doi:10.1038/nindia.2016.166 Published online 23 December 2016
Tropical cyclone Vardah, which wrecked the Indian city of Chennai and some coastal districts of Tamil Nadu and Andhra Pradesh states earlier this month, may have become more furious owing to what scientists describe as the ‘barrier layer’, a water-sheet created by fresh water from rains or rivers falling on top of the saline sea surface.
Ocean scientists studying tropical storms and cyclones, say that during such cyclones, fresh water from rains or river discharges falls on oceanic saltwater creating a barrier layer from the cold water below. The layer prevents mixing of waters, and thus reduces cooling. The heat keeps pumping fuel into the cyclone and hence they become very severe.
Cyclone Vardah, with a wind speed of up to 110 kilometres per hour (kmph) and gusting up to 120 kmph, pelted as much as 38 cm of rain in some areas. The India Meteorological Department called it a ‘very severe cyclone’.
Karthik Balaguru and his colleagues from the Pacific Northwest National Laboratory in Seattle, Washington have been studying links between such salinity stratification in ocean layers and cyclone intensity through 35 years (1979-2013) of data from north-western tropical Pacific Ocean cyclones. Hurricanes, typhoons and cyclones are the same weather phenomena, and are called so depending on the location where they occur. Their research1, which uses a combination of observations and coupled climate model simulations, shows that fresh water input into the ocean increases layering, reduces sea surface cooling and leads to stronger storms.
“The fresh water reduces the mixing induced by the cyclone and acts as a ‘barrier’ for cooler deeper water from getting into the surface warm layer,” Balaguru told Nature India. This process helps maintain the warm sea surface temperature and promotes intensification of cyclones, he explains.
Cyclones and storms are heat engines. They are fuelled by the water vapour evaporating off a warm sea or ocean. They keep going so long as water vapour is available to them, kicking up winds and pouring rains. The winds churn the surface water of the sea and dredge up colder waters from below, and help in mixing with one another. This brings down the moisture-making capacity of the heat, depleting the fuel and weakening the cyclone.
However, fresh water upends this process, helps draw upon the vapour coming off still warm waters, and makes cyclones all the more intense.
The researchers had earlier analysed2 587 storms and cyclones in vastly different zones such as the western tropical Atlantic, the western Pacific and the northern Indian Ocean. They found that the rate at which a storm strengthens increased by about 50% on an average when cyclones passed over barrier layers. Some other studies3, 4 also looked at fresh water impact on cyclones in the Bay of Bengal. They show that fresh water reduces storm-induced mixing leading to intense cyclones.
Fresh water effects and anomalies are significant for India as the country receives most rainfall from June to September, the monsoon months. The river discharge into the Bay of Bengal affects tropical cyclones more strongly post-monsoon. Most of the large rivers surrounding the Bay of Bengal – Ganges, Brahmaputra and Irrawaddy – discharge into it, Balaguru says.
Fresh water mechanism affecting the intensity of cyclones is one among many forces governing cyclones, the others being sea surface temperature, ocean warming, current La nina phase and local disturbances. “The monsoon helps feed these rivers and hence the fresh water barrier layer effect is typically larger during the post-monsoon months,” Balaguru adds.
1. Balaguru, K. et al. Global warming-induced upper-ocean freshening and the intensification of super typhoons. Nat. Communications (2016) doi: 10.1038/ncomms13670
2. Balaguru, K. et al. Ocean barrier layers’ effect on tropical cyclone intensification. P. Natl. Acad. Sci. USA (2012) doi: 10.1073/pnas.1201364109
3. Neetu, S. et al. Influence of upper-ocean stratification on tropical cyclone-induced surface cooling in the Bay of Bengal. J. Geophys. Res-Oceans (2012) doi: 10.1029/2012JC008433
4. Sengupta, D. et al. Cyclone-induced mixing does not cool SST in the post-monsoon north Bay of Bengal. Atmos. Sci. Lett. (2007) doi: 10.1002/asl.162