# Why does the El Niño Stop?

The reason for the initiation of the El Niño is unknown, but what is well known is how it progresses once initiated. The West Pacific warm pool begins to drift East, and the strength of the trade winds begins to decrease. In turn, the decreased trade wind strength further enhances the drift of the warm pool to the East, and this in turn further decreases the strength of the trade winds. It appears as though this forms a positive feedback cycle, so what is responsible for the cessation of the El Niño event?

• What exactly do you mean by stop? El Nino/La Nina is a cycle so it has natural times when it is more or less active in either direction. Do you mean stop as in the transitioning period between each main phase of the cycle? Apr 23 '14 at 4:08
• @IsopycnalOscillation, yes I think that is what I'm asking. I guess I'm saying the El Nino appears to be a positive feedback mechanism, so what mechanism is responsible for the cessation of this positive feedback, ultimately resulting in either a return to normal conditions, or even resulting in an La Nina. I believe the answer has something to do with the ocean rosby waves, but if you could clarify that would be great :) Apr 23 '14 at 4:19
• The question is still a bit confusing, have you looked at this, it might clarify things: tahan.com/charlie/research/physics/earth_science/nino/enso-ct/… ? Apr 24 '14 at 4:40
• @IsopycnalOscillation, even the introduction of that article describes the positive feedback mechanism I describe. The article also says that the feedback ends when the rossby waves travel to the West coast and rebound as Kelvin waves. I will accept an answer that explains this. Apr 24 '14 at 4:48

I think it is important to think of the El Niño as just one of the components of a large scale global atmospheric-oceanic phenomenon known as the El Niño and the Southern Oscillation (ENSO). ENSO takes shape as fluctuations of the rainfall, wind, ocean currents and waves. They are by nature very irregular.

The Southern Oscillation (SO) was first discovered by Sir Gilbert Walker who conducted seasonal averages for pressure and rainfall at a number of stations around the world. He found that a single spatial pattern was dominant where regions of high pressure in the central South Pacific Ocean and low pressure near Australia and India reverse and the highs turn into lows and the lows into highs. The SO is an atmospheric phenomenon, on the other hand, the El Niño is an oceanic process. Nonetheless, the interconnectedness between both phenomena is significant such that they cannot be considered alone, as pointed out by Bjerknes (1966). The combined action of ENSO is known to cause drought in Australia and the reduction of upwelling in the western coast of South America (which has disastrous consequences for nutrient delivery).

There is general agreement that one of the necessary ingredients for the initiation of the ENSO event is the reversal of the general equatorial Trade Wind pattern, an easterly steady wind due to a Walker cell. Westerly wind bursts spanning thousands of kilometers and acting over days or perhaps weeks at a time set off internal waves that transport energy westward and eventually poleward along the western coast of South America, some of that energy bounces off and radiates back east. See sketch below:

The equatorial isopycnal depression moves eastward along the equator as a Kelvin wave until reaching the western coast of South America relatively unchanged. These are shallow water waves so the speed is just $c = \sqrt{g'D}$ where $D$ is the depth and $g'$ is the reduced gravity, so they travel at about $c = 2.5$m/s plus 10-20% because they are literally advected by the Cromwell Undercurrent. They transport large amounts of energy over vast distances, depositing it far away from the generation zone in the western Pacific Ocean. Namely, after arrival on the east Pacific coast the equatorial Kelvin wave energy degenerates into two coastal Kelvin waves propagating poleward, and into new diffuse equatorial Rossby waves radiating towards the west Pacific. It is this removal of energy from the Western pacific through the development of these complicated wave motions and other more subtle processes that eventually brings an ENSO event to an end (chicken and egg?). Because the energy is removed, the SST drops, the trade winds resume thus ending ENSO.

"An El Niño event contains the seeds of its own demise."

Source:

Regional Oceanography: An Introduction By Matthias Tomczak

As explained on slide 9 of Dr. William S. Kessler's talk

The warm water pool in the western (Asian) pacific drains eastward and toward the poles.

This causes the thermocline in the western pacific to get closer to the surface, which in turn cools the surface temperature in the western pacific, ending the El Niño.

• Would you happen to know how long the transition period is before there is a transition to La Nina ? Dec 22 '14 at 18:37
• The same slide says El Nino lasts 9-12 months, but that doesn't mean the La Nina then occurs. For example El Nino in late 1991 to early 1992 and again late 1994 to early 1995, without any intervening La Nina ggweather.com/enso/oni.htm Dec 22 '14 at 19:02
• Would be helpful on how a "normal" state of the Pacific i.e. neither El Nino nor a La Nina affects the climate of the world. Oh well another question I suppose. Dec 23 '14 at 5:31