Science Results : Daily Update

Daily Update | Current SEA Research

June 23, 2010

By Giora Proskurowski

Today marked the eastern edge of our expedition as we changed course at 28°00'N x 40°36'W. We will be sailing west from here on out. Favorable winds and weather, combined with a well-planned cruise track, leave us well positioned – in terms of time and fuel – for the westward leg back to Bermuda. Our nets are still pulling up large numbers of plastics; and despite the daily forced confrontation with our species' negative impact on the environment, spirits are extremely high. It remains to be seen if the cataclysmic amounts of plastic we sampled on Monday will reappear during the next 2000 miles of our saw-toothed journey west.

I will now turn over the science report to Kara Lavender Law, the principle investigator on this project, who is providing shore support back on the East Coast. Kara has been instrumental in turning SEA's long-term plastics record into a useable dataset, and her physical oceanography background has provided the framework for our understanding of the distribution of plastics in the North Atlantic.

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By Kara Lavender Law

If you have been following the daily science updates you have probably noticed repeated references to a plastic "accumulation zone", or the "subtropical gyre" as the region where we expect to find high concentrations of plastic debris. Outside of this region concentrations of plastic are relatively low, or in many cases, even zero. Just what is this region and why does it exist?

To answer this question we must discuss the physics of the ocean and atmosphere. Before you decide to stop reading right here, let me say that it's not as complicated as it may sound. Let's start with a couple simple facts:
1- The sun heats the Earth unevenly, with the tropics receiving more heat than the poles.
2- The Earth rotates on its axis, completing one revolution per day.

It is because of the simple uneven heating of the Earth (fact #1) that we have winds. Warm, humid air heated at the Earth's surface in the tropics is "light" (less dense), and therefore rises above the colder, more dense air above it. Colder surface air at the poles moves towards the equator to replace this rising warm air. The movement of this air is wind.

Because we are on a rotating Earth (fact #2), the wind does not move in a straight line from the poles to the tropics. Instead, it curves due to the Coriolis effect – curving to the right of its original direction of motion in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect is not very intuitive, so you'll have to take my word for it.

This begins to explain why we observe easterlies (winds blowing from east to west) and westerlies (winds blowing from west to east) in different regions (latitude bands) of the globe. Easterlies typically occurring in the tropics are called the trade winds, while westerlies typically occur in mid-latitudes. The latitude that this expedition has been following, 30°N, is an approximate boundary between the trade winds and westerlies, and as such typically experiences weak winds. You may have noticed the relatively light and variable winds reported by the Cramer, except when a low pressure weather system passes through.

The wind is important to the Cramer – she is a tall sailing ship, after all, with a limited supply of fuel for the diesel engine. But the wind is also one of the major forces that cause ocean currents, the movement of water that is analogous to wind in the atmosphere. The wind blows over the surface of the ocean, and the friction between the moving air and the water below causes the sea surface to begin moving in the same direction as the wind. But, similar to wind, the water doesn't simply move in a straight line. Enter the Coriolis effect again...

Together, the Coriolis effect and the friction between "layers" of water in the upper few hundred feet of the ocean cause the water to move 90° to the right of the wind in the Northern Hemisphere (and to the left in the Southern Hemisphere). This means that water in the mid-latitudes (approximately 30 to 60°N) moves equatorward, and water in the tropics (about 0 to 30°N) moves poleward, causing water to "converge", or come together, at about 30°N latitude, the boundary between the two latitude bands. This is referred to Ekman transport, named after the oceanographer who first explained this phenomenon in 1905.

Not only does the water in the surface layer converge, but so does anything floating at the sea surface – such as Sargassum weed, plastic or other floating marine debris, or even natural debris such as wood that somehow found its way into the ocean. Therefore, we expect to find high concentrations of plastic debris in a region centered on 30°N. This is referred to as the subtropical convergence zone.

And this is what SEA has observed over the past 25 years. On more than 200 SEA Semester cruises between Woods Hole, Mass., and the Caribbean Sea, students and scientists have observed high concentrations of plastic in the subtropical gyre. The subtropical gyre is defined by clockwise currents flowing around the North Atlantic Ocean between roughly 15 and 45°N, with the subtropical convergence zone at its center. These currents are driven by the effect of Ekman transport on the water column below (which means these currents are ultimately driven by the wind).

The Plastics at SEA Expedition has continued to observe the phenomenon of plastic debris collecting in a region centered at a latitude of 30°N. This is why, with few exceptions, the neuston net tows have found plastic in concentrations of 50,000 pieces per square kilometer or more. Not every tow has had this much plastic. This illustrates how the ocean behaves in a far more complex manner than described by the simple theory above. While the basic prediction from theory holds true, the variation in winds and other parameters such as the sources of this plastic debris, cause the variation (or "patchiness") in plastic concentration the crew is observing.