If you watched many TV weathercasters report during the first week of January 2014, you heard explanations about how the polar vortex, and its associated pool of bitterly cold arctic air, controlled the weather across the eastern two-thirds of North America (Fig. 1). The counter-clockwise circulation around the polar vortex brought cold air southward on its western flanks and allowed warmer tropical air to surge northward on its eastern side. Between the two opposing air masses, several storm systems formed and brought heavy snow and/or icing to many places east of the Rockies. Fig. 2 shows one such low once it exited the U.S. into Canada.
Meteorologists tend to key on “cause-effect” relationships. For example, “the polar low brought the arctic air into the U.S.” The evidence certainly supports that hypothesis. However, what if the relationship goes the other way? Could cold air create a vortex that then allows cold air to move in certain directions?
And, what about some form of connectivity that lets both approaches contribute to the overall evolution of an arctic outbreak? Could there be a chicken-egg problem here (Fig. 3)?
Any of the three hypotheses could be at work. Which means that rather than cause-effect, meteorological colleagues should be addressing linkages and relationships.
First, it is important to realize that the so-called “polar vortex” is composed of a series of low-pressure systems rotating around a larger counterclockwise spinning gyre. This larger gyre is always located at high latitudes across the Northern and Southern Hemispheres (Fig. 1).
For the better part of the past week, one of the secondary circulation centers dropped southward across the U.S. This is the “polar vortex” about which everyone is talking. The real polar vortex, the center of the overall large-scale cyclonic circulation, is still somewhere near the North Pole.
Now, take a look at how the atmosphere operates. Cold air is denser and heavier than warm air. Hence, when a cold air mass develops over snow-covered, sun-deprived, northern climes (Northern Hemisphere discussion here), the atmospheric column above the cold air has to shrink downward. This causes pressures at certain altitudes aloft to lower, creating an upper level low-pressure system. In meteorological circles, we also note that at constant pressure surfaces, altitudes can lower, producing the same low-pressure configuration. The reverse holds when the air near the ground heats up. Then, upper level heights or pressures increase, creating a large upper level high. Much as cold waves and the polar vortex are linked in winter, so, too, are upper level highs and heat waves linked in the summer (Fig. 4).
Once the upper level low forms and/or intensifies, winds around the low can help steer the cold and warm air masses, allowing them to move to other locations. This, in turn, changes the atmospheric temperature, pressure and wind patterns further. Hence, the polar vortex can move, reform, and/or intensify/weaken (compare the sequence Fig. 1, Fig. 5 and Fig. 6 that spans the period Jan. 6 – 13, 2014). The iterative process is literally, “a work in progress.”
So, the next time a TV meteorologist (either on-air or on-line) starts talking about how an upper level pressure feature controls or steers weather patterns, put on your linkage headgear, please. You’ve seen two linkages here (winter lows and cold air and summer highs and warm air), but the atmosphere has many more linkages. With an open mind and some curiosity thrown in for good measure, you can start to see others involving hurricanes (wind shear or storm intensification), weather fronts (cold and warm) and Rocky Mountain lee troughs associated with upper level wind and pressure/height patterns.
© 2014 H. Michael Mogil