The hazardous, turbulent waters around Africa’s southern tip have sunk countless ships, but they also sustain plentiful fisheries, including abundant sardine and anchovy populations. Fish populations in the region have fluctuated sharply in the past, possibly because of changing ocean temperatures. Now, a study shows that shifting winds are the main driver of long-term temperature shifts in the shallow coastal waters, a finding that could improve fisheries management.
Two powerful ocean currents, the Agulhas Current and the Benguela Current, collide around the southernmost promontory of South Africa, creating the dangerous conditions that once earned the region the moniker “Graveyard of Ships.” The currents swirl over a shallow, triangular shelf called the Agulhas Bank, churning up nutrients that feed plankton blooms and a rich spawning ground for sardines and anchovies.
Fish populations in this area have fluctuated dramatically over the past century, but scientists aren’t entirely sure why. One possibility is changing temperatures along the coast: In 1996, for example, the anchovy population shifted rapidly east when temperatures around the Agulhas Bank dropped by 0.5°C. However, long-term observations of this ocean region are rare, making answers hard to find.
In the new study, Malan et al. used a computer simulation of atmospheric conditions, ocean currents, and wind patterns in the Agulhas Bank to investigate what factors might affect water temperatures from decade to decade. As they tweaked different variables in the model, they found that shifting wind belts, not ocean currents or heating from the atmosphere, were the most important driver of coastal temperatures.
As winds shift direction, their angle relative to the coastline changes. When the wind blows parallel to the coastline, cold, nutrient-dense water gets pushed up toward the ocean surface, a phenomenon called upwelling. When winds blow toward the shore, warm surface water is forced toward the coastline, causing temperatures to rise.
In the simulated model of the Agulhas Bank, altering wind direction forced the boundary of warm water—demarcated by the 17°C isotherm—to shift toward or away from the coastline by up to 100 kilometers, the team found.
The Southern Annular Mode, the north–south wobbling of a massive, westerly wind belt, has a profound impact on local wind patterns in this region. Understanding how large-scale wind belts may affect temperatures in the Agulhas Bank could help fisheries experts manage marine protected areas in the region, the scientists note. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2018JC014614, 2019)
—Emily Underwood, Freelance Writer