Guest Post by Willis Eschenbach
I was wandering through the graphics section of the TAO buoy data this evening. I noted that they have an outstanding animation of the most recent sixty months of tropical sea temperatures and surface heights. Go to their graphics page, click on “Animation”. Then click on “Animate”. When the new window comes up, click on “60 months”, and then click on the play button. Figure 1 shows a couple of screen captures of different points in time, an El Nino in early 2010 and the succeeding La Nina half a year later.
Figure 1. 3D section of the Pacific Ocean looking westward alone the equator. Each 3D section covers the area eight degrees north and south of the equator, from 137° East (far end) to 95° West (near end), and down to 500 metres depth. Click on image for larger size.
Let me go get a cup of coffee so y’all can go look at the animation, it is most informative. When you come back, allow me to point out a few things that are happening, and make a testable prediction
The main issue that the animation brings to mind for me is, what happens to all the warm water? In March of 2010, shown in the left hand 3D section, there is a thick layer of warm water covering the entire surface of the tropical Pacific. This is the El Nino condition. Half a year later, it’s gone, and its departure has exposed the cooler water which was previously below the surface.
What has happened is that when the Pacific gets to a certain threshold warmth (other conditions being equal), the rising air from the heated surface waters of the El Nino reinforces and strengthens the eastern trade winds. And these strengthened winds simply blow the warm surface water mass to the west, where it divides and goes towards both poles. This exposes the atmosphere to the cooler waters from below. At some point in this process, the exposure of the cool subsurface waters reduces the rising air over the Pacific. This reduces the trade winds, a neutral condition prevails, and the surface once again begins to warm.
Contrary to some students of the El Nino phenomenon, I do not see these two as separate phenomena. You could think of it as a two-stroke pump. In the first part, the tropical Pacific gets warmer and warmer from the sun. At some point, this starts reinforcing the easterly trades, leading to a high trade wind La Nina situation. This pumps the warm tropical surface water poleward. Finally, the Pacific returns to a neutral condition, and sets the stage for the next cycle.
There have been differences of opinion as to whether warmer conditions lead to more El Ninos, or to more La Ninas. Within the constraint that this is a natural system and that half-cycles are possible, I hold that warmer or colder conditions do not strongly favor either one. I say that the reason for the differences of opinion among students of the phenomena is that the data doesn’t clearly favor one position or the other.
Finally, I say the reason neither position is favored is that by and large, the El Nino and La Nina operate as a pair, a two-stroke pump that kicks in to move the hot water to the poles and expose the cooler water underneath. When the heating of the ocean reaches a certain point, it triggers the winds that power the pump. After the winds of the discharge stroke of the pump have moved the warm water first west and then poleward, the trade winds die down, a neutral condition prevails, the heating resumes, and the cycle starts over.
However, if I am correct this implies that there will be a difference between warmer and cooler times—in warmer times, the pump should cycle more frequently. That is to say, there should be more of both El Ninos and La Ninas during warmer times. In cooler times the Pacific should take longer to warm up, and the pump should cycle less frequently, so there should be less of both.
Now, I have made this prediction without recourse to any detailed information about how the relative frequencies of the El Ninos and La Ninas vary with temperature. If we have enough data to calculate it, this should make a good test of how good a student of the El Nino I actually am …
Testable predictions … don’t leave the lab without them. I’ll report back with my further findings on this question at a future date, unless someone beats me to it, because it’s 1 AM Monday morning and I’m back to pounding nails tomorrow … and what is the night doing?
Outside, a crisp night, the moon being at the zenith at 1 AM means it’s about a day past full. It has a large ring around it signaling the distant approach of a front, the smaller the ring the nearer the storm. A cool wind is sighing in from the ocean and backing to the west, it carries the sound of the Bodega Bay buoy six miles (10 km) inshore to tickle my sailor’s earbones, the sound nags and pulls at me like an outgoing tide. Big waves tonight, the buoy says ten foot swell at 13 seconds. I can just make out the distant susurration of the breakers ceaselessly striving to claw the cliffs and beaches back down to the ocean bottom … aaah, do what you love to do, my friends, skate on the underside of the ice, life is far too short …
PS—My explanation of the El Nino pump is a functional explanation, by which I mean I am focusing on what the El Nino does, what its function is in the larger climate system. The explanation leaves out a host of practical details of the mechanism of how it does it, not discussing Ekman transport and Kelvin waves and the effects of sloshing water in the Pacific basin and the influence of the Madden-Julian Oscillation and the like … so please don’t bust me for leaving them out. I am not ignorant of them by any means, I’m just ignoring them to focus on what I want to understand, which is what the El Nino is doing, and not the exact details of how it does it.