Marine debris pathways across the southern Indian Ocean

The distribution of marine debris both with the open oceans and along coastlines is largely governed by surface currents and thereby dependent upon seasonal circulation patterns and annual variability inherent in the major current systems. The recovery of a satellite-tracked current drifter on a South African beach some 22 months after release off Western Australia was an indicator of the ocean-wide processes operating in the southern Indian Ocean gyre. Using this single drift trajectory in combination with data from other current drifters released along the Western Australian coast between 2008 and 2013, from earlier satellite-tracked drifters and a selection of historical drift bottle returns, drift patterns across the southern Indian Ocean between Western Australia and east Africa were analysed to quantify inter-continental debris transport rates via the South Equatorial Current (northern east-west pathway) and the South Indian Ocean Current (southern west-east pathway). The northern east-west trans-ocean pathway can be considered in three segments: the complex and highly variable Leeuwin Current eddy system in the east, which can retain drifting debris for up to a year, the comparatively uniform westward travel in the South Equatorial Current, and the eddying components in the western boundary current system off southern Africa. A simple Eddy Activity Index is derived here to clearly distinguish the Leeuwin Current eddy region from the South Equatorial Current. The southern route to Australia is simpler via the Agulhas Return Current and the South Indian Ocean Current.