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The influence of the Madden-Julian oscillation (MJO) on global tropical cyclone (TC) activity has been well documented in many earlier studies. However, no prior studies have focused specifically on the MJO's impacts on TCs in the Bay of Bengal (BoB). Therefore, the present study examines the impact of the MJO on BoB TC activity during the two peak TC periods i.e. April–June (AMJ) and October–December (OND) from 1974 to 2015. The MJO considerably modulates various measures of TC activity in the BoB, including the number of TCs, the number of TC days, accumulated cyclone energy, the power dissipation index, TC genesis location and TC tracks. TC activity is significantly enhanced (suppressed) over the BoB when the convectively active MJO phase is positioned over the eastern Indian Ocean and the Maritime Continent (the Western Hemisphere and Africa). Alterations in the TC genesis locations and their tracks are more pronounced in OND than in AMJ. These changes in TC characteristics are mainly attributed to MJO-driven modulations in large-scale environmental conditions such as deep convection, mid-tropospheric relative humidity, sea surface temperature, sea level pressure, lower- and upper-level winds and vertical wind shear. A comprehensive knowledge of the MJO-TC relationship may be beneficial for short-term predictions of TC activity in the BoB. • The MJO considerably modulates TC activity i.e. number of TCs, duration, ACE, PDI and their genesis locations and tracks. • Changes in TC characteristics are mainly attributed to MJO-driven modulations in large-scale environmental conditions over the BoB. • TC activity is significantly enhanced over the BoB when the convectively active MJO phase is positioned over the eastern Indian Ocean. [ABSTRACT FROM AUTHOR]

Tropical cloud clusters (TCC) play a vital role in Earth's climate by not only releasing a large amount of latent heat into the atmosphere but also by forming the basis for the development of tropical cyclones (TC). However, not all TCCs can develop into cyclones; only a few develop into TC selectively. There are large uncertainties in the current understanding of why only certain TCCs develop into TC while others don't. The present study employs global TCC observations generated by GridSat and IBTrACS datasets from 1980 to 2009 to investigate the TCC distributions over various Oceanic basins such as the North Atlantic (NA), South Atlantic (SA), East-West and South Pacific (EP, WP and SP), as well as the North Indian (NI) and South Indian (SI) basins. The central objective of the present study is to characterize the size spectrum of TCCs and investigate their potential transformation into TCs. The TCCs are identified based on different IR temperature thresholds in each basin. The present results suggest that ∼ 5.5 % of TCCs were developed into TCs annually globally, and their trends in each oceanic basin are discussed. The size spectrum of TCCs showed a dominant peak at 100–200 km2. About 48 % of TCCs transform into TCs within 24 hr of being identified. Furthermore, 85 % of TCCs develop into TCs within 84 hr of the first identification, while only 5 % of TCCs develop into TCs after 84 hr. Further, we have also analyzed the background environmental conditions such as low-level wind speed, vorticity, divergence, vertical shear, upper-level relative humidly and latent heating (LH) for developing and non-developing TCCs over the NI basin, which have not been explored in detail in earlier studies. It is noted that the relative humidity in the developing composite is around 10–20 % higher than that in non-developing TCCs, and LH in developing TCCs is 0.15 K/hr, larger than that in non-developing TCCs. The significance of the present study lies in investigating the developing TCCs as a function of their size and lifetime, including their long-term trends, and bringing out favourable environmental conditions for developing TCCs in the NI Ocean. • The present results found that overall ~ 5.5 % of TCCs were developed into TCs per year across the globe. • About 85 % of TCCs develop in to TCs within 84 hr of the first identification and only 5 % of TCCs develop into TCs after 84 hr. • The number of developing TCCs in the size range of 100–200 km2 over the various oceanic basin showed an increasing trend in the NA, WP, NI and SI basins. • It is noted that the Latent Heating in developing TCCs is 0.15 K/hr larger than that in non-developing TCCs. • The present study brought out the distribution of TCCs including their size spectrum, life time and long-term trends. [ABSTRACT FROM AUTHOR]