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The significance of the oceans is that they store a much greater quantity of energy than the atmosphere. Heat transfers between the oceans and the atmosphere have a major influence on climates. In addition, the oceans are major reservoirs of moisture, and coastal zones in particular frequently experience humid and wet conditions that are common to maritime climates.

As in the atmosphere, energy in the oceans is circulated around the world, transferring heat from low latitudes to high latitudes. This energy is redistributed by a global system of ocean currents inter-connected to form the global ocean circulation. Surface ocean currents are driven by the atmospheric winds. Deep ocean currents are generated by the sinking and upwelling of water, due to density differences driven by temperature and salinity (saltiness) contrasts.

A well known and important surface ocean current is the Gulf Stream, which originates in the Gulf of Mexico and flows northeast across the Atlantic, driven by the prevailing southwest winds. The Gulf Stream is a warm ocean current that keeps the climates of UK and Northwest Europe mild in winter.

Unlike the Gulf Stream, which is a permanent feature of the global ocean circulation, the El Niño phenomenon develops roughly every 2 to 7 years, and usually lasts for several months, although on occasion has prevailed for several years. El Niño is the name given to this sporadic development of warm surface waters in equatorial Pacific Ocean. El Niño has a huge influence on regional climate, bringing with in heavy rainfall and flooding to the west coast of South America, and drought and bush fires to parts of Australasia. During strong El Niños, its influence may even extend globally. 1998, for example, became the warmest year on record, in large part due to the presence of a strong El Niño.