Variability of Tropical Cyclogenesis in the Bay of Bengal Across the Phase Space of the Madden–Julian Oscillation
Bradford S. Barrett, Nadim Mahmud, Md Masud‐Ul‐Alam, Pallav Ray, Sahadat Habib, Md Foysal, Most Israt Jahan Mili, Md Hanif BiswasABSTRACT
In this study, probabilities of tropical cyclogenesis in the Bay of Bengal (BoB) were analyzed across the full two‐dimensional phase space of the real‐time multivariate Madden–Julian oscillation (MJO) (RMM) index for pre‐ and post‐monsoon periods from 1990 to 2025. Results show that cyclogenesis anomalies span the phase space and depend largely on the sign of the first principal component of the index, RMM1. Moreover, on days when the intraseasonal index is in phases previously found favorable for cyclogenesis, the anomalies are statistically significant for only a limited range of amplitudes in those phases. Skill scores were calculated for vertical velocity (VV), relative humidity (RH), sea level pressure (SLP), vertical wind shear (VWS), and sea surface temperature (SST) to quantify how well each parameter identifies cyclogenesis in the intraseasonal phase space. Among these variables, 700‐hPa VV was most skillful at identifying both pre‐ and post‐monsoon cyclogenesis anomalies, outperforming RH, SLP, VWS, and SST, although all had high false alarm ratios. The 700‐hPa VV was also more skillful than the 500‐hPa VV, suggesting that it should be included in intraseasonal genesis indices. In the post‐monsoon period, SST showed little to no skill at identifying intraseasonal cyclogenesis anomalies, in agreement with recent work suggesting it be considered alongside other modes of variability. The results of this study fill a critical gap in our understanding of the MJO–tropical cyclone relationship in the BoB by establishing basin‐specific probabilities of cyclogenesis across multiple MJO states and identifying drivers of the variability.