Every-Changing Global Climate Patterns

Enlarge NOAA/Pacific Marine Environmental Laboratory/Tropical Atmosphere Ocean Project The 1997 El Nino featured abnormally warm sea-surface temperatures in the eastern Pacific Ocean (shown in red). This map shows the how much the sea-surface temperature differs from the normal value for that month (in degrees Celsius). Patterns of air pressure and winds high above the Arctic helped bring the snow that piled up in your driveway last winter. Similarly, abnormally cool sea surface temperatures over the central Pacific — known as La Niña — could help increase the number of Atlantic hurricanes this summer and fall. These are examples of long-term global climate patterns that can reach far around the globe to affect the day-to-day weather. Meteorologists often call these climate patterns "oscillations," since they fluctuate on time scales ranging from days to decades. Fifty years ago, scientists and meteorologists had little grasp of the importance and influence of these regional and global oscillations on weather. However, increased interest and research funding, along with improved technology, have helped meteorologists sharpen their understanding of many of the most important oscillations. Interactions between the atmosphere and the oceans are at the core of most of the patterns. You may think of the atmosphere and ocean interacting much like two partners dancing — mirroring each other's movements, though not always at the exact same time. The atmosphere and ocean push and pull against one another. When one makes a move, it affects the other. Since the ocean is much slower to respond to environmental changes than the atmosphere, the two players are unequal partners. The discovery of one of the most important global climate oscillations, El Niño-Southern Oscillation (ENSO, for short) began with observations the British meteorologist Gilbert Walker made in the 1920s. Walker was stationed in India and wanted to know what caused fluctuations in the strength and effects of the South Asian monsoon. He noticed that a strong monsoon season in India often occurred at the same time as severe droughts in Australia, Indonesia, and even parts of Africa. After investigating further, he noted correlations between periods of high air pressure in the eastern Pacific occurring at the same time as periods of low air pressure in the western Pacific. Walker proposed a climate pattern called the "Southern Oscillation" to describe this seesaw in global atmospheric pressure and weather patterns. It was not until the 1950s and 1960s, however, when meteorologists linked Walker's hypotheses with observations of fluctuations in ocean temperatures off the west coast of South America and Walker's "Southern Oscillation" became the "El Niño-Southern Oscillation." El Niño refers to one extreme in the oscillation — with above-average sea surface temperatures in the central Pacific ocean — while La Niña refers to the other extreme — below-average sea surface temperatures in the central Pacific. Since the discovery of the El Niño-Southern Oscillation, scientists have found several other important global and regional climate patterns, including the Arctic Oscillation (AO), the closely linked North Atlantic Oscillation (NAO), and the Pacific Decadal Oscillation (PDO). There are other global climate patterns, such as the Pacific-North American Oscillation (PNA) and the Madden-Julian Oscillation (MJO), among others, but less is known about these oscillations, and they typically have less of an impact on global weather. The Arctic Oscillation (AO) has a significant influence on winter weather in the U.S. — the northern and eastern U.S., especially — as well as Western Europe. Recent research has also demonstrated a link between the AO and tropical cyclone formation during the Atlantic hurricane season. The AO refers to a seesaw pattern in atmospheric pressure between the polar regions and the middle latitudes. It fluctuates on a different time scale than ENSO, on the order of weeks and months, though it also shows some tendency to favor one phase or another for years at a time. The AO features a negative (cold) phase, which brings higher-than-normal pressure over the polar regions and lower-than-normal pressure over the middle latitudes; the positive (warm) phase brings the opposite conditions. Most climate scientists consider the North Atlantic Oscillation (NAO) to be a regional manifestation of the AO. In short, they both refer to the same climate phenomenon. When the AO/NAO are in their positive phase, much of the U.S. experiences mild winter weather; when the AO/NAO are in their negative phase, much of the U.S., especially the North and East, experiences cold and stormy (often snowy) weather. The Pacific Decadal Oscillation (PDO) is important as well, but scientists don't know as much about it as other climate patterns. In fact, some researchers argue that the PDO is not its own climate oscillation, but simply ENSO fluctuating on a much longer time scale. In any case, the PDO is similar to ENSO, though on a time scale of decades instead of seasons. It is marked by fluctuating sea surface temperatures in the north-central Pacific as well as near the Gulf of Alaska. The PDO mainly affects weather patterns in the U.S. Pacific Northwest. This was especially true of the two PDO events that occurred in the 20th century and lasted 20 to 30 years. One phase of the PDO is called the "cold" phase, the other the "warm" phase. Not all of the weather that the USA and the world experiences is a result of large-scale global climate oscillations, but if you looked down your snowy driveway last winter in the Northeast, or marvel about how rainy it is in Florida one spring, chances are that one, or more, of these global climate patterns is hard at work.

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