Marine heatwaves exhibit pronounced three-dimensional evolution that influences ocean heat storage, air-sea interactions, and marine ecosystems, yet conventional sea surface temperature (SST)-based metrics do not adequately capture their subsurface structure. Using a water mass transformation (WMT) framework, we investigate the life cycle of volume-integrated marine heatwave (VMHW) events in the North Indian Ocean during April–June (AMJ), when VMHWs occur most frequently. Solar shortwave radiation is the primary driver of VMHW evolution in both the Arabian Sea (ArS) and the Bay of Bengal (BoB), while anomalous atmospheric circulation modulates this forcing through changes in cloud cover. During the development phase, the subtropical high suppresses cloud formation, enhances downward shortwave radiation, and promotes rapid VMHW growth. The onset of the southwest monsoon marks a fundamental transition from VMHW development to decay. As the South Asian High and the southwest monsoon circulation intensify, increased cloud cover reduces downward shortwave radiation, leading to the progressive decay of VMHWs. Despite these common large-scale controls, VMHWs in the BoB persist substantially longer and attain greater thickness than those in the ArS. This enhanced persistence results from stronger monsoon rainfall and weaker surface winds in the BoB, which maintain a thicker barrier layer, strengthen upper-ocean stratification, and suppress the entrainment of cooler subsurface water into the mixed layer, thereby delaying BoB VMHW decay. These results demonstrate that VMHW evolution in the North Indian Ocean is jointly regulated by radiative forcing, southwest monsoon evolution, and upper-ocean stratification. Importantly, the onset of the southwest monsoon governs the transition from VMHW development to decay, whereas barrier-layer variability controls basin-scale differences in VMHW persistence.