Quasi-Biennial Oscillation (QBO)

The Quasi-Biennial Oscillation (QBO) is an oscillation measured by the direction and intensity of winds in the middle of the stratosphere, usually around the 30 millibar level. When the QBO is in its positive phase, winds are called 'westerly', as they flow from west to east at an anomalously high speed. The positive QBO typically strengthens the polar vortex, thus resulting in a warmer than normal winter for lower latitudes. The negative QBO is when winds at the 30mb level are 'easterly', or going from east to west. Since this is the opposite direction as typical low pressure circulation in the Northern Hemisphere, the polar vortex is weakened, and cold weather tends to permeate the lower latitudes more frequently in the winter.