## 1 Introduction
Furthermore, due to warming, the intensity of landfalling cyclones, from severe to very severe cyclonic storms (SCSs/VSCSs), escalated 24 h before landfall. The number of such cyclones increased three times during 2011-2020 compared to 2001-2010 over the BoB (Table 1). Such changes are attributed to the enhancement of the necessary and sufficient conditions for cyclone formation in the region, including abundant moisture and warmer SST in recent years. The comparison of SST, moisture, and vertical wind shear over the past two decades shows favorable conditions that facilitated the formation of VSCSs. Table 1 provides details on the formation of DD and VSC storms in recent decades. [PERSON] et al. [15] found that \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Arabian Sea**} & \multicolumn{4}{c}{**Bay of Bengal**} \\ \hline
**Decade** & **D** & **SC** & **VSC** & **Total** & **D** & **SC** & **VSC** & **Total** \\ \hline
1992–2001 & 4 & 4 & 4 & 12 & 13 & 4 & 14 & 31 \\ \hline
2002–2011 & 6 & 3 & 2 & 11 & 14 & 9 & 6 & 29 \\ \hline
2012–2021 & 9 & 2 & 7 & 18 & 14 & 5 & 12 & 31 \\ \hline \hline \end{tabular} where D = depression; SC = severe cyclone; and VSC = very severe cyclone.  
\end{table} 
Table 1: Decadal variability of tropical cyclonic storms over the BoB and AS during OND season during 1992–2021 obtained from RSMC, IMD New Delhi. an increasing trend in SST in the eastern BoB (90-95\({}^{\circ}\) E, 10-20\({}^{\circ}\) N) has led to the formation of stronger tropical storms during the post-monsoon season. There has been a notable shift in the total number of severe to extremely severe cyclonic storms over the past 20 years, occurring more frequently over the southeast and central BoB rather than the south-central region. Further explanation for this shift may involve regional or remote influences, as discussed by [PERSON] et al. [16]. There may be a decadal or large-scale shift in the fundamental atmospheric and oceanic conditions that facilitate tropical cyclone formation. In addition to SST, other factors such as atmospheric latent heat flux, vorticity, wind shear, and moisture content also play important roles in TC genesis and intensification. Low-level tropospheric relative vorticity measures atmospheric rotation, and cyclonic vorticity is essential for TC formation. Wind shear, the change in wind speed and direction with height, can inhibit TC development or intensification. Moisture availability is also crucial, as it fuels TC development, and low vertical wind shear triggers severe cyclonic storms. Studies by [PERSON] and [PERSON] [17], [PERSON] et al. [18], and [PERSON] et al. [19] have suggested that aerosol concentrations in the middle atmosphere play an important role in latent heat release, which probably reduces basin-wide vertical wind shear, creating a more favorable environment for intense tropical cyclones over the NIO. During the past two decades (2000-2020), La Nina years experienced almost twice the number of intense cyclones compared to El Nino years. Additionally, during the La Nina years, the positional shifts in the average cyclogenesis of intense cyclones in the Bay of Bengal align with observations for the western North Pacific Ocean basin. The decadal variability in TC activity over the BoB and AS during the post-monsoon season (OND) is a critical concern, given the increasing frequency and intensity of cyclones, especially in recent decades. This study evaluates the influence of key genesis factors such as sea surface temperature (SST), vertical wind shear (VWS), vorticity, relative humidity (RH), and potential instability (PI) across three decades: 1991-2001, 2002-2011, and 2012-2021. Warm SST anomalies, along with increased atmospheric moisture, have significantly contributed to the rapid intensification of TCs before landfall, particularly in the BoB region, which has witnessed an increase in cyclonic storms, including very severe cyclones (VSCs), during 2009-2019.