|Physical coupling with the atmosphere
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."