occurrence of such storms in the northern coastlines of Gulf of Oman are more possible.