This study presents an analysis of observed data sets from multiple sources, including observations from a network of Argo floats during (2002--2003), with the aim of investigating the role of the southwest monsoon circulation in affecting the interactions between the oceanic mixed layer and the underlying thermocline in the northern Indian Ocean. Examination of the seasonal cycle of the upper-ocean thermal structure shows that the surface cooling of the Arabian Sea, during the southwest monsoon season, is accompanied by significant warming of the thermocline. It is seen that the thermocline is warmer by about 1.2¿C in the south-central Arabian Sea during the southwest monsoon season relative to other months. Offline computations of the profiles of vertical diffusivity of heat reveal stronger and deeper penetration of heat into the Arabian Sea during the southwest monsoon season. The results presented in the paper demonstrate that the combined effects of strong wind-driven mixing by the monsoonal winds, weak density stratification in the upper-ocean, and downwelling in south-central Arabian Sea, along with strong vertical diffusivity, favor downward transfer of warm waters from the surface into the thermocline. Besides the climatological seasonal cycle, the present study also examines the impact of monsoon interannual variability on the upper-ocean response, by analysis of long-term observed records during (1955--2001) as well as the Argo observations for (2002--2003). It is found that the interannual variations in the ocean response reveal signatures of the influence of strong and weak southwest monsoons on the mixed layer and thermocline variabilities.