ASK TOM WHY: How does the Arctic Oscillation affect Chicago's winter weather?

Dear Tom,


Please explain the Arctic Oscillation. How does it affect Chicago’s winter weather?


William J. Ooms Jr. Alsip


Dear William,


The Arctic Oscillation (AO) is an index that varies irregularly with time, usually weeks, between positive and negative values. It compares departures from average air pressure between the Arctic and mid-latitudes (about 45 degrees North). The AO is negative when air pressure is relatively high over the polar region and low at mid-latitudes; it’s positive with relatively low Arctic air pressure and high air pressure at mid-latitudes. The polar jet stream tends to be stronger during positive AO, inhibiting the southward movement of frigid arctic air. Chicago’s winter weather is relatively mild. The polar jet is weaker and variable during negative AO, allowing more southward penetration of frigid air; Chicago’s winter weather is colder and more stormy.

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