Trajectory experiments in a thermocline layer of an Indian Ocean model are used to investigate the role of different meridional transport mechanisms and quantify spreading pathways and rates under different forcing. Particles are introduced along two boundaries: the south Indian Ocean at 30°S and the Indonesian Throughflow. Particles are advected horizontally within the layer by archived model velocity fields (⅓° × ⅓° resolution) for a period of 50 years. The velocity fields are the result of forcing the model by monthly mean climatology (case A). The distribution of particles within the Tropics suggests efficient water mass blending; model results show a mixture of three parts South Indian Central Water to one part Indonesian Throughflow. In agreement with chlorofluorocarbon (CFC) observations, transport of thermocline waters along the western boundary into the northern Indian Ocean occurs on timescales of less than two decades. Additional Lagrangian experiments carried out with the seasonality ... Abstract Trajectory experiments in a thermocline layer of an Indian Ocean model are used to investigate the role of different meridional transport mechanisms and quantify spreading pathways and rates under different forcing. Particles are introduced along two boundaries: the south Indian Ocean at 30°S and the Indonesian Throughflow. Particles are advected horizontally within the layer by archived model velocity fields (⅓° × ⅓° resolution) for a period of 50 years. The velocity fields are the result of forcing the model by monthly mean climatology (case A). The distribution of particles within the Tropics suggests efficient water mass blending; model results show a mixture of three parts South Indian Central Water to one part Indonesian Throughflow. In agreement with chlorofluorocarbon (CFC) observations, transport of thermocline waters along the western boundary into the northern Indian Ocean occurs on timescales of less than two decades. Additional Lagrangian experiments carried out with the seasonality ...