{"id":301,"date":"2006-05-17T09:49:31","date_gmt":"2006-05-17T13:49:31","guid":{"rendered":"\/?p=301"},"modified":"2007-01-08T10:08:38","modified_gmt":"2007-01-08T15:08:38","slug":"el-nino-global-warming","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2006\/05\/el-nino-global-warming\/","title":{"rendered":"El Ni\u00f1o and Global Warming"},"content":{"rendered":"
\n

By Rasmus Benestad & Raymond Pierrehumbert<\/small><\/p>\n

This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.<\/p>\n

<\/p>\n

The name ‘El Ni\u00f1o’ originally was given to a change in the coastal current (usually flowing from south to north) near the Peruvian coast during anos de abundancia<\/em>. \"SirPaita<\/a> sailors who used to sail north-south direction along the coast called the counter-current ‘El Ni\u00f1o’, after the Child Jesus because it had a tendency to appear soon after Christmas (the reason for this seasonality is not yet fully understood, and some of the strongest events peaked earlier in the year). The counter-current (reversal to north-to-south flow) usually would appear in concert with rains in otherwise dry regions. El Ni\u00f1o is also associated with warm surface water in the eastern tropical Pacific. <\/p>\n

However, the name ‘El Ni\u00f1o’, which originally has its origin from changes in the ocean, is linked to changes in the atmospheric circulation. The understanding of the atmospheric circulation changes, later to be discovered to be connected to the appearance of El Ni\u00f1o events, was originally motivated by reasons other than ocean currents. Sir Gilbert Walker<\/a> (photo to the left) was motivated by the question why the South Asian Monsoon sometimes failed from one year to another. There was a catastrophic drought and a subsequent famine in 1877, and the question was why do such events take place? Walker discovered that the sea level pressure fluctuations over the Indian Ocean and tropical Pacific tend to vary with opposite phase. He named this dipole of opposite variations ‘the Southern Oscillation’. The atmospheric circulation associated with this dipole pattern is known as the Walker circulation<\/a>. The Walker circulation refers to the mean (steady) ciculation where air over the warm pool in the western part of the tropical Pacific rises, being fed by the easterly surface trade winds across the Pacific, and subsidence over eastern Pacific. The Southern Oscillation refers to the inter-annual variations in this circulation.<\/p>\n

\n<\/a><\/a>
\nFigure showing the Walker circulation (left) and the Hadley circulation (right).<\/em><\/p>\n

It is important not to confuse the Walker Circulation with the Hadley Circulation<\/a> (also known as the ‘Hadley Cell’), which also involves deep convection in the tropics. Whereas the Walker Circulation (or ‘Walker Cell’) refers to an air flow parallel with the equator – all in the tropics – the Hadley Cell involves air rising in the tropics (follows the solar equator<\/a> and gives rise to the Inter-Tropical Convergence Zone<\/a> (ITCZ) which then flows polewards before sinking in the subtropics. The Walker Circulation involves an east-west asymmetry, whereas the Hadley Cell in principle does not. <\/p>\n

\"ProfessorIt was not until 1969 that Jacob Bjerknes (photo to the left) proposed that there was a physical connection between the oceanographic and atmospheric variations on the year-to-year (inter-annual) time scales, and now the oceanic and atmospheric aspects are combined in the term ‘El Ni\u00f1o Southern Oscillation’ (ENSO<\/a>) that encompasses both the ocean and the atmosphere (also see the IRI link<\/a>, the Australian Bureau of Meteorology<\/a>, National Geographic<\/a>, and a discussion on Wikipedia<\/a>). El Ni\u00f1o events tend to recur every 3-8 years. The last El Ni\u00f1o as of today was in 1997-98, and was the strongest or second strongest (after 1982-83, depending on what you look at) event observed in modern times. The Bureau of Meteorology (BOM) in Australia provides an Internet page on ENSO with a nice ENSO wrap-up<\/a> for up-dated information. Another resource for keeping up-to-date with ENSO is the TAO-array<\/a>. The seasonal migration of both the ITCZ<\/a> and the South Pacific Convergence Zone (SPCZ) are affected by the presence of El Ni\u00f1o<\/a>. <\/p>\n

What is La Ni\u00f1a?<\/strong>
\n<\/a>La Ni\u00f1a usually refers to the opposite state to an El Ni\u00f1o<\/a>: low sea surface temperatures in the eastern part of the eastern tropical Pacific. Intense trade winds and strong uppwelling along a region near the equator, known as the cold tongue<\/a> and caused by Ekman pumping<\/a>, bringing up cold and nutrient water from the deep sea. Note that Ekman pumping does not penetrate deep into the oceanic interior, but since the trades advect the surface waters westward, the upper layer of warm sea water is deeper in the west than in the east. Underneath this layer lies cold ocean water, and the Ekman pumping reaches sufficients depths in the east to bring some of this up to the surface.<\/p>\n

As an aside, it’s amusing to note that in some early papers, the opposite of El Ni\u00f1o was described as the ‘anti-El Ni\u00f1o’ but given the religious connotations described above, this usage did not get a lot of support…<\/p>\n

Why does ENSO arise?<\/strong>
\nThe background conditions are essential for the existence of ENSO. The prevailing surface winds over the tropical Pacific blow from east-to-west (easterlies), and tend drive a surface current, pushing (advecting) the warm surface water westward. The winds are known as ‘trade winds’ and have played an important role in the world history in terms of the ship routes for sailing vessels<\/a>. The western tropical Pacific is known as the ‘warm pool’ with the highest sea surface temperature (SST) in the world (on average). The trade winds ‘pile’ up water masses in the west, resulting in a slightly higher sea level in the west. The higher sea level near the western ocean boundary creates a west-east pressure difference in the ocean, that results in the equatorial undercurrent flowing from west to east below the surface. <\/p>\n

Mechanisms:<\/strong>
\nThe usual pattern is that an El Ni\u00f1o event gradually builds up between June and December, peaking around Christmas time (the reason for this seasonality is not yet fully understood). There are several explanations<\/a> as to why there are fluctuations in the Walker circulations\/trade winds, and the ocean currents. The different explanations do not exclude the others, and it is possible that more than one of these may take place. Here is a simple desciption of the main mechanisms:
\n–Delayed Oscillator<\/a><\/em>: Adjustments in the oceans as a whole, such as response to changes in the surface wind stress, happens through wave propagation. Ocean serves as the memory whereby slow oceanic Rossby waves<\/a> and Kelvin waves<\/a> propagate through the basin and affect the depth of the oceanic surface layer of warm water. Through waves, wind forcing in one location may have an effect on a remote location. Kelvin waves can travel eastward along the equator and poleward on the coasts along the eastern boundary of the ocean basins, but not in the ocean interior. Rossby waves, however, can travel westward and away from the equator and the coasts. Model results have shown that these waves can account for some of the irregular appearances of ENSO.
\n–Coupled Mode<\/em>: The notion involves unstable air-sea coupling, where a local region deep convection enhances the trades to the east and reduces the surface winds to the west. These local wind alterations produce changes in the Ekman pumping.
\n–Noise forced and volcanoes<\/em>: The idea is that weather events kick off El Ni\u00f1os due to unstable conditions. The system is ‘charged up’, and El Ni\u00f1o events are then initiated by a ‘kick’ (eg the Madden-Julian Oscillation or westerly wind bursts).<\/p>\n

What are the effects of ENSO?<\/strong>
\nAlthough ENSO represent ‘modest’ shifts in the weather patterns world-wide, El Ni\u00f1o events may, according to the ‘Report to the Nation’<\/a>, have dire social and economical consequences. Increased rain in the Andean states due to El Ni\u00f1o can have devastating effects, as can the increase in corresponding increase in drought in Indoneasia – and this can lead to big increases in forest fires and air pollution. There is also a clear statistical relationship between El Ni\u00f1os and hurricanes\/typhoons. Whereas the number of hurricanes in the Atlantic basin tend to drop (~50%) during El Ni\u00f1o years, the number of typhoons tend to increase in the Pacific. Drought in the American West is associated with La Ni\u00f1a conditions. The effects can also be felt as far away as Antarctica where a ‘dipole’ of sea level pressure between the Bellinghausen and Weddell Seas is highly correlated to the ENSO phase and can have important effects on sea ice and Antarctic ecology.<\/p>\n

When will next El Ni\u00f1o take place?<\/strong>
\nThis is always the exciting question around June. After the notorious spring barrier<\/a> the predictive skill of ENSO models increase, and our confidence in the ENSO prognoses are higher. The BOM provides a nice summary of a number of different model prognoses<\/a>. At the moment, there are no indication of an El Nino for the end of this year.<\/p>\n

How will the El Ni\u00f1o phenomenon be affected by a global warming?<\/strong>
\nThis is what the Dutch Meteorological Institute (KNMI)<\/a> and the Max-Planck Institute (Germany)<\/a>, Matt Collins of Univ. Reading<\/a> (U.K.) think. There is even a short entry about global warming and ENSO in Wikipedia<\/a>. The brevity of this entry may reflect the fact that the question about how ENSO will respond to a global warming is still not settled. However, it seems that one common trait among some<\/em> climate models is the indication that a global warming may result in a more a general El Ni\u00f1o-type average state<\/a> (eg. Collins et al. 2005, Climate Dynamics<\/em>, 24<\/strong>, 89-104. 19 and here<\/a>). <\/p>\n

How could a change in the background state affect ENSO? There are a number of different theories which go in different directions, but part of the difficulty is that we cannot put our finger on one mechanism and say ‘this is the one!’. If we look into the ocean, then changes in the vertical temperature profiles may plausibly affect oceanic wave propagation, thus perturbing the conditions to which the delayed mechanism is sensitive. Furthermore, a deeper upper layer of warm surface water may weaken the cold tongue if the Ekman pumping doesn’t reach down below the thermocline to bring up colder water, and weakened trade winds would have a similar effect through reduced Ekman pumping near the equator. Some ENSO experts, such as Mark Cane, points out that the upwelling impact on SST in the East implies that in the transient warming, the warming might happen faster in the West than the East thus strengthening the Walker circulation – some of the supposed volcanic-El Ni\u00f1o connections<\/a> would support that. Changes in convection and cloud formations through altered air moisture (CAPE<\/a>) could have implications for the coupled mode mechanisms, as would a deeper thermocline<\/a> (usually situated near the bottom of the warm surface layer). Another question is for the noise-forcing mechanism, whether the high frequency forcing will change or whether the state at which the ocean becomes sensitive to such forcing will be altered (eg ‘charging’ at a different rate or reaching ‘charged level’ at a different threshold). The picture may be even more complex, and a response in ENSO may even involve perturbations of the carbon cycle. The models are all over the place, and most climate models yield patterns with some bias either in geographical character, amplitude or time scales. Analysis by Collins of climate model simulations indicated that increased CO2 may result in ENSO events becoming larger in amplitude and more frequent than under present conditions. This conclusion was based on version 2 of the Hadley Centre Coupled Model (HadCM2). However, in a subsequent analysis based on version 3 of the Hadley Model (HadCM3), Collins found that he could not detect a change in magnitude or frequency of ENSO as greenhouse gases increased, thus contradicting the results of his earlier study. These differences highlight the level of uncertainty associated with ENSO and global warming.<\/p>\n

A Science paper by Huber and Caballero<\/a> (2003) on Eocene El Nino reports on similar ENSO in the Eocene warm climate as the model has in the modern climate. They suggested that the transient changes in El Nino (before the deeper water tapped by upwelling has warmed) may be different from the state of El Nino after the ocean has come into equilibrium. <\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

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