While gigantic eddies trap floating particles at the heart of each ocean basin in subtropical regions, other currents link the Southern Indian Ocean to the South Pacific. Stable connection over 8,000 km!
“Variability is key to a stable connection”, assures Christophe Maes. At least when it comes to ocean currents. The oceanographer and physicist has just revealed the existence of a convergence front between two oceans, the Southern Indian Ocean and the South Pacific (1). Paradoxically based on the variable nature of the oceans’ surface currents, this connection allows various floating particles – such as plastic particles which pollute the seas – to be continuously carried from the Southern Indian Ocean to an area in the South Pacific.
Let’s start at the beginning. The effect of winds on the sea surface, combined with the Coriolis force - generated by the Earth’s rotation on itself - directly affects ocean currents and creates gigantic eddies in the oceans: these areas, which cover thousands of kilometres, are referred to as ocean gyres, from the Greek word gyros, meaning “turn”. Five vortices of this type can be found in the North South Atlantic, North and South Pacific and Southern Indian Ocean. And at the heart of each vortex, carried year after year, massive amounts of objects and debris are trapped and form potential accumulation zones for debris of all kinds, the misnamed “continents” of plastic. Even though there are no borders between them, the oceans seem to have enclosed spaces separate from one another.
An 8,000km road
Research by Christophe Maes and his colleagues sheds new light on this situation. Based on computer simulations, the researcher from Brest revealed the existence of outlet currents, allowing debris to escape, notably from the South Pacific gyre to the coasts of South America. “These currents stretched a few hundred kilometres, the researcher indicates. This time, we showed a stable connection over 8,000 km!”. Specifically, researchers confirmed the existence of the five convergence areas, specific to each ocean basin and, more importantly, revealed the presence of a new “route” linking the Southern Indian Ocean to the South Pacific Ocean. To achieve this result, scientists used an ocean circulation model at a spatial resolution of a few tens of kilometres across the globe. “In addition, to simulate the path of the particles, we had realistic data on surface currents for the 1985-2003 period”, points out the oceanographer.
“This is not to say that everything moves from one ocean to another; it is one of many paths”, moderates Christophe Maes. These results undermine the static vision of the oceans and their convergence areas. “These areas should no longer be seen as closed systems. Above all, insists the researcher, the forces at play here are different from those which cause the gyres. They are based on the variability of swirling currents, not on the influence of the wind on surface currents”. The researcher believes this variability should be taken into account in future models to study the dispersion of floating particles, and decide how to deal with this pollution which threatens marine biodiversity and human health by carrying - and spreading - viruses and bacteria.
1. Maes C., Grima N., Blanke B., Martinez E., Paviet-Salomon T., Huck T., A surface “super-convergence” pathway connecting the South Indian Ocean to the subtropical South Pacific gyre.Geophysical Research Letters , 2 février 2018