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Ocean temperatures are not constant. They vary from predominantly warm to predominantly cold over large areas in multidecadal cycles, or oscillations. Two oscillations among many that influence North America's climate are the Pacific Decadal Oscillation (PDO) in the North Pacific and the Atlantic Multidecadal Oscillation (AMO) in the North Atlantic.
When warm water dominates the eastern tropical Pacific, heat is carried both northward and southward along the coastline of the Americas. The prevailing winds that carry this warmth east into North America cause increased warming along the West Coast. (This describes El Niño, an oscillation within the PDO.)
Conversely, when cool water dominates the eastern tropical Pacific, cool air is carried both northward and southward along the coastline, and then east by the prevailing winds. In North America, this results in cooling along the West Coast. (This phenomenon describes La Niña, El Niño's opposite.)
In the Atlantic Ocean, the AMO experiences similar warm and cool periods, or oscillations. When waters of the north Atlantic are warm, the East Coast is at risk of more hurricanes, more strong hurricanes, and more hurricanes that make landfall. In addition, warm temperatures may develop in the ocean and coastal areas of Greenland, Iceland, Scandinavia, and other parts of the Arctic, however slightly.
The AMO is in a warm phase that began in 1995 and could last several decades. The PDO was in a warm phase from 1978 to, at least, the late 1990s. In 2007, the PDO reentered its cold phase (on schedule, many believe), and the National Oceanic and Atmospheric Administration (NOAA), among other sources, announced the presence of a La Niña.
When the AMO and PDO are considered together, they can be used as an ocean-warming index. A strong correlation exists between this index and annual mean U.S. temperatures (see graph at left)—one that is roughly two to four times stronger than the correlation of carbon dioxide with the annual U.S. temperatures.
The high correlations of temperatures with both solar cycles (see graph above) and ocean cycles (see graph at left) suggest that the Sun and oceans are themselves correlated. It is possible that the increased heat produced during active solar periods and felt most intensely in the tropics where the Sun is highest in the sky induces the oceans to enter their warm phases. Conversely, a less active Sun can result in less heat and thus cooler ocean waters.
Weather and climate are driven by imbalances. The atmosphere and the oceans compensate for the variability of solar activity (and the resulting variability in heat energy) by moving any tropical heat received during active solar periods to higher latitudes through warm water currents, more hurricanes, and strong winter storms (as occurred in eastern North America earlier in this decade). These events reduce some of the imbalance in temperature, or heat energy, and produce warming in higher latitudes.