Ocean gyres are vast, rotating systems of ocean currents that span thousands of kilometers across the world’s oceans. These immense features are fundamental components of global ocean circulation, playing a significant role in distributing water and influencing various oceanic processes. Their scale and continuous motion impact regional weather and marine ecosystems.
Understanding Ocean Gyres
An ocean gyre is a large system of circulating ocean currents with a generally circular motion. These systems are immense, often encompassing entire ocean basins, and typically feature a relatively calm area at their center. There are five major ocean gyres: the North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Ocean gyres.
Gyres are categorized into types based on their location and characteristics. Subtropical gyres are the most prominent, found ringing subtropical high-pressure systems. Subpolar gyres form at higher latitudes, around 60 degrees, and are characterized by a counterclockwise rotation in the Northern Hemisphere and clockwise in the Southern Hemisphere around a low-pressure area. Tropical gyres are also present, exhibiting a more east-west flow closer to the equator.
The Forces Shaping Ocean Gyres
The formation and maintenance of ocean gyres result from the complex interplay of several physical forces. A primary driver is the Coriolis effect, which arises from Earth’s rotation and deflects moving objects, including ocean currents. In the Northern Hemisphere, this effect causes currents to deflect to the right, leading to a clockwise rotation in gyres, while in the Southern Hemisphere, deflection to the left results in a counterclockwise rotation. This deflection is particularly noticeable over the long distances covered by ocean currents.
Prevailing winds also exert a strong influence on surface waters, pushing them in specific directions. For example, trade winds near the equator drive westward-flowing equatorial currents, while westerlies at mid-latitudes push surface water eastward. Only about 2% of the wind’s energy is transferred to the water, meaning a strong wind is needed to create a noticeable current. These wind-driven surface currents primarily affect the top 100-200 meters of the ocean.
Continental landmasses act as barriers, guiding and shaping the flow of these wind-driven currents into their characteristic circular patterns. When equatorial currents encounter continents, they are diverted away from the equator by the Coriolis effect, contributing to the gyre’s circular motion. This combination of Earth’s rotation, global wind patterns, and continental boundaries creates and sustains ocean gyres.
The Ecological and Climatic Importance of Ocean Gyres
Ocean gyres play a significant role in Earth’s natural processes, influencing marine ecosystems and global climate patterns. One of their functions is distributing heat around the globe. Gyres transport warm water from tropical regions towards the poles and cooler water towards the equator, which helps moderate global temperatures and influences regional weather patterns.
The circulation within gyres also impacts marine productivity by transporting nutrients. While the centers of subtropical gyres can be relatively nutrient-poor, the edges often feature upwelling zones where nutrient-rich waters are brought to the surface, fostering abundant marine life, particularly phytoplankton growth. This primary production forms the base of the marine food web, supporting diverse ecosystems.
Gyres also influence the migration patterns and dispersal of marine life. Many marine species, such as sea turtles, rely on gyre currents as navigational aids during their extensive migrations. Fish and marine mammals also utilize these currents to find food or suitable spawning grounds.
Ocean Gyres and Marine Debris
The circulating currents of ocean gyres act as “collection points” for marine debris, primarily plastics. This accumulation leads to the formation of what are commonly referred to as “garbage patches,” though this term can be misleading. These patches, such as the widely known Great Pacific Garbage Patch located within the North Pacific Gyre, are not solid islands of trash but rather vast areas of high plastic concentration. The debris spreads across the water surface, throughout the water column, and even on the ocean floor, often appearing like a “peppery soup” of tiny microplastics mixed with larger items.
The environmental consequences of this accumulated debris are harmful to marine life. Marine animals mistake plastic pieces, especially microplastics (plastic pieces smaller than 5mm), for food, leading to ingestion. This can cause internal injuries, starvation, and the transfer of harmful chemicals up the food chain. Larger debris, such as abandoned fishing nets, also poses an entanglement risk for marine animals.
Addressing this issue presents challenges, as the debris constantly moves with currents and winds, making exact sizing difficult. The Great Pacific Garbage Patch, for example, is estimated to cover an area of 1.6 million square kilometers, roughly three times the size of France, and contains an estimated 1.8 trillion pieces of plastic. Cleanup efforts are complex and expensive. Organizations like The Ocean Cleanup are developing technologies, such as large U-shaped systems, to collect plastic and are exploring ways to convert collected plastic into new products. The continued increase in global plastic production suggests that garbage patches are expected to grow, making prevention of plastic entry into the ocean a key concern.