Why Does Western Indian Ocean Circulation Connectivity Matter?

Marine circulation connectivity describes the pathways and timescales over which spatially separated parts of the ocean are connected by oceanic currents. In the Western Indian Ocean (WIO), these pathways and associated timescales are characterised by pronounced seasonal and interannual variability, including monsoon-driven reversal of surface currents in the northern part of the basin.

Understanding the connectivity timescales in the WIO – and their variability – is important for a multitude of reasons. Ecological connectivity between coral reefs is necessary to maintain their biodiversity, understanding downstream connectivity from marine resource exploitation sites is important to understand which areas are likely to be affected, and circulation connectivity is a key concern when designing marine conservation measures. For example, establishing an effective network of marine protected areas (MPAs) requires that they are connected on ecologically relevant timescales (e.g. the duration of species’ pelagic larval stages), but gaps in the existing MPA network mean that decisions need to be undertaken about which areas to prioritise for future protection. Therefore, knowledge of the advective pathways connecting the WIO over these timescales is essential for effective management of the region.

Here, a Lagrangian particle tracking method is used in conjunction with a 1/12° resolution ocean model to elucidate the advective pathways mediated by major surface currents in the WIO. Model experiments are performed with virtual particles released into several major WIO currents and tracked for 100 days, and the resulting trajectories are analysed. Significant variability was found, with advective pathways and timescales sensitive to both season and year of release. The main differences are associated with the different monsoon regimes driving changes in connectivity timescales, and reversing direction of advective pathways in the north of the WIO. In addition to this seasonal variability, interannual changes are explored. Case studies of anomalous connectivity pathways / timescales are presented and discussed in the context of extremes in forcing and larger scale variability, including the Indian Ocean Dipole.