Significant Natural Climate Fluctuations
As seen in other sections, there are many factors influencing Earth’s climate. However even within a relatively stable period, the systems that make up and influence the global climate still naturally fluctuate. These fluctuations or “oscillations” as they are often called (because they oscillate between two main states) can have a large affect on the climate, both locally and on a global scale.
For centuries, fishing people of the coastal communities of northern Peru and Ecuador have used the term El Niño (Spanish for little boy) to describe an annual warming of the offshore waters during December. El Niño is now used to describe extensive warming of the ocean surface across the eastern and central equatorial Pacific lasting three or more seasons. When this oceanic region switches to below normal temperatures, it is called La Niña (Spanish for little girl). La Niña conditions are shown in the image to the right. The Southern Oscillation which is a fluctuation of atmospheric pressure over the tropical indo-pacific region and El Niño are closely linked and are collectively known as the El Niño/Southern Oscillation phenomenon (ENSO). The ENSO swings between warm (El Niño) and cold (La Niña) conditions.
Modern thinking about ENSO rests on a hypothesis first put forward by Jacob Bjerknes in the mid-1960s. He noted that in normal conditions, the persistent tropical Trade Winds ‘push’ the ocean’s surface water westward causing upwelling of cold subsurface water off the coast of Peru. During an El Niño event, the appearance of positive sea surface temperature anomalies over the eastern equatorial Pacific Ocean is accompanied by falls of atmospheric pressure and a reduction of the normal sea-level pressure gradient that drives the Trade Winds. The Trade Winds are weakened and the upwelling of cold water off the coast of Peru is reduced, thus reinforcing the initial positive temperature anomaly. The net effect of these interactions gives the appearance of large quantities of warm water slowly sloshing back and forth across the equatorial Pacific and a large east-west oscillation in the heat supply to the atmosphere from the Pacific Ocean. At the peak of an El Niño event, the tropical Pacific Ocean is warmer than normal and the global near-surface air temperature warms up as the ocean gives up heat to the atmosphere.
The effect of the ENSO phenomenon on global climates is complex, however below is an image summarizing the typical impacts of El Niño.
[Further information] on El Niño/La Niña can be found under the Climate Prediction and Outlook webpages.
[More in depth information] The IPCC Fourth Assessment Report has a detailed section on the ENSO and its role in climate change
[More in depth information] Information of the current status of the ENSO can be found at the WMO El Niño/La Niña update An Archive of the past reports are also available.
In 1971-72, Roland Madden and Paul Julian reported that there was a roughly periodic nature to tropical convection. Over the Indian and Pacific Oceans, in particular, tropical convection was often active for about a week or so and then there was a relatively quiet period of around a month. The active regions of convection travel from west to east, close to the equator, with a period of roughly 30-60 days. Much of the rainfall in the tropics occurs during the active convection phases of this cycle, many tropical cyclones are also formed.
This cycle is now known as the Madden-Julian Oscillation (MJO) or Intra-seasonal Oscillation. The MJO is at its strongest from September to May. Moreover, if one of these bouts of activity hits the western Pacific just as an El Niño event is ready to hatch, it can stimulate a rapid development of the El Niño. The trigger factors for the MJO are not fully understood and they are largely chaotic like other aspects of atmospheric weather behaviour. However, once an active area is identified, satellite imagery can be used to predict the short-term future movement of the active regions. The MJO itself can be strong in some years and almost absent in others, thus making the forecasting of its behaviour that more difficult.
In the South West Pacific, there is an interesting region where the generally warm sea surface temperatures produce lower surface air pressure, and where converging, rising air produces cloud and rainfall. Its position and the associated rainfall have a major impact on the peoples of many Pacific Island countries in the region. Known as the South Pacific Convergence Zone, it runs diagonally southeast from the Solomon Islands to Samoa and beyond. While it usually shifts little during the year, its position is linked to ENSO variations. During El Niño events it is displaced east, and during La Niña events west, of its mean position. Over the longer term the zone has shown a striking eastward displacement since 1977, compared with the period 1948–76, again reflecting the more prevalent occurrences of El Niño events during the later period.
In the tropical regions the planet is girdled by a large belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ), that shifts with the seasonal latitude of maximum solar heating. During the northern summer the ITCZ develops into the more sustained expansion of the monsoon over much of southern Asia. Activity around the ITCZ can erupt into tropical cyclones. Similar processes operate in the Southern Hemisphere during the southern summer, however the definition of the ITCZ is less pronounced and the number of tropical cyclones is only about half the northern figure.
In the tropics, particularly in the ITCZ, the air primarily rises buoyantly within short-lived convective clouds (called ‘hot towers’) and rainfall is often very intense. When the rising air reaches an altitude of 12 to 15 km and virtually all the moisture has been extracted, it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ.
An important mode of variability in the extratropics of the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO is a measure of the surface westerlies across the Atlantic. Positive values of the NAO index indicate stronger-than-average westerlies over the middle latitudes with low pressure anomalies in the Icelandic region and high pressure anomalies across the subtropical Atlantic. The positive phase is associated with cold winters over the northwest Atlantic and mild winters over Europe, as well as wet conditions from Iceland to Scandinavia and drier winter conditions over southern Europe. A negative index indicates weaker westerlies, a more meandering circulation pattern, often with blocking anticyclones occurring over Iceland or Scandinavia, and colder winters over northern Europe.
Over the last two decades of the 20th century the pattern of wintertime atmospheric circulation variability over the North Atlantic apparently shifted in an unprecedented manner. A sharp change in the index began around 1980. Since then the NAO has tended to remain in a highly positive phase. The recent upward trend in the NAO accounts for much of the regional surface warming over Europe and Asia, as well as the cooling over the northwest Atlantic. There are also clear indications that the thermohaline circulation of the North Atlantic Ocean varies significantly with the variations in the circulation patterns of the overlying atmosphere. These changes are linked not only to the NAO but also to wider circulation patterns involving both the North Pacific and ENSO, and including variations on decadal timescales.
The Interdecadal Pacific Oscillation (IPO) is a lengthy interdecadal fluctuation in atmospheric pressure. When the IPO is low, cooler than average sea surface temperatures occur over the central North Pacific, and vice versa. These changes extend over the entire Pacific Basin. During the 20th century the IPO exhibited three major phases. The IPO had positive phases (southeastern tropical Pacific warm) from 1922 to 1946 and 1978 to 1998, and a negative phase between 1947 and 1976. The two phases of the IPO appear to modulate year-to-year ENSO precipitation variability over Australia, and bi-decadal climate shifts in New Zealand. The positive phase enhances the prevailing west to southwest atmospheric circulation in the region, and the negative phase weakens this circulation.
The Antarctic Oscillation (AAO) is an oscillation of atmospheric pressure in the far southern latitudes. It is characterized by pressure anomalies of one sign centred in the Antarctic and anomalies of the opposite sign centred about 40-50°S. The AAO is also referred to as the Southern Annular Mode (SAM). There is a Northern Hemisphere equivalent called the Arctic Oscillation or Northern Annular Mode.
The fact that Antarctica appears to have cooled during the 1990s is claimed to relate to the fact that the AAO was largely in its positive phase during that time. Typically the Antarctic Oscillation alternates between phases about every month. But in the 1990s the positive phase occurred more often. Without the influence of the Antarctic Oscillation, it is likely the Antarctic would show the same kind of warming as the rest of the Southern Hemisphere. Before 1975, Antarctica appears to have warmed at about the same rate as the rest of the hemisphere, about 0.25 degree C per century. But since 1975, while the Antarctic showed overall cooling, the Southern Hemisphere has warmed at a rate of about 1.4 degrees per century. It has been claimed that ozone depletion in the Southern Hemisphere is keeping the Antarctic Oscillation in its positive phase for longer periods.
The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean. It is normally characterized by anomalous cooling of the sea surface in the south-eastern equatorial Indian Ocean and anomalous warming of the sea surface in the western equatorial Indian Ocean. Associated with these changes the normal convection situated over the eastern Indian Ocean warm pool shifts to the west and brings heavy rainfall over east Africa and severe droughts/forest fires over the Indonesian region.
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