24 November 2010

ILLUSTRATED: THE EKMAN SPIRAL

(The ocean off Tasmania, Australia. This is an unfiltered false-color MODIS {Moderate Resolution Imaging Spectroradiometer} satellite view designed to enhance the reflections from phytoplankton, dissolved organic matter, sediments, and bubbles in the sea—though it may have also picked up reflections from Earth's atmosphere and even from the spectroradiometer itself. Normally, the MODIS images we see have been filtered clean of most of these data. This unfiltered image gives a sense of the ocean's dynamic complexity. HT Discovery Earth. Credit: NASA/GeoEye.)

I've been getting feedback from readers of my book DEEP BLUE HOME wishing for photos or illustrations of some of the species and subjects I wrote about. So I thought I'd post some clarification here on the blog.

First up, the Ekman Spiral, from Chapter 7, "Whorls." Here's the excerpt from the book, now illustrated:

Upwellings are the most biologically productive of all currents: vertical conveyor belts rising from the abyss to the surface, bearing the sunken components of dead plants and animals in the form of dissolved organic matter. This rich broth is destined to fertilize the phytoplankton in the sunlit zone, whence much of the dissolved organic matter originally came. 

(Upwelling in the Northern Hemisphere. Image courtesy of Sanctuary Quest 2002, NOAA/OER.)

Upwellings occur anywhere, including in midocean, though the superproductive ones develop along coastlines, where prevailing winds blow parallel to the shore, pushing the surface waters ahead of them. But because the flow of wind-driven water is also influenced by the Coriolis effect (a spin-off of the Earth’s rotation), the wind-driven current is deflected to the right of the wind in the Northern Hemisphere and to the left in the South. 


In the image above you can see how the Coriolis effect causes ocean gyres in the Northern Hemisphere to spin in a clockwise direction and those in the Southern Hemisphere to spin in a counterclockwise direction as a result of Earth's spinning rotation.


(Credit: Eumetsat.)

The same process drives low pressure storm systems in the atmosphere. You can see a low sweeping across Ireland and Britain in the top of this image, and numerous southern hemispheres lows swirling around Antarctica at the bottom.

BTW, if you really want to trip out in this image, check out this super high-res close-up of it.

The Coriolis effect is a deceptively complex process, as you can see when BBC filmmakers spring the question on some unsuspecting experts in the video below.



Annoyingly, you may have to go here to be amused by this BBC clip.

Back to the excerpt:

And because the ocean is stratified into density layers, the Coriolis effect redirects these tiers too. The surface-driven current, spun by the Coriolis effect, tugs on the layer below it, which tugs on the layer below it, to successively lesser degrees. The end result is a downward whorl known as an Ekman spiral, which corkscrews miles below its originator, the wind. The net work of all the layers is known as the Ekman transport, with a theoretical power to deflect water ninety degrees off the wind. 
























(This schematic shows how ocean currents change direction in relation to the wind direction as a result of the Coriolis effect: (1) wind (2) force of water from above (3) direction of current prior to the Coriolis effect (4) direction of water after the Coriolis effect. Each layer of ocean water exerts pressure on the layer below it and the process is repeated downward. Courtesy Wikimedia Commons.)

Consequently, breezes blowing parallel to a coastline actually result in water flowing offshore at a right angle. The offshore flow is then replaced by water rising from the deep, bearing its Miracle-Gro of nutrients destined to feed blooms of phytoplankton, which feed zooplankton, who feed the sardines, who feed the tuna, dolphins, whales, and seabirds. The plankters that escape being eaten sink to the seafloor upon death, their ghosts eventually resurrected back to the surface to fertilize their own kind, maybe their own kin, some generations hence. Thus the deep blue home recycles matter and energy with impeccable efficiency. 

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