Role of multiple equatorial waves on cyclogenesis over Bay of Bengal

Abstract

Tropical variations on intraseasonal time scales like Madden–Julian Oscillations (MJO), tropical depressions and convectively coupled equatorial waves (CCEWs) are known to modulate the probability of cyclogenesis over different ocean basins. In the present study, a detailed analysis of the role of MJO, Kelvin waves, equatorial Rossby (ER) waves, and combined Mixed Rossby gravity waves and tropical depressions (MT) on cyclogenesis over the Bay of Bengal (BoB) is carried out for a period of 1979–2013. The effect of different waves occurring individually and their combinations on the cyclogenesis over BoB is studied. It is shown that the cyclogenesis is favoured over BoB when multiple waves are present simultaneously than in the presence of a single wave. Individually, ER is most favourable for the cyclogenesis followed by MJO, while MT is the least favourable. A combination of ER and MJO is associated with the highest number of cyclogenesis and the least number of cyclones are formed when Kelvin and MT are present together. Modulation of the atmospheric factors contributing to the cyclogenesis potential, namely vorticity, relative humidity (RH) and vertical wind shear by different waves is discussed. It is shown that the MJO increases the cyclogenesis by increasing RH, while the Rossby wave contributes by increasing the relative vorticity and MT contributes by reducing the vertical wind shear. On the other hand, MT has the least RH while Kelvin wave is associated with the least vorticity, thus contributing to lesser number of cyclogenesis.