While global warming is triggering climate change and changing wind patterns, there is more disturbing news for India. The rapid increase in sea surface temperature (SST) and the associated five-fold increase in Tropical Cyclone Heat Potential (TCHP) will trigger an increase in occurrence of higher categories and higher number of cyclones in the Arabian Sea, says a study titled "Is a warming Northern Indian Ocean Generating more tropical cyclones", by R S Abhinav, Jayu, Narvekar, S Prasannakumar and Evelin Francis, published by the Journal of Operational Oceanography on May 8.
Explaining the findings, National Institute of Oceanography former director S Prasannakumar told TNIE that the fish productivity of Arabian Sea will be affected.
"There has been a decline in the rate of warming of the Bay of Bengal after 1995. An analysis of 60-month data of the Sea Surface Temperature of both the Arabian Sea and Bay of Bengal showed an approximate decadal cycle. Though the overall rate of warming in both the Arabian Sea (0.0118°C per year and the Bay of Bengal (0.0117°C per year was almost the same, the post-1995 slopes were quite different. In the Arabian Sea the rate of warming during post-1995 (0.014°C per year) was higher than that of pre-1995 (0.008°C per year). In contrast, the Bay of Bengal’s post-1995 rate of warming (0.009°C per year) was lower than that of the pre-1995 period (0.013°C per year),” he said.
According to Prasannakumar, the Arabian Sea will witness more number of cyclones with high intensity while there will be a decline in the number of cyclones in the Bay of Bengal. “The Arabian Sea is warming faster than the Bay of Bengal. There will be a biological implication which can affect fishermen. The productivity of the Arabian Sea is linked to upwelling and winter convection. The warming of the Arabian Sea will trigger stratification and the area of upwelling will decrease. Upwelling is the process which brings up cold, nutrient-rich waters to the surface, which encourage seaweed growth and support blooms of phytoplankton. This supports fish productivity. As the sea gets warmer, it will impact the wind shear and the winter convection will decrease,” he explained.
Though the North Indian Ocean contributes to only 6% of the world’s tropical cyclones, its impact in terms of loss of life and property is large as one-third of the global population lives in the countries that adjoin the rim of the ocean.
A study on the temporal distribution of the total number of cyclonic systems and different categories of tropical cyclones in the Arabian Sea and the Bay of Bengal raises two pertinent questions. One, why is there a difference in the long-term trend of total number of cyclonic systems and the occurrence of different categories of tropical cyclones between the Arabian Sea and the Bay of Bengal, and two, why higher category cyclones are occurring more frequently after the mid-nineties, especially in the Arabian Sea? An analysis of the Sea Surface Temperature data from 1960 to 2011 showed that the Arabian Sea was experiencing an accelerated rate of warming after the mid-nineties. In contrast, the post-1995 rate of warming in the Bay of Bengal showed a reduction in warming.
The correlation of carbon dioxide with sea surface temperature showed an increase in the Arabian Sea in the post-1995 period, while in the Bay of Bengal, the correlations showed a marginal decline post-1995. A higher correlation of sea surface temperature with carbon dioxide during the study period implied a dominant role of carbon dioxide in the warming of the sea in both basins. Thus, the overall warming in the North Indian Ocean could be linked to the global effect of anthropogenic greenhouse gas emissions.
In the Bay of Bengal, though the overall warming rates during the study period were similar to that of the Arabian Sea, the trend and variability of the ocean-atmospheric parameters were more complex and often opposing one another. The decreasing trend of the rate of warming of SST in the Bay of Bengal post-1995 could be explained in the context of the cooling of the eastern Indian Ocean by the increased occurrence of positive Indian Ocean Dipole (IOD). The IOD is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.
This led to the overall reduction in the total number of cyclonic systems in the Bay of Bengal. The occurrence of higher category cyclones after 1975 and the appearance of Category 5 cyclones after 1987 must be linked to the availability of higher magnitudes of heat storage in the upper ocean along with higher quantity of moisture availability in the atmosphere in comparison to the Arabian Sea. Hence, whenever the ocean-atmospheric dynamics and thermodynamics were congenial the Bay of Bengal could generate higher category cyclones.
The lack of trend in the total number of cyclonic systems on a decadal time scale in the Bay of Bengal is in part due to the lack of a supportive trend in the relative vorticity and vertical wind shear. These findings will have important implications for the coastal population living in the north Indian Ocean rim countries as it would affect their life and livelihood, the study said.