the ocean and climate change
  Physical coupling with the atmosphere

Physical coupling with the atmosphere
The ocean couples with the atmosphere in two main ways. The first way is physically, through the exchange of heat, water, and momentum. Covering more than 70 percent of the Earth's surface and containing about 97 percent of its surface water, the ocean stores vast amounts of energy in the form of heat. The ocean receives most of its heat along the equator, where incoming solar radiation is about double that received at the poles. Hence, sea surfaces are much warmer along the equator than at the poles.

Ocean and atmosphere move because they are fluid. The speed and direction of air and sea currents are determined primarily by air temperature gradients. As heat rises and eventually escapes the ocean to warm the overlying atmosphere, it creates air temperature gradients and, consequently, winds. In turn, winds push against the sea surface and drive ocean current patterns. Over time, a complex system of currents was established whereby the ocean transports a tremendous amount of heat toward the poles. Because heat escapes more readily into a cold atmosphere than a warm one, the northward flow of ocean and air currents is enhanced by the flow of heat escaping into the atmosphere and, ultimately, into outer space.

The ocean has a high temperature and momentum "inertia," or resistance to change. Relative to the atmosphere, it has a very slow circulation system, so changes in its systems generally occur over much longer timescales than in the atmosphere, where storms can form and dissipate in a single day. The ocean changes over periods from months to years to decades, whereas the atmosphere changes over periods of minutes to hours to days. The interactions between ocean and atmosphere are fully nonlinear, and occur over decades, which is why their "dialogue" is so hard to interpret.

New climate models provide an excellent way to crack the code. Recently, scientists have suggested that the atmosphere provides the means for the ocean to extend its reach globally and set off, like dominoes, chains of meteorological events. During the 1997-98 El Niño, for example, the we witnessed record levels of rainfall in southern California—where it is normally arid desert. Just before the onset of an El Niño, we see the tropical Indian Ocean warm dramatically. Then the warming seems to propagate across the Pacific. About 9 months into an El Niño, the new trade wind patterns cross over South America and change the current patterns of the tropical Atlantic, bringing drought to Brazil, where there is normally lush rain forest, and the African Sahel. Other scientists have theorized that ocean temperatures oscillate between hot and cold, like some decadal climatic pendulum swing. It becomes clear that there is an almost mechanistic system by which the ocean drives climate change, which is why it was dubbed by scientists as the "global heat engine."

next: The ocean's chemical interactions with the atmosphere
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As the ocean absorbs incoming sunlight, its surface warms. The ocean emits some of its heat up into the atmosphere, both in the form of thermal energy and water vapor, creating winds and rain clouds. In turn, surface winds push against the surface of the ocean, creating currents that help control the distribution of warm and cold waters. Where surface waters are cooler, they allow even colder, deeper depths to upwell. Where sea surface temperatures are cold, local air temperatures also tend to be cooler due to the surface winds dragging across the water. On the other hand, where sea surface temperatures are warm, local air temperatures tend to be warmer due the heat emitted by the water. In short, ocean and atmosphere are intertwined in a complex and perpetual dance—with each following the other's lead. (Illustration by David Herring)

Other Ocean Fact Sheets:
What are phytoplankton?
What are coccolithophores?
El Niño
La Niña