Forecasting the southwest monsoon variability in the Bay of Bengal (BoB) requires coupled air-sea numerical models with an accurate representation of the atmosphere and ocean physics. Here we additionally explore the importance of biophysical interactions, where we consider how intraseasonal oscillations can modulate primary productivity, and how primary productivity could influence monsoonal variability. The cloudy southwest monsoon decreases light availability, inducing a temporary transition from nutrient- to light-limited productivity wherein subsurface irradiance and primary productivity co-vary, derived from diel cycles in co-located measurements of downwelling irradiance and subsurface chlorophyll-a fluorescence. This has implications for carbon fixation within the BoB, in addition to higher food chain impacts. As phytoplankton concentrations increase, light attenuation also increases, modifying the vertical distribution of solar heating over the upper water column. By employing one-dimensional air-sea numerical models forced with the observed atmospheric forcing, we investigate the sensitivity of sea surface temperature and mixed layer depth to the time- and depth-variable light attenuation. In summary, primary productivity, light attenuation, and sea surface temperature are potentially interdependent during the southwest monsoon, highlighting the need for realistic estimates of light attenuation when forecasting monsoon variability.