Particle trajectories in an Indian Ocean model and sensitivity to seasonalforcing

Trajectory experiments in a thermocline layer of an Indian Ocean model are used to investigate the role of different meridional transport mechanisms a nd quantify spreading pathways and rates under different forcing. Particles are introduced along two boundaries: the south Indian Ocean at 30 degrees S and the Indonesian Throughflow. Particles are advected horizontally withi n the layer by archived model velocity fields (1/3 degrees X 1/3 degrees re solution) for a period of 50 years. The velocity fields are the result of f orcing the model by monthly mean climatology (case A). The distribution of particles within the Tropics suggests efficient ater mass blending; model r esults show a mixture of three parts South Indian Central Water to one part Indonesian Throughflow. In agreement with chlorofluorocarbon (CFC) observa tions. transport of thermocline waters along the western boundary into the northern Indian Ocean occurs on timescales of less than two decades. Additi onal Lagrangian experiments carried out with the seasonality removed from t he velocity fields directly (taking the mean in case B) and from the forcin g (case C) allow the role of horizontal eddy transport to be evaluated. Sig nificant northward transport of southern subtropical gyre waters along the western boundary does not occur unless there is eddy transport, even though the mean flow appears to dominate the cross-equatorial transport in the im mediate vicinity of the equator. Particles reach northward of 10 degrees N on shorter timescales (<20 yr) in case A: compared with case C (>20 yr). Bo th the mean and seasonal forcing components are important for the meridiona l flux of particles. The results suggest that to adequately simulate meridi onal transport of mass and water mass properties in the Indian Ocean, model s should include the full annual cycle. In a new methodology, CFC-II concen trations along trajectories are calculated using observed CFC-11 concentrat ions for boundary conditions. Additional CFC observations allow model-data comparisons to be made in the interior of the domain. The method may be use ful in other studies of transport rates and processes where both computing power and good quality high-resolution observations are available.