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Using trajectory tracing method from 308 long-lived surface drifters, the subtropical surface circulations in the southern Indian Ocean (SIO) are investigated in the views of both Lagrangian and Euler.Analysis of the results from the Lagrangian view confirm the two pathways of seawater transport from high to low latitude based on different types of drifter trajectory. One pathway is that the seawater formed in high latitude flows northward along the eastern boundary of SIO, joints the westward South Equatorial Current (SEC), and reaches east of the Mozambique along the north side of the Madagascar Island, then bifurcates into northward and southward branches: the northward branch cross the equator and finally joints the north Indian Ocean circulation system; the southward branch meanders along the Mozambique Channel and forms a series of mesoscale eddies. The other pathway is that the seawater flows northward through the central SIO, then turns to west following the SEC, and flows back to the south when reaches the east of the Madagascar Island. Based on the trajectory of drifters, the mean life cycle of subtropical circulation is about 43±20 months. In Euler view, zonal and meridional velocity, mean kinetic energy (MKE), and eddy kinetic energy (EKE) are calculated in 0.5°×0.5° grid using Poulain’s method. The high MKE reaches over 1000 (cm/s)2 in the regions of South Equatorial Current, Agulhas Current/Agulhas Return Current, Eastern Madagascar Current, and Eastern African Coastal Current, the two latter of which are western boundary currents. The EKE has similar spatial distribution with the MKE in the region west of Australia. The spatial variations of current in several areas are significantly modulated by mesoscale eddies, such as in the regions of Agulhas Retroflection, western basin of SIO, and west of Australia. Agulhas system, especially in the area of Agulhas Retroflection, has highest MKE and EKE, due to the interaction with the bottom topography and the wind stress curl induced instability. West of Australia, the EKE is remarkably larger than the MKE due to active mesoscale eddies. In particular, Leeuwin Current can be detected by drifters in the region. For the South Indian Ocean Countercurrent (SICC), although previous studies suggested an eastward current near 25°S, this study does not find drifters following eastward along this proposed current. However, similar result in EKE distribution as previous studies suggest that the eddy activities are much more significant that the mean current in this proposed SICC region. Ocean surface drifters have important role in track the missing crafts. The long-lived drifters passing around the Reunion Island are collected and traced back to their origins and to rebuild the track of missing MH370’s wrecks. Three possible pathways are rebuilt and two possible crash sites are identified around 35°S, 95°E and 38°S, 92°E, which are consistent with former results from satellite but show more evidences and details. The latest news said that some wrecks were found in the coast of Mozambique, which can be further validated by analysis of drifter trajectories in this study. Along both pathways, passing by north or south of Madagascar Island, surface drifters can reach the coast of Mozambique in different landing time. Both the currents/circulations in the SIO basin and Mozambique Channel contribute to the routes of the wrecks. In this study, the subtropical surface circulation in the SIO is investigated from surface drifters, which help us to understand the mass transport between high and low latitude. A few puzzles still remain, such as there is none of trajectory supporting the SICC at the sea surface. The use of surface drifters on tracing the MH370’s wrecks provides a good try to search missing craft in the ocean, and thus suggests more possible applications in the future in this isolated ocean.

Subtropical surface circulation in the southern Indian Ocean derived from surface drifters