A giant vortex that forms in the waters off east Africa each year has been tracked in unprecedented detail. This ‘Great Whirl’ might play an important part in the start of India’s southwest monsoon, which drenches the subcontinent in rain but can be difficult to predict.

The Great Whirl typically appears off the Somali coast around May — usually a few weeks before the monsoon kicks in — and lasts until December. Its currents prevent relatively fresh water flowing along the coast from penetrating the Arabian Sea.

Bryce Melzer with Perspecta at the Stennis Space Center, Mississippi, and his colleagues developed an algorithm to track the Great Whirl by monitoring its effect on sea-surface height, which satellites can measure. Piracy off Somalia’s coast makes research trips to the region almost impossible.

The authors studied 23 years of satellite observations and found that the vortex lasted an average of 198 days per year, about a month longer than previous observations had shown. The vortex also covered a bigger area than thought: roughly 275,000 square kilometres, an area greater than that of the United Kingdom.

Learning more about the whirl could help researchers to understand the factors that feed into the monsoon.