By modifying mixed layer thickness, the barrier layer (BL) may have a significant impact on air--sea interactions. In the tropical Indian Ocean, thermohaline observations are lacking over most of the basin, despite a few BL observations in the Bay of Bengal and off Sumatra. A 15-yearlong model simulation is used to investigate the seasonal variations and formation mechanisms of the BL. In the eastern equatorial area, the model thermohaline structure is in close agreement with observations and simulates a realistic seasonal variability. A robust and thick BL (of more than 40 m) is observed seasonally with a maximum horizontal extent (of about 1500 km) in November. This BL corresponds to the merging of two BLs that undergo different formation mechanisms. The first is formed as the Wyrtki Jet propagates eastward, deepening the thermocline while zonally advecting a salinity subsurface maximum. At the same time, northward wind stresses induce a northward advection of a thin fresh water layer toward the equator. This combination decouples salinity from temperature, and a BL is formed. Later, as the jet penetrates further toward the fresh pool, a second BL appears off Sumatra. This one is controlled by two different mechanisms. A westward surface current trapped in the thin mixed layer off Sumatra advects fresh water from the coastal precipitation area. Then, significant upwelling associated with the westward current further raises the halocline. In situ observations confirm the realism of these simulated formation mechanisms. From year to year, the timing, thickness, and shape of the BLs follow the strong interannual variability of the Wyrtki Jet. Despite these variations, the formation mechanisms remain unchanged.