On the basis of S4max data retrieved from COSMIC GPS radio occultation measurements, the long-term climatology of the intensity of the ionospheric sporadic E layer is investigated. Global maps of Es intensity show the high-spatial-resolution geographical distribution and strong seasonal dependence of Es layers. The simulation results show that the convergence of vertical ion velocity could partially explain the seasonal dependence of Es intensity. Furthermore, some disagreements between the distributions of the calculated divergence of vertical ion velocity and the observed Es layers are found. Other processes in the formation of Es layer may be neglected. We suggest that the large-scale horizontal transport of Es should be considered.