Gyres and climate change
Gyres are large-scale oceanic circulation patterns that influence climate and marine ecosystems. They are characterized by circular movements of water, shaped by wind and the Earth's rotation, with major gyres located in both the Atlantic and Pacific Oceans. Each gyre consists of distinct currents, including narrow, deep western boundary currents like the Gulf Stream, and wider, shallower eastern boundary currents such as the California Current. These currents play a crucial role in heat distribution, transferring warmth from the equator to higher latitudes and affecting regional climates, such as contributing to milder temperatures in Europe compared to North America.
Climate change significantly impacts gyres and their functions. Increased freshwater influx into the Arctic due to global warming threatens to disrupt the warm water flow of currents like the Gulf Stream, potentially cooling the Northern Hemisphere. Additionally, variations in gyral strength can influence hurricane intensity, coastal upwelling, and nutrient availability, affecting marine biodiversity and fisheries. As global temperatures rise, studies indicate that oligotrophic regions—areas with low productivity—are expanding, leading to the formation of larger oceanic deserts. This expansion, which has been observed to exceed predictions, raises concerns about the long-term health of marine ecosystems and their capacity to support marine life.
Subject Terms
Gyres and climate change
Definition
The oceans’ gyres are basin-scale circulation patterns in which the net flow of water occurs in a circular pattern around the basin. Each is generally made up of four distinct currents that are driven by wind stresses, and its circulation direction is governed by the Coriolis effect. The major, subtropical gyres in the Atlantic and Pacific Oceans are located between the equator and approximately 45° north latitude, while the smaller, subpolar gyres lie north of that latitude. The Antarctic Circumpolar Current is a gyre that flows continuously around Antarctica, because there are no landmasses to impede it.
Individual currents that make up a gyre have different characteristics. Because of the and resulting differences in sea surface elevation, the western boundary currents (currents that flow on the western side of the and flow northward from the equator) are narrow, are relatively deep (up to 1,200 meters), and have velocities of up to 178 kilometers per day. Examples of western boundary currents include the Gulf Stream in the Atlantic Ocean and the Kuroshio Current in the Pacific Ocean.
Eastern boundary currents have widths of up to almost 1,000 kilometers but are only 500 meters deep. Examples of eastern boundary currents include the California Current in the Pacific Ocean and the Canary Current in the Atlantic Ocean. Offshore Ekman transport associated with the eastern boundary currents leads to upwelling and high levels of productivity in these surface waters. Circulation of the gyres is completed by transverse currents that flow east and west across the ocean basins, connecting the western and eastern boundary currents.
The subtropical gyres are associated with persistent high-pressure regions in the atmosphere, which leads to a net motion of to the center of the gyre—a process known as convergence. These regions of high pressure are associated with low annual rainfall totals, so the of water in the center of the gyres is somewhat elevated. Elevated salinity and convergence lead to downwelling, so the central gyres are regions of low productivity.
Significance for Climate Change
The gyres transfer large amounts of heat from the equator to the poles. For example, the carries heat north from the Caribbean Ocean, travels along the East Coast of the United States, and then curves eastward toward Europe. The heat released from the Gulf Stream may lead to warmer average temperatures in Europe than those found at similar latitudes in North America. It has been hypothesized that global warming will lead to an increased influx of freshwater to the Arctic Ocean, which could block the northward flow of the warm, salty water of the Gulf Stream. In turn, this could prevent heat transport from the equator and could lead to cooling of the Northern Hemisphere.
The strength of the gyral currents can vary on decadal timescales, such as is observed in the Atlantic Multidecadal Oscillation (AMO), which manifests as cycles in average sea surface temperatures (SSTs), as lesser or greater amounts of warm water are transported from equatorial regions. The intensity of hurricanes is strongly dependent on SST, with stronger, more frequent hurricanes occurring when SSTs are higher. Thus, if global temperatures increase, the amplitude of the AMO will increase, leading to the circulation of warmer water and increased hurricane intensity.
Decadal variations in gyral flow can also affect the extent of coastal upwelling, salinity, and nutrient concentrations. Scientists from the Georgia Institute of Technology have discovered the existence of the North Pacific Gyre Oscillation. Variations in the mode of the oscillation are thought to cause historical variations in fish populations that are critical to Pacific fisheries. While such oscillations are part of the natural climate cycle, evidence indicates that the amplitude of the oscillations may increase with global warming.
Water in the center of the gyres is isolated from the rest of the world’s oceans. An important consequence of this isolation is the low levels of phytoplankton productivity of the water—a situation described as “oligotrophic.” The major source of nutrients to support productivity in the central gyres is upwelling of deep waters. However, downwelling, not upwelling, occurs in the central gyres, making these waters the equivalent of large, open-ocean deserts. As global temperatures have increased, the size of these ocean deserts has increased. Scientists from the National Oceanic and Atmospheric Administration and the University of Hawaii have examined a nine-year series of remotely sensed ocean color data from the SeaWiFS satellite and concluded that these open-ocean deserts have expanded by up to 15 percent. The expansion of the oligotrophic regions is consistent with computer models of oceanic vertical stratification in the gyres, but the rate of expansion exceeds all model predictions.
Bibliography
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Polovna, Jeffrey J., Evan A. Howell, and Melanie Abecassis. “Ocean’s Least Productive Waters Are Expanding.” Geophysical Research Letters 35, no. 103618 (February 2008).
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