Unlocking the mysteries of Red Sea eddies
First-time reports of the statistical properties of ‘whirlpools’ show they are frequent and seasonal.
The Red Sea is a narrow basin with warm and highly saline water that supports a rich and diverse marine ecosystem. It is of immense ecological and commercial importance, yet little is known of a crucial oceanographic aspect— the properties of its eddies.
Eddies are clockwise or counter-clockwise circular movements of water that play a significant role in transporting heat, nutrients and organic material in the ocean. Now a KAUST and US research team shows that these marine ‘whirlpools’ are more frequent than had been supposed and that they follow a distinct seasonal pattern.
“In the Red Sea, eddies profoundly affect the social and economic lives of people living in the surrounding countries,” explains Ibrahim Hoteit from KAUST’s Division of Physical Science and Engineering. “Knowledge of how regularly these eddies occur and behave would help, for instance, improve local ocean forecasts, as well as help the coastguard undertake search-and-rescues; responses to oil spills or concentrate discharges; or marine planners to formulate conservation plans.”
To gather information on eddy properties, the KAUST researchers and colleagues at Scripps Institution of Oceanography in San Diego, USA considered satellite data collected since 1992 along with numerical model simulations of the Red Sea circulation1.
The team analyzed the number, size, and frequency of eddies observed. Over the 20-year data period, the team was surprised to find 4,998 eddies — a higher number than they expected — given the long narrow shape of the Red Sea. Eddies are usually observed in the open ocean where strong winds and currents form. The data revealed most eddies occurred in the central basin and the average eddy life span was six weeks.
The team also identified a significant seasonal cycle of eddy intensity. Peaking in February — perhaps due to the cooling effects of the winter wind jets blowing to the north — the intensity and spin tended to gradually decrease until August. After a sharp drop, eddy frequency then increased to a second peak from September to December.
Eddy size — measured in radii from its center to edge — ranged from 35km to 200km, with an average radius of 94km. While eddies may vary greatly in size, the researchers were interested to find that eddy radii was proportionate to the width of the Red Sea basin which meant the larger eddies occurred in the sea’s wider central region.
“As the Red Sea is a relatively narrow basin, most eddies can only occupy about half of its width, providing rapid transport of organisms and nutrients from one coast to the other,” says Hoteit. Next steps for the team is to quantify eddy transport of heat and nutrients in the region.
Zhan, P., Subramanian, A.C., F. Yao, F. & Hoteit, I. Eddies in the Red Sea: A statistical and dynamical study. Journal of Geophysical. Research: Oceans 119, 3909–3925 (2014).| article