The polar vortex is a permanent low- pressure system that surrounds the geographical north and south poles. “Vortex” refers to the circulation around the poles. The term also describes smaller vortices that occur within lobes of the primary vortex.
It is not a new phenomenon, but it has gained fame again this winter as it holds the Midwestern and Eastern U.S. hostage with deep snow in the Northeast, snow into the Deep South and subzero temperatures in the Midwest.
The vortex is held in place by the polar jet stream, which snakes its way around the globe, an intense, narrow river of air in the upper troposphere. It is a result of Earth’s rotation and the Coriolis effect.
Because it often meanders into the temperate zone and sometimes into the subtropics, it is a significant factor influencing the weather of most of the U.S. and Canada. Part of it can break off as a smaller vortex and migrate southward, bringing cold air with it to areas as far south as Florida.
The sun powers the circulation of the atmosphere. If Earth were covered with water and not rotating, the atmosphere would form a simple convection cell with heating at the equator and cooling at the poles. Warm air would rise in the tropics, flow at upper levels as it cooled, then sink in the polar regions to flow along the surface back to the equator.
A rotating Earth changes the picture. The Coriolis effect occurs because a point anywhere on the surface is moving faster than any point poleward of it. This is because each degree of longitude spans a narrower arc nearer the poles. This causes anything in motion on a rotating Earth to curve to the right in the counterclockwise-rotating Northern Hemisphere or to the left in the clockwise- rotating Southern Hemisphere.
The Coriolis effect causes the convection from heating to break into three cells: one in the tropics, one in midlatitudes and the polar vortex. The interactions that are responsible for most weather occur at the boundary between the midlatitude cell and polar vortex.
Along that boundary flows the polar jet stream.
Air flowing poleward in the midlatitude cell is warm and moist, having traveled across the surface from the subtropical zone. Air in the polar vortex is cold and dry. At the surface, the boundary between them is the polar front. In the upper atmosphere, the jet stream is the boundary.
Like a river of water on the surface, the jet stream meanders in a wavelike pattern. As seen from above the North Pole, the troughs of the waves form lobes of cold air extending into the midlatitude regions. This sets up a counterclockwise rotation within the lobe, giving rise to smaller vortices that can break off and move on their own.
This winter a strong vortex brought cold air south, intensifying a severe nor’easter that dumped snow on New England, while snow and freezing temperatures affected 3 million people in Texas. Meanwhile, winter storm Viola is sweeping across the continent, bringing snow and freezing temperatures to the South and Southeast.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
