Why El Niño and not the AMO?

Guest Post by Willis Eschenbach

On another thread, a poster got me thinking about the common practice of using the El Nino 3.4 Index to remove some of the variability from the historical global average surface temperature record. The theory, as I have heard it propounded, is that the temperature of the Earth is “signal”, whereas the El Nino cycles are natural swings and as such are just “noise”. So if you remove the El Nino swings from the temperature, the theory goes, then we can see more of the underlying temperature signal by removing the noise.

Figure 1. Various “Nino regions” used in the study of the El Nino / La Nina phenomenon. Each area has its own index, with one of the most commonly used being the Nino 3.4 Index. SOURCE. See also the NOAA page

The more I thought about the practice of subtracting the Nino 3.4 variations from the global average temperature anomalies, the more questions came up for me. I don’t have the answers, hence this post. The first question that came up is, how do we decide that the Nino 3.4 Index represents noise instead of signal?

The Nino 3.4 region covers about 2.4% of the planet’s surface, a bit bigger than the USA. So … why isn’t the temperature of the USA “noise”? Or perhaps, is the temperature of the US “noise” but no one ever checked? And how would you check? What mathematical procedure would allow us to discriminate? What test would we use to say well, Nino 3.4 is noise so we can safely subtract its effects from the global temperature signal, but, for example Nino 1+2 is not noise, it’s part of the signal?

My next question about the situation revolves around the fact that the Nino 3.4 Index is merely a linear transform of the sea surface temperature of the Nino 3.4 area. So what we are doing is taking a linear transformation of the surface temperature anomaly in one part of the world, and subtracting it from the global average surface temperature anomaly.

As a result the question is, is this a legitimate operation? Subtracting a linear transform of something from the whole of which it is a part? Like, say, taking the average temperature variations in the whole US including Texas, but then subtracting out some linear transform of the temperature variation in Texas? What is the meaning of that procedure, subtracting something from itself? And if we are going to subtract a transform of say the Nino 3.4 temperature from the global average, should we include the Nino 3.4 temperature to begin with when we calculate the global average, or not?

Next question is, is this a legitimate operation in a system with a thermostat? Like for example, taking the variations in my body temperature, but subtracting out some linear transform of the temperature variations in my foot? What does that procedure give us, what does the result mean?

Next question. If we’re going to remove the transform of the El Nino Index from the global average temperature record, then should we remove the other indices as well? Should we remove the AMO (Atlantic Multidecadal Oscillation) Index? The PDO (Pacific Decadal Oscillation) Index? The Madden-Julian Oscillation Index? Some combination of them? All of them?

Final question. From my perspective, the El Nino/La Nina oscillation actively regulates heat loss, and thus is part of the planetary temperature regulation system. It regulates the heat loss by way of both the ocean and the atmosphere. Let me give a functional explanation of how it works. The explanation is slightly but not significantly simplified.

During La Nina conditions, in the upper part of Figure 2 below, the warm blanket of water normally covering the Pacific has been blown to the west by the strong eastern trade winds. From there, that mass of warm Pacific surface water splits and moves north and south along the coasts of Asia and Australia towards the Poles. The mass of water is radiating and losing heat as it travels. Functionally, the El Nino/La Nina alteration serves as a huge, slow-cycling, thermally regulated Pacific-wide pump. The La Nina pump stroke moves warm Pacific surface water poleward to lose its heat through conduction, radiation, and evaporation.

Figure 2. La Nina and El Nino conditions. North and South America are the brown areas in the upper right. Australia is at the lower left. Black arrows in the atmosphere show the direction of atmospheric circulation. White arrows show surface ocean currents SOURCE: NOAA El Nino Theme Page

In addition to moving warm Pacific water poleward, the removal of the warm Pacific tropical surface waters exposes the atmosphere to huge amounts of cooler sub-surface Pacific water. This lowers the air temperature over that whole area of the tropical Pacific. Soon, however, the surface of the Pacific starts to warm again. One effect of this is that it slows down the eastern trade winds. As a result of reduced winds and reduced clouds, the warming of the surface of the Pacific continues. In addition, some of the warm surface water in the Western Pacific moves back out east. Soon, with the sun beating down on an ocean with reduced clouds, it warms up all across the Eastern Pacific. This leads to neutral conditions, which can last a while.

However, if the tropical Pacific surface temperature warms enough, then El Nino conditions develop. After the El Nino conditions come into being, at some point as the surface of the Pacific continues to warm, and the El Nino thunderstorms drive the surface air upwards, the eastern trade winds start to strengthen. Soon the eastern trade winds start pushing the warm tropical surface waters and their associated thunderstorms and clouds to the west across the Pacific and eventually poleward again. This is the power stroke of the pump, when the trade winds strip the warm surface waters off and push them westwards. In this process, the full La Nina conditions come into existence. Finally, the La Nina conditions eventually peter out to a neutral condition once again.

Note that this system is triggered by temperature. If the temperature doesn’t build up across the surface of the eastern Pacific for some reason, then things stay neutral, neither El Nino or La Nina. In that case, the El Nino doesn’t form, and so the eastern trade winds don’t build up to pump the warm water across the Pacific and towards the poles.

But when the surface waters of the Pacific do heat up beyond a certain point, El Nino conditions arise, the eastern trade winds strengthen and pump the warm tropical surface water, first across the Pacific and then to the poles. It also exposes the atmosphere to a large area of cooler subsurface water.

Note the effect of this amazing temperature regulating heat pump. It functions to prevent any long-term buildup of heat in the waters of the surface Pacific. If the water in the surface of the Pacific stays cooler, the heat pump doesn’t kick in. But as soon as a certain amount of heat builds up in the surface Pacific waters, the El Nino/La Nina alteration occurs, pumping the surface water west to be flushed out toward the poles. The layer of warm surface water that was blown west is then replaced by cooler water from the subsurface, cooling the entire tropical Pacific.

This mechanism, this El Nino/La Nina pump skimming off the hot Pacific water and pumping it to the poles, prevents long-term Pacific heat buildup and thus actively keeps the planet from both overheating and excessive cooling. It is one of the many interacting thermoregulating mechanisms that keep the earth from either overheating or becoming too cool.

So … this brings up the final question regarding the theme of this post.

Since the variations in the Nino 3.4 index are indicative of the functioning of one of the Earth’s major thermoregulating mechanisms, namely the giant El Nino/La Nina pump that magically materializes to move warm tropical Pacific water to the poles whenever the planet gets too hot and sweaty … then under what possible construction could the Nino 3.4 Index variations be called “noise”?

Like I said … lots of questions, I don’t have the answers, all courteous contributions welcomed.

Regards to all,

w.