Of Weather and Climate

Weather has always been a significant concern to humankind, and our inability to control it has led us down through the ages to try to measure it, compare it to previous years, and predict it. Prediction, however, requires a lot of information about conditions in different locations as well as a way to convey that information between distant places. In the latter half of the nineteenth century, the telegraph made it possible for meteorological data from stations scattered over a huge area to be collected rapidly, leading to the creation of several national weather services. The global observational network grew in sophistication during the twentieth century, especially after launch of the first satellite in 1957. Today, satellites, commercial airlines, and ships at sea take measurements. Information also comes from balloons that are released twice a day into the upper atmosphere by meteorological stations around the globe, as well as by fixed buoys that record temperature several hundred meters deep in the ocean.

Even with all this high-tech help--including sophisticated computer models--we can predict the weather with reasonable accuracy only a few days in advance. How, then, has it become possible for climatologists to anticipate the onset of the El Niño phase of ENSO several months ahead? The answer has to do with how interactions between the ocean and the atmosphere play out over time.

Fundamentally, many argue that the engine that drives long-term "climate" is the heating and cooling of the tropical Pacific Ocean. The sea breeze is a familiar example. On a sunny afternoon the land heats up faster than the ocean; as the air over the land warms and rises, the air over the cooler surface of the ocean flows toward the shore to take its place. Aloft, the warm air returns to the sea, then subsides over the ocean to complete the circuit. The same principles apply to the planet as a whole. Over the course of the year, the sun's rays strike more vertically in the tropical zones than at midlatitudes or at the poles; as a result, the tropical oceans absorb a great deal more heat than do waters elsewhere. As air near the ocean surface is warmed by the equatorial waters, it expands, rises (carrying heat with it), and drifts toward the poles; cooler denser air from the subtropics and the poles moves toward the equator to take its place.

In other words, the atmosphere and ocean together act like a global heat engine. This continual redistribution of heat, modified by the planet's west-to-east rotation, gives rise to the high jet streams and the prevailing westward-blowing trade winds. The winds in turn, along with Earth's rotation, drive large ocean currents such as the Gulf Stream in the North Atlantic, the Humboldt Current in the South Pacific, and the North and South Equatorial Currents. In the tropical ocean, westward-blowing trade winds harvest water vapor over the ocean, carrying it away from one part of the world and depositing it somewhere else. The result of this ocean-atmosphere dynamic is that the Pacific coast of South America, for example, is generally dry, while on the opposite side of that ocean basin Indonesia and New Guinea contain lush jungles. The trade winds also push the warm water in the upper layer of the tropical ocean westward. As warm water piles up in the western Pacific, the cool water in the lower layers of the eastern Pacific rises to the surface.

As researchers have gradually learned, if they have information about subsurface temperatures in certain parts of the tropical Pacific, they can improve their predictions of the behavior of trade winds several months hence. Conversely, if they have information about the behavior of the trade winds, they can predict sea surface temperatures.

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