so anyone who attributes this to human-caused global warming is immediate suspect in my book. Not to say that climate is a linear system, but the best answer I've found is on the NOAA website (FAQ #16).

Here's an interesting tidbit from the NOAA El Nino website FAQ page

Has there been any research to cause a small manmade El Niño or the replication of some of its effects to control weather systems? If it were possible to change the temperature of the water, would it reverse an El Niño?

Asked by a college student.

To answer this question we need to consider how much heat change is involved in climate oscillations such as El Niño.

The increase in SST during January through July 1997, averaged over the equatorial Pacific (10S-10N, 150E-75W) was about 1.62°C.

But the quantity we need is heat content, which proportional to the mass of water times its temperature. This can be estimated from the TAO buoy array, by multiplying the density of water (1030 kg/m3) times the heat capacity of seawater (4000 J/kg/K) times the area of the TAO array times the vertical integral of buoy temperature with depth. (see the note on units and symbols below) (The area was estimated by dividing the array into regions each assumed to be represented by one buoy. Each day, the above calculation was made summing over the area of only those buoys with data for that day). Performing this calculation with TAO temperatures, the time series of heat content in the upper 500m shows that this quantity increased by about 3.5e22 J during July 1996 through March 1997, then decreased slightly. (See also other related calculations).

Now we can compare that heating to human engineering power. One of the most powerful devices humans have created is the H-bomb. The heating during El Niño can be related to the power of the H-bomb as follows. One kiloton of TNT is about 4.18e12 J (Reference). Therefore. 3.5e22 J is about equivalent to 8e9 kilotons, or about 400,000 20-Mt bombs. That assumes the power of those bombs could be directed entirely to heating (i.e. not to moving) the water.

Another way to look at this is to estimate the heating that would be done by a bomb if all the energy went into heating. Consider a 1-Mt H-bomb, releasing 4.18e15 J of energy. That energy would heat a volume of about 1 cubic kilometer of seawater 1°C. That certainly seems like a lot of energy, but the volume of just the upper water of the equatorial Pacific (10°S-10°N, upper 200m) is about 6 million cubic kilometers. Compared to H-bombs, climate changes are a different order of beast entirely.

Another way to look at it is in terms of power plants. Increasing the heat content by 3.5e22 J over the period July 1996 to March 1997 is a heating rate of about 1.5e15 W (1.5 PW). A large power plant can generate about 1000 MW, so the El Niño-related heating is equivalent to the total output of roughly 1,500,000 power plants working continuously for 8 months.

Of course, this argument is misleading in that El Niño probably does not actually heat that much water. What it does is redistribute the heat already in the ocean, first moving it in to the TAO array region, then out. However, it probably is the case that the work required to move that much water is similar in magnitude to the work required to do the heating as above. This calculation is harder to do because we can't measure the currents nearly as well as the temperatures.

Even if we could make the kind of deliberate changes of ocean temperature suggested above, it would have many unpredictable effects, and we do not understand the complex processes well enough to know what would happen if it was done. It is important to remember that El Niño is not the only climate fluctuation that is occurring. El Niños occur on a changing background state, which is probably the reason that the various El Niños are somewhat different. So it would not be simple to figure out how to modify the ocean temperatures to get the changes we wanted, even if we had the technological means to do that.

In any case, even if we had a way to channel these effects into direct heating of ocean water, it is clear from the magnitudes involved that people are nowhere near the ability to modify the climate on this scale.

There is one exception. By burning fossil fuels at the rate we do, we increase the CO2 concentration in the atmosphere in a way that could well cause climate changes (global warming), as most scientists think is now happening. The difference is that this change represents the "efforts" of all the industrial world acting over a century to just get the change under way.

A note on units used here:
kg = kilogram.
m = meter.
K = degrees Kelvin (same units as °C, but starting from absolute zero or -273.15°C)
J = Joule. This is a unit to measure work or energy.
W = Watt. 1 W = 1 J/second. This is the power (rate of doing work).
Abbreviations for large numbers: M = meta (10e6 or 1 million). P = peta (10e15 or 1 million billion).

Definition of the terms from the NOAA website includes the following:

"El Niño events occur irregularly at intervals of 2-7 years, although the average is about once every 3-4 years. They typically last 12-18 months, and are accompanied by swings in the Southern Oscillation (SO), an interannual see-saw in tropical sea level pressure between the eastern and western hemispheres."