365 Days of Climate Awareness 64 - The Polar Vortex

Above each pole is a large dome of cold, high-pressure air (under high pressure because cold air is denser than warm air). The polar vortices themselves--both north and south--are stratospheric winds 15-50 km (10-30 mi) above the ground and traveling east (cyclonically, in the same direction as earth's rotation). They are generally above 50°N latitude, and are stronger in the winter.

Polar vortex, ordinary state.

Frequently the jet stream, an eastward tropospheric (0-15 km) is taken to refer to the polar vortex. Like the stratospheric vortex, the jet stream intensifies in winter, though it is less stable in the northern hemisphere than in the south. This is due to the continent Antarctica covering the south pole, providing a stable zone of extremely low temperature and high pressure. The north pole being ocean, allows for more heat to intrude to extremely high latitudes and break the vortex up into smaller pressure centers which then migrate south.

A "gradient" (latin "gradior", walk) refers to a change over distance. A mountain slope is a gradient. If you travel from a warm to a cold area, or vice versa, you have experienced a temperature gradient. The concept is used throughout science.

Polar vortex, having veered far to the south.

In this case, the polar vortex and jet stream are strongest when the temperature (and pressure) gradients from pole to midlatitudes are strongest. Normally, the vortex is strongest in late fall and early winter. The vortex is centered over the Arctic Ocean, remains roughly circular, and begins to break up by March or April.

But when warmer air approaches the pole, either due to an out-of- season storm or unusually warm temperature over land (such as in Siberia, Alaska or Greenland)--when the normal seasonal gradient is upset--the jet stream begins to oscillate north-to-south (in large, slow fluctuations called Rossby waves, named for Swedish meteorologist Carl Rossby).

These fluctuations can lead to arms of cold arctic air venturing much farther south than normal. In extreme cases they can extrude as far as southern Europe or the Gulf Coast of the United States. In this way the strange circumstance of snow and ice in Texas is a very clear signal of global warming and the growing instability of seasons as we have known them.

Tomorrow: biospheres.

Be well!