Ekman transport and pumping
Ekman transport and pumping are oceanic phenomena resulting from wind interactions with water surfaces. When wind blows over the ocean, it causes surface water to move, which drags deeper layers of water due to friction, creating a spiral effect known as the Ekman spiral. This effect demonstrates that the direction of water currents is influenced by the Coriolis effect, resulting in currents that veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As wind patterns vary horizontally, they induce vertical movements of water, a process termed Ekman pumping, which occurs when displaced water is replaced by water moving upwards against gravity.
These mechanisms play significant roles in ocean dynamics, leading to upwelling and downwelling, which are crucial for redistributing heat and nutrients within the ocean. They also have implications for climate variability, impacting patterns such as the Northern and Southern Annular Modes and the El Niño-Southern Oscillation. Changes in these oceanic processes can be influenced by atmospheric conditions, including increases in carbon dioxide, which may alter wind-driven ocean currents and thermal distributions. Understanding Ekman transport and pumping is vital for studying ocean circulation and its broader effects on climate systems worldwide.
Subject Terms
Ekman transport and pumping
Definition
When the wind blows over a body of water, it exerts a force that causes the water on and near the surface to move. Each layer of moving water pulls or drags the layer immediately underneath it. This continues into the depth of the body of water, until the drag reaches the bottom or becomes vanishingly small, whichever occurs first. This process is known as Ekman transport, and it is related to the so-called Ekman spiral. Although this spiral had been observed earlier by Fridtjof Nansen (1861-1930), the Norwegian explorer, diplomat, scientist, and 1922 Nobel Peace laureate, both the transport and the spiral were named after Vagn Walfrid Ekman (1874-1954), the Swedish oceanographer. It was the latter who conducted the first scientific study of this phenomenon and published his results. The research project itself was identified and assigned to Ekman by his mentor and teacher, Vilhelm Bjerknes (1862-1951), the Norwegian physicist and meteorologist.
The Ekman spiral is a twisting structure of liquid or gas currents that arises near a horizontal boundary. The net effect is that, as the flow moves away from the horizontal boundary, its direction rotates, thereby creating the physical structure of a spiral. The Ekman spiral is related to the Coriolis effect (named for Gaspard-Gustave de Coriolis, 1792-1843), a phenomenon that is due to the rotation of the Earth and used to explain why it is that objects moving on the surface of the Earth, or in its atmosphere, do so at an angle to the forces that one applies directly to them. Theory and experiment show that, in the Northern Hemisphere, objects move to the right of applied forces, while in the Southern Hemisphere, they move to their left. The helps explain part of the Ekman spiral.
When a wind that blows on the surface of the sea varies in the horizontal direction, it induces horizontal variability in the Ekman transports, which creates vertical velocities at the top of the Ekman layer. The creation of vertical velocities is necessary, because the mass of ocean water that flows through a fixed region of space must be conserved (displaced water is replaced). This effect forces water to move up the Ekman layer, against the downward pull of gravity. That action is known as Ekman pumping. In other words, the existence of horizontal divergence in the Ekman transports creates vertical velocities in the upper boundary layer of the ocean.
Significance for Climate Change
When the wind blows on the ocean’s surface, the direction of surface currents that are so created do not line up with the direction of the wind. Instead, they move at an angle to it: In the Northern Hemisphere, they move to the right of the wind, and in the Southern Hemisphere, they move to its left. As the effect of the wind moves deeper and deeper into the water, the angle between the direction of the wind and that of the water current of each succeeding layer increases in size.
Consequently, if the currents at the different levels of depth could the viewed from above the ocean surface in the time sequence of their occurrence, then, in the Northern Hemisphere, one would see a water current twisting itself progressively to the right with deeper and deeper penetration, while in the Southern Hemisphere, a similar current would twist itself progressively to the left with deeper and deeper penetration of the ocean. In each hemisphere, the average angle across all depths between the direction of the wind and that of the water current is 90°. Given that the surface of the Earth is covered mostly by oceans, these large-scale movements of ocean water have an influence on the climate.
Patterns of large-scale climate variability in each hemisphere of the Earth are studied using annular modes: a Northern (NAM) and a Southern annular mode (SAM). They are used to explain the variance in atmospheric flow with time that is not associated with the changes in seasons. The El Niño-Southern Oscillation (ENSO) is a third example of a large-scale pattern of climate variability that is historically tied to the interactions between the ocean and the atmosphere in the tropical Pacific. All three patterns are affected by what happens in the oceans, as well as in the atmosphere, and they particularly reflect the sea surface temperature fields of their respective hemispheres, which are affected by the surface fluxes of latent heat, sensible heat, and heat due to Ekman pumping.
Ekman transport and pumping are very important in the study of general circulation in the world’s oceans, because they create upwelling and downwelling of ocean water. Upwelling occurs when water from below the surface of the ocean is forced to come to the top. Downwelling is the reverse of upwelling. Spatial variability in wind current leads to upwelling near the shore. In the open ocean, it leads to both upwelling and downwelling, which redistribute the mass of water in the ocean. Therefore, upwelling and downwelling due to Ekman transport and pumping are leading mechanisms in the variability of the heat contents in the upper oceans.
Furthermore, simulations indicate that Ekman pumping and oceanic wind-driven circulations respond to increases in atmospheric carbon dioxide (CO2), one of the greenhouse gases. These simulations have shown that modest increases in CO2 in the atmosphere have several effects on wind-driven circulations in the oceans in the Southern Hemisphere. They change and intensify the distribution of wind stresses; increase the rate of water circulation, Ekman pumping, and deep-water upwelling in the southern oceans; and expand the subtropical gyres toward the poles in both hemispheres.
Bibliography
Ahrens, C. Donald. Meteorology Today. 9th ed. Pacific Grove, Calif.: Thomson/Brooks/Cole, 2009.
Price, James F., Robert A. Weller, and Rebecca R. Schudlich. “Wind-Driven Ocean Currents and Ekman Transport.” Science 238, no. 4833 (1987): 1534-1538.
Sverdrup, Keith A., Alison Duxbury, and Alyn C. Duxbury. Fundamentals of Oceanography. New York: McGraw-Hill, 2006.
Zhang, Linfang, Yaokun Li, and Jianping Le. "Impact of Equatorial Wind Stress on Ekman Transport During the Mature Phase of the Indian Ocean Dipole." Climate Dynamics, vol. 59, 21 Feb. 2023, doi.org/10.1007/s00382-022-06183-7. Accessed 20 Dec. 2024.