Abstract:
The influence of Sea Surface Temperature (SST) on tropical cyclone formed in the Bay of Bengal was examined, using 314 months (November 1981•December 2007) of National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation version 2 weekly mean SST data. The study area was from 5.5-21.5ºN to 80.5-95.5°E; with a total 272 grid points at 1º × 1º grid spans were found. During the study period, 162 disturbances were formed over the Bay of Bengal. 91 were cyclones, among these cyclones 38 were cyclonic storms (CS), 23 were severe cyclonic storms (SCS), 28 were very severe cyclonic storms (VSCS) and 2 were super cyclones (SC). More than 86% cyclones are formed in the observed months having positive SST anomalies. The average of the contemporaneous SST at the formation time of CS, SCS, VSCS and SC was 28.93, 29.08, 29.27 and 29.41ºC, respectively. The frequency of cyclone shows positive trend in pre-monsoon season and negative trends all other seasons with increasing SST. The duration of cyclone shows positive trends in winter and pre-monsoon seasons and negative trends in monsoon and post-monsoon seasons with increasing SST. The formation of SCS, VSCS and SC starts after 27.50ºC and increases with increasing SST but discontinuously. However, the intensity of cyclone has a step-like rather than continuous relationship with SST. It is seen that the depressions which are formed in April have 100% probability of intensification to convert into VSCS or SC. The most active zone for powerful cyclone (VSCS and SC) formation is located within area-3 (7.5ºN ≤ latitude < 13.5ºN) where the temporal average SST is higher (around 28.70ºC) and the rate of declining temperature with increasing latitude is nearly constant (0.01ºC/latitude). The retention time of the disturbances within the area-3 shows the highest value. At the initial stage, the speed of the disturbance remains less. So the consumption of heat energy from the reservoir, which has nearly constant 0.010ºC/latitude and higher SST, remains lower. As the heat acts as fuel for cyclone, adequate heat energy lingers the cyclone to survive in area-3. It is found that when the disturbance moves to the higher latitudinal direction the speed gradually increases and the SST decreases. As the SST decreases the heat energy also decreases. It may be due to the augmentation of wind speed as the supplied energy does not cope with the burning up of heat energy. For this reason the highest number of cyclones die out within area-1 (within 17.5ºN ≤ latitude < 21.5ºN) where the SST decrease was the highest of value 0.36ºC/latitude.
