How Debris Accumulates
Many different ocean features can cause debris to accumulate, and it can do so on a very large scale or small scale. For example, gyres, which spin over huge swaths of ocean, can aggregate debris in their centers. Smaller scale debris aggregations can result from eddies or other tiny features.
What is a gyre?
A gyre is a large system of rotating ocean currents that spiral around a central point, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Worldwide, there are five major subtropical oceanic gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre.
The most studied and notable gyre, the North Pacific Subtropical Gyre, has the tendency, similar to any circulating body of water, to collect debris near its center. This anticyclonic gyre rotates in a clockwise direction and is comprised of four major ocean currents – North Pacific current, California current, North Equatorial current, and Kuroshio current. Because it is a dynamic system, a gyre’s exact size is difficult to measure, but the North Pacific Subtropical Gyre is estimated to be 7 to 9 million square miles. This is equivalent to approximately three times the area of the continental United States (three million square miles).
Marine debris items can become entrained within the North Pacific Subtropical Gyre and accumulate in the center, based on its overall convergence. Gyres can accumulate debris on a very large scale, but they do not produce “garbage patch”-like concentrations.
Another area known to concentrate marine debris is the North Pacific Subtropical Convergence Zone (STCZ). This convergence feature concentrates food sources for many species, making it a high productivity area for feeding and breeding. However, the same forces concentrate marine debris, which comes ashore on the uninhabited Northwestern Hawaiian Islands as the Convergence Zone shifts north and south, passing debris through the island chain.
Meanders, eddies, and other medium-to-small features
Within the transition and convergence zones there are a multitude of medium-scale features such as oceanic eddies and frontal meanders. Think of meanders as the deviation from a straight line. As energy (wind/currents) hit the front, there are undulations and "curvature" which are described as frontal meanders (movements to the north and south along the front).
These features can concentrate and scatter debris within the convergence zone depending on the location and type of feature. They produce the accumulations, such as the one that occurs in the North Pacific Subtropical High, that are sometimes referred to as “garbage patches.”
On a small-scale, marine debris may be concentrated by Langmuir circulation. Langmuir circulation is the result of the interaction between wind-driven surface currents and surface waves. Though Langmuir circulation may be present in weak or no-wind situations, it is most often seen when the wind speed is 1.5 m/s or greater.