The interphase momentum coupling and growth in interfacial mass flow rate corresponding to liquid jet injection into a quiescent gas are studied in the present work via highly-resolved simulations. It is shown that the spray formation process is composed of three regimes. In the first regime, which is closest to the nozzle, the surface of the jet undergoes interfacial perturbations and breakup; however, the internal liquid core remains intact, and the interphase momentum exchange is weak. In the second regime, primary atomization occurs, resulting in the breakup of the entire jet. Besides the creation of relatively large amounts of interfacial area, this second regime is characterized by a huge increase in the momentum coupling between the gas and liquid phases. In the last regime, both phases merge into a quasi-equilibrium state, where the atomization process is essentially complete, and both phasic velocities converge to a unique average value. In a subsequent analysis, this unique value is shown to be given by (U-inj rho(L)Omega(L)) / (rho(L)Omega(L) + rho(G)Omega(G)).