NOTE: Sea Level Pressure data presented in this post are actually negative. The correlation is that as 200hPa temperature increases, sea level pressure falls. I’ll fix the images soon.
This is a short post in which I will present monthly data that shows how El Nino events originate at 200hPa.
First a graph that shows how 200hPa temperatures drive Sea Level Pressure (SLP) in the Southeast Pacific.
The ’82, ’86/7, and ’97/8 El Ninos saw increases in both SLP and 200hPa temperature. The two time series correlate well during other years as well. Now there are three ways to explain this: 1. SLP drives 200hPa temperature. 2. 200hPa temperatures drive SLP. 3. Something else drives both 200hPa temperatures and SLP.
Option 1 is impossible. Option 3 is wrong as you will soon see that variation in 200hPa temperature precedes any sea surface temperature (SST) variation that could cause the observed atmospheric temperature variation.
I boxed the El Nino events. The right edge of the box is placed at the peak of each SST rise. As you can see, 200hPa temperature begins rising and peaks a few months before the pressure change is translated into a rise in equatorial SST. Therefore, it is impossible that SST is driving 200hPa temperature; 200hPa temperatures must be driven by an outside source. This is a topic for another post.
The graph below shows that SST in the Cold Tongue of the Pacific usually lags about four months behind SLP in the critical zone.
It is worthy to note that during the three largest El Ninos since the global climate shift, the Cold Tongue responded immediately to the pressure change.
Now that we have shown that 200hPa temperature is driving SLP in the Southeast Pacific critical zone, and that SLP in the critical zone drives SST in the Cold Tongue with up to a 4-month lag, it is clear that 200hPa temperatures are driving ENSO.
The connection between 200hPa temperatures and solar activity since the great climate shift is a topic for another post.