The prior posting, “Some Background Regarding An El Nino” began like this: “Currently, the weather is being strongly affected by an El Nino. El Nino is but one part of a weather/climate system known as the El Nino Southern Oscillation (ENSO). There are three phases of ENSO — El Nino, La Nina and Neutral. ENSO is important because of its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe. The global atmospheric circulation is called the Walker
This posting examines the Walker Circulation.(I have seen both cycle and circulation used but much of my sourcing for this posting uses Circulation.)
First lets talk about high and low pressure centers. Fair weather generally accompanies a high-pressure center while clouds and precipitation generally accompany a low-pressure center. Low-pressure centers are formed by a hot surface. For example, the hot Pacific Ocean water that is driven to the Maritime Continent by the trade winds along the equator. The air is hot and moisture laden and as it rises, it cools and the moisture becomes rain. It reaches high-level winds that drive it to the west or east. This air is now dry and cool. It begins to fall forming a high-pressure center. The air in the high-pressure center begins to flow toward the low pressure center residing above the hot seawater located in the Maritime Continent. Along the way it begins to warm and pick up moisture and then rise. This completes the circulation.
When the Cycle is neutral, the hot pool is roughly located at the Maritime Continent. When the hot pool moves from the Maritime Continent to the eastern Pacific along the coast of South America, forming an El Nino, the main up flow of hot moist air is over South America. When a La Nina occurs, the hot pool is again in the vicinity of the Maritime Continent but more westerly than when the cycle is neutral. The more westerly location is due to an upwelling of colder, deeper ocean water along the South American Coast. This upwelling occurs when the hot spot begins to move back to the western Pacific. This colder water increases the strength of the high-pressure center and that in turn increases the strength of the Easterly winds that push the hot spot further west.
Now looking at representations of these three ENSO states are shown below:
In Neutral, the major low-pressure center is shown over the Maritime Continent. With smaller lower pressure centers over South America and Africa. The graphic shows the hot air moving up, providing much rain to SE Asia, Northern Australia and the Maritime Continent.
When the hot spot moves easterly to the coast of South America, the major low-pressure center is now there, accompanied by a smaller low-pressure center over Africa. The west coasts of North and South America experience more rain and departure of much of the drought stricken areas.
La Nina results as the hot spot moves west. The Major low-pressure center is back in the Maritime Continent. A smaller center is created over the South American continent. Dry and potentially drought conditions reappear in the western US and South America. Note that the artist has increased the size of the easterly wind arrow to indicate greater wind speeds that occur with a La Nina.
These three charts were lifted from NOAA’s Climate For You. They have a foot note with respect to the license taken in drawing of these charts and it is:
“Rising and sinking branches of the Walker Circulation generally occur over land as opposed to the ocean. This is due to a variety of other forcing mechanisms such as land-sea temperature contrasts where land areas warm and cool to a larger degree than the oceans nearby which is one reason why rising and sinking air is focused over land.”
There are other things that affect the ENSO that are not included in this simple presentation. The reason for the wind shift that allows the hot pool to move toward South America has many fathers. Is it really ocean waters mixing or is it Kelvin Waves or what ever. I am not competent to know what causes the movement to the east. All we know for certain is that it does move east, without any repeating cycle other than between 2 and 8 years.
I have a colleague that believes the heat to make the hot pool at the Maritime Continent is from volcanism in the Marianas Trench. Again, I am not well enough versed to say that is not so, but when I look at the ocean surface temperature maps during a La Nina, I don’t see any hot areas around the Trench and besides the Trench is pretty far north of where the pool seems to form.
What I have posted here and in the previous one is probably close enough to the factual to feel comfortable that you know generally how it all works.
I made a correction where I changed WEST to EAST in the third from the bottom paragraph. 15 January 2016