In 1893, Fridtjof Nansen, a Norwegian explorer, embarked on an ambitious expedition. He intentionally trapped his ship, the Fram, in Arctic sea ice, aiming to drift with the ice across the North Pole. While Nansen didn’t reach the geographical North Pole, his three-year voyage provided invaluable scientific observations. One notable discovery was that ice drift didn’t align with wind direction; instead, it consistently deviated to the right. This intriguing phenomenon would later be recognized and named Ekman Transport.
Wind blowing across the ocean surface exerts a force due to friction, setting the water in motion. However, the Earth’s rotation introduces a crucial factor: the Coriolis effect. In the Northern Hemisphere, this effect deflects moving water to the right of the wind direction. Conversely, in the Southern Hemisphere, the deflection is to the left. This initial deflection at the surface is just the beginning of a more complex vertical pattern.
The movement of ocean water isn’t uniform with depth. The surface layer moves at an angle to the wind, influenced by the Coriolis effect. Water just beneath the surface also moves, but with a slightly greater deflection and reduced speed. This pattern continues downwards, with each deeper layer turning further and slowing down, creating a spiral effect known as the Ekman spiral. This spiral of moving water typically extends to a depth of 100 to 150 meters (330 to 500 feet).
An Ekman spiral (A) illustrates the rotating water column caused by the Coriolis Effect deflecting water movement from the wind direction. Ekman transport (B) shows the net water movement at a right angle to the wind direction in the Northern Hemisphere. In the Southern Hemisphere, the deflection is to the left. Source: NOAA.
Ekman transport is defined as the net or average direction of water movement throughout the entire Ekman spiral. Remarkably, this average transport direction is approximately 90 degrees to the wind direction – to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It signifies that while surface currents might initially follow the wind’s general direction, the overall effect of wind on ocean water, considering depth and the Coriolis effect, is a perpendicular movement of water masses.
The explanation and mathematical model for the Ekman spiral and Ekman transport were developed in 1905 by Swedish scientist V. Walfrid Ekman. Crucially, Ekman’s theoretical work was inspired and grounded in the empirical observations made by Fridtjof Nansen during his Arctic expedition. Nansen’s meticulous records of ice drift provided the real-world evidence that prompted Ekman to investigate and ultimately describe this fundamental oceanographic process. Ekman transport is a cornerstone concept in understanding large-scale ocean circulation and the intricate interplay between wind, the Earth’s rotation, and ocean currents.
