Particle Trajectories in an Indian Ocean Model and Sensitivity to Seasonal Forcing

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 (?° ? ?° 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 removed from the velocity fields directly (taking the mean in case B) and from the forcing (case C) allow the role of horizontal eddy transport to be evaluated. Significant 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 immediate vicinity of the equator. Particles reach northward of 10°N on shorter timescales (<20 yr) in case A compared with case C (>20 yr). Both the mean and seasonal forcing components are important for the meridional flux of particles. The results suggest that to adequately simulate meridional transport of mass and water mass properties in the Indian Ocean, models should include the full annual cycle. In a new methodology, CFC-11 concentrations along trajectories are calculated using observed CFC-11 concentrations for boundary conditions. Additional CFC observations allow model?data comparisons to be made in the interior of the domain. The method may be useful in other studies of transport rates and processes where both computing power and good quality high-resolution observations are available.