The annual intensification of the Somali jet, which accompanies the onset of the Indian summer monsoon, is rapid compared to the evolution of the seasonal insolation forcing. Using observationally-based data sets, the dynamic and thermodynamic changes accompanying the onset of this jet are presented. The abrupt component of jet onset is shown to occur over ocean about 1000 km east of the East African highlands, and is accompanied by increases in both deep convection and baroclinic flow over the off-equatorial Arabian Sea. These abrupt changes are well separated from the core of the cross-equatorial jet, which is located over land adjacent to the East African highlands. The onset of the Somali jet and the associated monsoon are then examined in a convective quasi-equilibrium framework. Such a framework is consistent with the mean summer state, in that peak free-tropospheric temperatures are nearly collocated with peak subcloud layer entropies over the northern Bay of Bengal and adjacent coastal regions. Jet onset is accompanied by a large (O 100 W m-2) increase in surface enthalpy flux over the Arabian Sea that is nearly collocated with, and linearly related to, the concurrent increase in deep tropospheric ascent. At the same time, the highest subcloud entropies shift inland slightly from the Bay of Bengal, and the region of warmest free-tropospheric temperatures expands poleward. The consistency of all of these changes with a wind - evaporation feedback and several other hypothesized mechanisms of abrupt monsoon onset is discussed.