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El Niño is associated with the weakening of the ‘normal’ pressure/temperature gradients between the western and eastern Pacific Oceans.

My question is this: why does it only exist in the Pacific Ocean?

Could it be that the Atlantic is always in an ‘El Niño’ state too, given that the pressure/temperature gradients in the Atlantic aren't as steep as those of the Pacific to begin with?

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    $\begingroup$ ENSO existence in the Pacific has a lot to do with the relatively shallow water in the western part of that basin, which is easier to warm than if it were deeper. $\endgroup$ – casey May 5 '14 at 14:51
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    $\begingroup$ @casey I agree. It is also likely that the size of the Pacific basin makes the circulation near the Equator much less constrained by boundary currents compared to smaller basins (Atlantic and Indian). $\endgroup$ – milancurcic May 5 '14 at 17:25
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First, to describe El Niño as a Pacific Ocean event seems overly simplistic. The effects of El Niño are felt worldwide as can be seemed by a principal component (EOF) analysis (e.g., Alexander et al., 2002). Also, using the term "El Niño" to describe the entire oscillation (ENSO, El Niño Southern oscillation) can lead to confusions as the term "El Niño" corresponds to one of the phases of the oscillation.

The size of the Pacific Ocean and the lack of equatorial constraints makes it a prime candidate for large oceanic fluctuations such as ENSO. The large distance between the western and eastern sides of the basin allows for a more dynamic equilibrium with the presence of large scale waves (Kelvin and Rossby; e.g., Kirtman, 1997). This equilibrium can be altered by external (atmospheric) forcing resulting in the different modes of oscillation of ENSO.

Apart from that, there are a number of significant modes of oscillations (see http://www.esrl.noaa.gov/psd/data/climateindices/list/ for a very good list) that occur in basins outside the Pacific ocean. A good example is the North Atlantic Oscillation (NAO; Hurrel [1985, Science]; http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml) that affects not only the North Atlantic but also the climate of Europe and North America.

Other examples of modes of climate variability include the Pacific Decadal Oscillation (PDO; Zhang et al., 1997), the Atlantic Multidecadal Oscillation (AMO; Schlesinger & Ramankutty, 1994), or the Arctic Oscillation (AO; Thompson & Wallace, 1998).

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    $\begingroup$ That might be the problem. The way the question is formulated, that is not clear. By the way, the anomalous warm water upwelling does not initiate ENSO or El Niño, it is the result of a weakening of the trade winds. When the trade winds relax in the central and western Pacific, the thermocline deepens in the eastern Pacific and becomes shallower in the west. $\endgroup$ – arkaia May 4 '14 at 22:51
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    $\begingroup$ @IRO-bot I'm not sufficiently expert to be sure, but it sounds as though the answer you're after is along the lines of Q:"Why does El Nino not form in other oceans?" A: "Because the parts of the global oscillations that form in other oceans are not called El Nino"? ;-) $\endgroup$ – Semidiurnal Simon May 5 '14 at 7:56
  • $\begingroup$ +1 For the answer at this point. $\endgroup$ – milancurcic May 7 '14 at 16:54
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Likely factor to be the underlying ring of fire underneath the sea, but there are other natural interactions causING the cycles to start and finish. Cold Antarctic sea moving up the South American coast mixing with tropical warm sea water near the equator is the other major cause and affect.

*"El Niño results from interaction between the surface layers of the ocean and the overlying atmosphere in tropical Pacific. It is the internal dynamics of the coupled ocean-atmosphere system that determine the onset and termination of El Niño events. The physical processes are complicated, but they involve unstable air-sea interaction and planetary scale oceanic waves. The system oscillates between warm (El Niño) to neutral (or cold) conditions with a natural periodicity of roughly 3-4 years.

http://www.pmel.noaa.gov/tao/elnino/faq.html#what

According to another source "http://www.amazon.com/Oryx-Resource-Guide-Nino-Nina/dp/1573563781" External forcing heat from volcanic eruptions (submarine or terrestrial) could have connection with El Niño.

enter image description here

Image source: http://www.universetoday.com/31139/ring-of-fire-volcanoes/

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    $\begingroup$ Your quote says External forcing from volcanic eruptions (submarine or terrestrial) have no connection with El Niño, but you open your answer with Likely factor to be the underlying ring of fire underneath the sea. That appears inconsistent to me. $\endgroup$ – gerrit May 6 '14 at 14:18
  • $\begingroup$ heat rises and mixes with cold water, the source maybe misleading but the on a geological scale the sea floor is rising. Read Oryx Resource Guide ot El Nino and La Nina. $\endgroup$ – Mapperz May 6 '14 at 14:24
  • $\begingroup$ Could you add a related quote to your answer? I find it confusing as it currently stands (I realise that not all geologic activity equates volcanic eruptions). $\endgroup$ – gerrit May 6 '14 at 14:25
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    $\begingroup$ I agree, I'd like more clarification on this. what do you mean "on a geological scale the sea floor is rising"? is this in reference to the oceanic crust? or uplift of continental crust over subduction zones? or mid ocean ridge uplift? and how are you, or your reference stating the ring of fire has a bearing on el-nino? is it due to basin topography effects? are subduction trenches part of what you're/they're claiming? Also the ring of fire isn't under the sea, it's the volcanism seen surrounding the ocean basin, what role are you claiming the ring of fire has in el-nino events? $\endgroup$ – Siv May 6 '14 at 16:55
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    $\begingroup$ For the "external forcing from volcanic eruptions" argument to be true, volcanic eruptions should show a similar periodicity to ENSO and as far as I know there hasn't been any study showing that. $\endgroup$ – arkaia May 6 '14 at 17:57

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