There are cyclic circulation phenomena that occur in the ocean that play a direct role in climate over large areas of Earth. The most significant are the North Atlantic Oscillation (NAO) and ENSO. The most powerful of these cyclic anomalies is NAO. While it does not produce the violent weather effects as dramatic as those of El Niño, NAO is much more consistent. NAO is a system involving two separate pressure systems and their relationship. The first is a high-pressure system that sits over the Azores, a group of islands 900 miles (1,448 km) west of Portugal; the second is a low-pressure system over Iceland. Both of these pressure systems generally exist on a year-round basis. During the spring, summer, and fall, the two systems generally stay weak and do not interact. During the winter, however, they both come to life, and it is their direct interaction that controls the climate over the Atlantic Ocean and all the surrounding continents.
According to NOAA, during the winter months, both pressure systems fluctuate in pressure relative to each other. The greatest effect of NAO is on the storms passing into Europe. Between cyclone circulation patterns over the Iceland and Azores pressure cells, there is an area where they come together to form a steady, strong, forward-moving current of air that channels weather systems from the United States directly into Europe. NAO has the most dramatic effect on storms from December through March each year.
When the pressure difference between the two systems is large, northern Europe has higher temperatures, the Middle East has droughts, and the northeastern United States has warm temperatures. Rainfall in Europe can increase by 0.14 inch (0.36 cm) per day, and temperatures can rise by 5°F (3°C). If the pattern persists, it can extend the growing season up to 20 days in Sweden. When the pressure difference between the two systems is small, the Mediterranean is rainy, Scandinavia is extremely cold, and the East Coast of the United States is cooler.
According to scientists at NASA, NAO varies in a rhythmic pattern from decade to decade. They have determined that since the 1960s, the difference in pressure has noticeably increased for three to five years, and then has noticeably decreased for three to five years, setting up a recognizable cycle. Ocean currents or the formation of sea ice have been suggested as possibilities for this cyclic pattern that has prevailed now for almost half a century. Scientists at NASA are currently working on models to better understand NAO, which would allow them to predict its behavior and allow affected countries time to plan ahead; for example, if a year for poor agricultural productivity seems probable, more food could be stored in reserve for those lean years.
Based on computer modeling work done so far, scientists at NASA believe that changes in NAO are directly related to sea surface temperatures. They are currently working on models to integrate the effects of ocean currents and sea ice, as well as trying to determine whether global warming is playing a part.
El Niño is an abnormal warming of surface ocean waters in the eastern tropical Pacific. It is one part of a larger system called the Southern Oscillation. The Southern Oscillation is a back-and-forth pattern of reversing air pressure that occurs between the eastern and western tropical Pacific and affects the weather worldwide.
When the surface pressure is high over the eastern tropical Pacific, it is low in the western tropical Pacific; conversely, when it is high over the western tropical Pacific, it is low over the eastern side. The phenomenon works like a switch, with the pressure system alternating back and forth. This is where it gets the name oscillation.
Usually, winds blow from the east to the west along the equatorial region in the Pacific. Because the wind pushes the water, it actually piles up the water about 1.7 feet (0.5 m) in the western portion of the Pacific. As a result, the ocean on the east (next to South America) has the deeper, colder water pulled up from below to replace the water that was pushed westward (toward Indonesia). This means that in normal years, the western Pacific has warmer waters and the eastern Pacific has colder waters.
During an El Niño, however, the winds that push this mass of water weaken, which causes some of the warm western water to flow back toward the east, which, in turn, means that less cold water is brought up from the depths. As a result, part of the buildup of water in the west flows back to the east. Because of this and the fact that the cold upwell-ing is minimized, the water in the eastern Pacific becomes warmer, a major component of ENSO.
Once this process occurs, the winds pushing the water get weaker, causing some of the water pushed to the west to flow back to the east. Once the ocean water's temperature rises, it causes the winds to significantly weaken, making the ocean even warmer. This sets up a positive feedback cycle, perpetuating an El Niño pattern. When ENSOs stay strong, the winters in the southern United States are usually very wet, and Indonesia experiences drought conditions. Australia has experienced increased rainfall and flooding.
ENSOs can persist for a year or more and historically have occurred on a three- to seven-year cycle. Computer modeling algorithms today are able to predict an ENSO up to a year and a half in advance. In order to predict occurrences, NOAA operates a network of buoys that measure currents, temperature, and winds. The data collected by the buoys is transmitted to NOAA research facilities in real time.
ENSOs can be detected through the analysis of sea surface temperatures and other collected data. During normal conditions, the western Pacific Ocean is warm, and the eastern Pacific Ocean is cool, with a cold tongue on its extreme eastern edge. The winds in the western Pacific are very weak, and the winds of the eastern Pacific are blowing toward the west. During El Niño, warm water spreads from the western Pacific Ocean (Indonesia) toward the east (South America). The normal cold tongue has weakened, and the winds in the western Pacific, which are usually weak, are blowing strongly toward the east, pushing warm water with them. This makes the water in the center of the Pacific Ocean warmer than normal. Sometimes an event called La Niña follows El Niño. La Niña is a cold event; the cold tongue is cooler than usual by around 5°F (3°C).
Although El Niño has been studied extensively, the exact mechanisms that control it are still not completely understood. According to NASA, not all ENSOs are identical, and the atmosphere does not always react in the same way. Therefore, each episode's characteristics are not predictable, which is why El Niño seems to get blamed for so many unexpected, sometimes violent weather events. Climatologists are becoming better at determining when El Niño events are going to occur, about a year ahead of time.
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