A 4.5-layer model of the Indian Ocean coupled to a four-component (NPZD) ecosystem model is used to study the phytoplankton bloom in the Bay of Bengal during the Northeast Monsoon of 1996. The model realistically simulates the surface chlorophyll bloom observed by satellite-derived ocean-color images in the western Bay of Bengal during that time. It also reproduces the subsurface chlorophyll maximum present earlier in the fall. The terms in the phytoplankton equation for the model's mixed layer show that the surface bloom is caused by the entrainment not only of subsurface nutrients but also of phytoplankton from the subsurface maximum. To our knowledge, this study is the first to report enrichment of surface chlorophyll by entrainment of the subsurface maximum. Upwelling caused by Ekman pumping and mixed layer entrainment occurring over an already shallow thermocline drives the vertical fluxes. An intense wind event from 26 November to 5 December also generated favorable conditions for the formation of a phytoplankton bloom.