UCT student discovers new current off Madagascar

UCT doctoral candidate Juliano Ramanantsoa’s discovery of a new current off south-west Madagascar has rounded off his research and brought him and his co-authors international commendation.

The Southwest Madagascar Coastal Current (Smacc) was described in Ramanantsoa’s recent journal article in Geophysical Research Letters.

According to the observational-system and computer-modelling data, this wind-driven, poleward-flowing surface current is relatively narrow and shallow – 300m deep and 100km wide – and salty.

It flows more intensely in summer and its physical impact on the ocean is noticeable in a rich upwelling of nutrient-dense waters at the southern end of Madagascar. This has implications for the commercial and subsistence fisheries as well as for the Agulhas Current along South Africa’s eastern shores.

The current transports an average of 1.3 million cubic metres of water a second, and is comparable with the poleward-flowing Leeuwin Current off Western Australia.

The discovery was a major boost for Ramanantsoa’s PhD. With Master’s degrees from the Institute of Halieutics and Marine Science at the University of Toliara, in his home town in Madagascar, and the University of Reunion Island under his belt, Ramanantsoa came to UCT to further his studies in 2015.

Initially it was the mysterious variability in the ocean upwelling off the south-western part of the island that had puzzled Ramanantsoa.

“The only explanation was a poleward, warm surface current moving to the southern tip of Madagascar which influences the upwelling.”

The researchers analysed data, including shipboard observations (water speed and direction, salinity, depth and temperature), satellite observations of sea surface temperatures, surface drifter trajectories from Global Drifter data and a computational model of ocean dynamics in the region.

The analysis proved they were dealing with a previously unknown, warm surface current heading south towards the pole.

The water wasn’t emanating from the East Madagascar Current but from the Mozambique Channel, the 400km stretch of ocean between the island and the Africa mainland.

“Big fisheries blame the seasonal scarcity of fish on subsistence fishers. My work has suggested that production variability is not their fault; it’s the current that varies and affects the fisheries. So understanding the current helps with planning,” he said.

Ramanantsoa said he wants to learn more about the waters surrounding Madagascar, including the south-west Indian Ocean.

He plans to sign up for a post-doctoral thesis and work with his supervisors to develop physical oceanography around Madagascar. His studies will extend his focus, specifically on the south-eastern side.

“This is a huge, well-known current, but it’s unstable and highly exposed to climate change. I want to understand its dynamics. It’s also one of the main contributors to the Agulhas Current,” he said. – Staff Writer