Ask the Expert: Global warming and the laws of process control

Q: Can global warming cause cooling?

Can the use of the laws of process control help to explain why global warming can cause cooling? Is the size of polar vortex events increased by global warming? Do dead times, time constants, capacitance or feedback effects play a role here?

Z. Friedmann
Instrument Engineer
[email protected]

A: I should start by saying that on the one hand I am not a weather scientist, but a process control engineer. On the other hand, I am used to analyzing the behavior of complex processes, and the weather is one of those. The most basic rule of process control is that we must fully understand the process in question. In case of the polar vortex, we have to analyze the combined effects of the following processes:

The effects of Earth's rotation around a tiled axis and the albedo effect
The travel of heat and the directions of the winds on the global surface
The Coriolis effect

The effects of Earth's rotation around a tilted axis and the albedo effect: If we look at it from the north pole, Earth rotates counter clockwise (east to west) and if it was not warmed by the sun, gravity would hold the atmosphere to the planet's surface and it would travel with it. There would be no winds. Because the atmosphere is heated by the sun, the insolated regions’ warm water evaporate, which makes the air even lighter, and there the light air lifts. In the areas that do not receive solar energy, the is colder (heavier air) moves in below the warm air and winds evolve. The direction of the winds vary, because the air always moves to the areas where low pressures develop, which most often are the warmer locations or regions.

Figure 1: In the winter the Arctic not only receives less solar heat, but also reflects more of it back into space (albedo effect) as it cools and it's ice cover extends to the south.

The fact that the axis of Earth is tilted away from the sun, during the winter the arctic receives practically no sunlight and the little it does, is mostly reflected back into outer space by the highly reflective snow (left side of Figure 1). In the summer, the sunlight reaches the arctic and melts most of the reflective snow cover (right of Figure 1) such that most of the surface becomes heat absorbing water and land (albedo effect). This is why the global warming in the arctic is four times faster than the global average.

Figure 2: The lower polar vortex is stable in the summer, when the temperature difference (ΔT) between the polar and the mid-latitude zones is high.

The travel of heat and the directions of the winds:This second major process involves thermodynamic forces acting on the atmosphere. Earth rotates east to west (counter-clockwise) if looking at it from the north pole and as the warm air is lighter, it rises, the heavier cold moves in below and the resulting wind carries heat to the north through three zones (also called cells):

1. In the tropical zone (between the Equator and 30° latitude), the trade winds are coming from the northeast dominate as they move heat towards the north.

2. In the mid-altitude zone (between 30° and 60° latitudes) the dominant winds that carry the heat further north are blowing from the west.

3. In the polar zone (between 60° latitude and the north pole) easternly winds carry the heat towards the north pole.

The Coriolis effect: If there was no Coriolis effect, the air would rise at the equator, move to the north pole and sink there. The Coriolis effect is caused by the rotation and the global shape of Earth. This is because as the air moves towards the poles, the diameter of the globe drops and therefore a point on its surface moves (rotates) slower and slower as it moves towards the poles. At the equator a point on Earth's surface rotates at 1,000 miles per hour (1,600 km/h), while at the north (or south) pole at zero. Therefore, the counter-clockwise rotation of the colder air on the northern hemisphere, while moving towards the equator, is picking up speed as if it was moving downhill while the rotational speed of the warm air that is moving north was slowing down, like moving uphill. As the cold air circulates faster as it moves to the south, it is deflected to the left (west) and as the warm air above it moves to the north, its path is deflected to the right (east). It is for this reason that in the tropical zone, (also called the Headley cell, which is between the equator and latitude 30°north in Figure 2), the "trade" winds blow from the northeast and at around 30°N, where the mid-latitude starts, the hot air from the equator cools, descends back toward the ground and moves back toward the equator, while the heat is transported forward north by the westerly wind in the mid-latitude, as it moves towards the polar latitude starting at 60°N.

Figure 3: During the winter the polar jet stream can become unstable as the force (ΔT) that holds it to the pole weakens.

Global alarming and the polar vortex:Having described the three main processes which combine and form into the overall weather process, we can now discuss the "polar vortex" phenomenon. The polar zone is a large, low pressure and cold area, a wide expanse of swirling cold air around the poles. The term "vortex" refers to the counter-clockwise flow of air that keeps the colder air close to the poles. Actually, there are two polar vortexes, one in the stratosphere at an altitude of 10-30 miles above Earth's surface, which operates only in the winter and the second below in the troposphere at an altitude of five to nine miles from the surface, which operates year around. This second vortex is larger in diameter and it's boundary winds are usually referred to as the "polar jet stream"(Figure 3). This jet stream is stable, strong and in the summer it rotates at the "polar front latitude", where the easterly polar winds meet the westerly winds in the mid-latitude zone. In the winter, this path is near the border between the United States and Canada.

Béla Lipták
liptakbela@aol.com