Upper ocean freshening and intensification of Tropical Cyclone Bansi in the South West Indian Ocean

In the South Indian Ocean, most cyclones originate just south of the Inter Tropical Convergence Zone (ITCZ) and move westward until they approach Madagascar before recurving east and back into the open South Indian Ocean. However, severe Tropical Cyclone Bansi formed near northern Madagascar, intensified and moved toward the east-southeast during 11–18 January 2015. Several weeks of very heavy rainfall occurred over Malawi and Mozambique, associated with the passage of TC Bansi and tropical storm Chedza, caused widespread flooding, which displaced more than 100,000 people and led to several deaths and considerable damage (http://www.bbc.com/news/magazine-30980324, http://edition.cnn.com/2015/01/17/africa/malawi-flooding/). 

The 850 hPa Geopotential height anomaly prior to Bansi, showed that strong anticyclonic conditions were present to the west-northwest of the formation region of Bansi, thus making propagation toward Madagascar unlikely. The movement of Bansi may be explained by the strengthening of the monsoonal north westerlies toward northern Madagascar, steering the storm toward the southeast.

On the 13 January 2015, Bansi intensified to a Category 5 cyclone with maximum sustained wind speed of 220 km/h and minimum pressure of 923 hPa as monitored by Meteo France. Under WMO agreements, Meteo France through the RMSC at La Reunion, is responsible for forecasting TCs in the South West Indian Ocean. On 14 January 2015, the eye of Bansi was about twice as large as the size of Mauritius due to eyewall replacement (Moderate Resolution Imaging Spectroradiometer aboard NASA’s Aqua satellite) and decreased its intensity to Category 2 before strengthening back to Category 4 within 24 h. On the 15 January, the maximum sustained winds reached 185 km/h, with a minimum pressure of 940 hPa (http://www.meteofrance.re/cyclone/saisons-passees/2014-2015/dirre/BANSI). 



The rapid intensification of TC Bansi might be due to what scientists describe as the ‘barrier layer’, a layer created by fresh water from rains or rivers lying on top of the saline sea surface. Karthik Balaguru, Ocean Scientist at the Department of Energy's Pacific Northwest National Laboratory 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. This intermediate layer prevents mixing of waters, and thus reduces cooling. The heat keeps pumping fuel into the cyclone and hence they become very severe. Mawren and Reason [2017] presented a case study of TC Bansi with the objective of investigating how changes in the barrier layer may have influenced its evolution. In the Southwest Indian Ocean, the Barrier layer is sensitive to freshwater inputs by both precipitation and the advection of low-salinity waters by the South Equatorial Current. 2015-2016 was a very strong positive El Nino year and the westward advection of low salinity water during this period might have given rise to a thicker barrier layer, thus contributing to the strengthening of TC Bansi.


References :
[1] Mawren, D., and C. J. C. Reason (2017), Variability of upper-ocean characteristics and tropical cyclones in the South West Indian Ocean, J. Geophys. Res. Oceans, 122,doi:10.1002/2016JC012028.

[2] Balaguru, K., P. Change, R. Saravanan, L. R. Leung, Z. Xu, M. Li, and J.-S. Hsieh (2012), Ocean barrier layer's effect on tropical cyclone intensification, Proc. Natl.Acad. Sci. U. S. A., 109(36), 14,343-14,347


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©2017 Oceanographer Daneeja Mawren