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In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon. We consider three epochs; namely, the early (1955–74), middle (1975–94) and recent (1995–2014) epochs. It is found that the number of cyclones in the Bay of Bengal (BOB) decreases throughout the three epochs. Over the Arabian Sea (ARB), however, there is a decrease in the early epoch, before then reaching a minimum in the middle epoch followed by an increase in the recent epoch, thus exhibiting epochal variability. Dynamic and thermodynamic parameters along with Genesis Potential Index (GPI) are examined to understand the frequency variation in cyclogenesis over the ARB and BOB. Over the ARB, thermodynamic factors such as mid-level moisture, surface latent heat flux and sensible heat flux, and dynamic parameters such as lower-level convergence and upper-level divergence, are favorable during the early and recent epochs but unfavorable during the middle epoch, and these results are found to be consistent with the observed epochal variability in the frequency of cyclogenesis. However, all these influential parameters are found to have decreased over the BOB during the entire 60-year period. [ABSTRACT FROM AUTHOR]

Located in a monsoon domain, East Asia suffers devastating natural hazards induced by anomalous monsoon behaviors. East Asian monsoon (EAM) research has traditionally been a high priority for the Chinese climate community and is particularly challenging in a changing climate where the global mean temperature has been rising. Recent advances in studies of the variabilities and mechanisms of the EAM are reviewed in this paper, focusing on the interannual to interdecadal time scales. Some new results have been achieved in understanding the behaviors of the EAM, such as the evolution of the East Asian summer monsoon (EASM), including both its onset and withdrawal over the South China Sea, the changes in the northern boundary activity of the EASM, or the transitional climate zone in East Asia, and the cycle of the EASM and the East Asian winter monsoon and their linkages. In addition, understanding of the mechanism of the EAM variability has improved in several aspects, including the impacts of different types of ENSO on the EAM, the impacts from the Indian Ocean and Atlantic Ocean, and the roles of mid- to high-latitude processes. Finally, some scientific issues regarding our understanding of the EAM are proposed for future investigation. [ABSTRACT FROM AUTHOR]