عنوان مقاله [English]
North Indian Ocean (NIO), which includes the Bay of Bengal(BoB) and the Arabian Sea (AS),is one of the tropical oceans and therefore, prone to the formation of the tropical cyclones (TC). On a global scale, approximately 7% of the tropical cyclones are formed in this area. Studies indicate an increase in the frequency of remarkably powerful cyclonesin the Arabian Sea in recent years.In the period between May 16 and 27, 2018, two very strong cyclones called Sagar and Mekunu, affected southwestern and western regions of the Arabian Sea. The present study aims to determine the role of large-scale environmental parameters affecting the tropical cyclogenesis during the life period of these two storms.
Data and Methodology
The current study collects data, including the location of cyclones occurrence, tropical cyclone track, the minimum sea level pressure, and maximum wind speed from the report prepared by the India Meteorological Department. Requiredoceanic and atmospheric parameters, including U and V components of wind (at 200 and 850 hPa levels), relative humidity (at 600 hPa level), sea surface temperature (SST), sea level pressure (SLP), air temperature, pressure, and specific humidity at 23 levels of pressure (levels of 1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 150, 200, 250, 300, 400, 500, 600, 700, 775, 850, 925, 1000 hPa) were also extracted from the reanalyzed dataof ECMWF (European Centre for Medium-Range Weather Forecasts)on a daily basis and with the spatial resolution of 0.5°longitude and 0.5° latitude. In order to achieve the goal of the research, first, the values of large-scale environmental parametersplaying a crucial role in TC formation, including absolute vorticity (at 850 hPa level), vertical wind shear, potential intensity, and relative humidity, were calculatedusingGRADS and MATLAB. The related maps were also plotted and analyzed. Then, the genesis potential index of days before the storms occurrence wascalculated for different regions of the Arabian Sea, and the likely areas for cyclone occurrence were predicted based on the index. Finally, some anomaly maps were produced for the atmospheric parameters affecting cyclogenesis, and changes in these parameters were examined in the life period of the storms as compared to the normal climatological conditions.
Results and Discussion
Results indicated that the storms track coincided with the regions in which maximum relative humidity and maximum absolute vorticity occur.During cycloneSagar, relative humidity in areas affected by the cyclone reached over 80%. During the formation period ofcycloneMekunu,maximum relative humidity was observed in the area between 0°N to 10°N and 50°E to 80°E- the area dominated byMekunucyclone. Spatial distribution of environmental variables, such as temperature, sea level pressure, and vertical wind shear indicates that the favorable values of these parameters have been concentrated in the areas affected by the cyclones in all three phases of their formation, intensification, and dissipation.
Although, vertical wind shear did not considerably change in different parts of the Arabian Seaduring the life cycle of Sagar, its minimum levelwas reported in the Gulf of Aden. Similarly, with the increase in wind speed duringcyclone Mekunu on May 25, the minimum vertical wind shear moved to the northern latitudes and its value ranged from 6 to 12 m/s in the western Arabian Sea. The maximum absolute vorticity is observed in the Gulf of Aden during the life cycle of Sagarcyclone, and these conditions continue until cyclone’s dissipation. Also duringcycloneMekunu, maximum absolute vorticity was observed in the areas affected by thecyclone. Affected by the maximum sea surface temperature, potential intensity indexreached a value of more than 70 m/s in regions affected by the storms (20-degree north latitude). Spatial distribution of GPI values collected from the days before the cyclones occurrence indicated that there is a strong correlation between the spatial distribution of this index and the occurrence of cyclones. Furthermore, the storm track also coincided with the increase in this index,so that highest GPI values were concentrated in areas dominated by cyclones Sagar and Mekunu.Analysis of anomaly maps revealed that compared to the long-term average,sea surface temperature and relative humidity have increased in the area affected by tropical cyclones and sea level pressure and vertical wind shear have decreased.
Findings of the present research indicated that dynamic and thermodynamic parameters have provided the most favorable cyclogenesis conditions in the areas affected by the storms. In other words, the cyclone had moved to the direction in whichenvironmental parametersexhibited the best threshold levels. Therefore, it is possible to predict the occurrence of tropical cyclones in the northern latitudes of the Arabian Sea, especially in the Gulf of Oman,based on the changes in large-scale environmental parameters in different parts of the Arabian Sea.