A vessel returns to the traditional fishing village Lavanono, inshore of the upwelling zone at the southern end of Madagascar, in a region where the newly discovered coastal current could play a key role for the local fisheries. Courtesy/Mathieu Rouault
While investigating coastal upwelling that nourishes marine wildlife and fisheries off the coast of Madagascar, Ramanantsoa et al. identified a previously unrecognized current.
The Southwest Madagascar Coastal Current (SMACC) is a rare example of a subtropical surface current flowing in a poleward direction off the western coast of a continent or a large island—in this case, Madagascar.
Previous studies have provided a general picture of currents near Madagascar: The Indian Ocean’s South Equatorial Current, which flows east to west, splits in two at the east coast of the island. The resulting North Madagascar Current flows toward the northern tip of the island, while the East Madagascar Current flows along the island’s eastern coast to its southern tip. However, finer details of ocean circulation around Madagascar have remained unclear.
To better characterize ocean dynamics in the region, the researchers investigated upwelling off Madagascar’s southern coast. They analyzed shipboard observations of water speed, direction, salinity, depth, temperature, and more. They also evaluated satellite observations of sea surface temperature, data on temperature and salinity from free-floating Argo instruments, and a computational model of ocean dynamics in the area.
The analysis revealed that much of the water involved in upwelling in the region is not transported by the East Madagascar Current as previously thought. Instead, it comes from the western side of the island, traveling down the southwest coast of Madagascar. This discovery suggests the presence of a previously unknown, warm surface current: SMACC.
According to the observational and computer modeling data, SMACC’s average length is about 500 kilometers, and its average width ranges between 50 and 100 kilometers. It extends from the surface to a depth of about 150 meters upstream and about 70 meters downstream.
Driven by winds, SMACC flows faster in warmer seasons, but its average speed is 20 centimeters per second. It transports an average of about 1.3 million cubic meters of water per second, almost comparable to the Leeuwin Current, a similarly rare poleward-flowing warm current off Australia’s western coast.
Because SMACC plays a central role in the ocean upwelling system, which “fertilizes” coastal marine life with nutrient-rich waters, its identification could help improve ecosystem and fisheries management in the region. With further research, it could also help explain other ocean features recently observed in the region. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075900, 2018).