Planetary Waves

Planetary-scale waves have their origin relating to the Earth’s shape and rotation; the waves are so large that some of them wrap around the whole Earth and can be observed in the atmosphere through the meandering of the jet stream. A long wave or planetary wave is a weather system that circles the world, with one to three waves forming a looping path around the Earth at any given time and displacing air north and south. Planetary waves have ridges (high points) and troughs (low points). Warmer upper air is associated with an increasing number of waves or stronger waves.

Planetary waves form in the lowest part of the atmosphere, called the troposphere, and propagate upward, transferring energy into the stratosphere and heating polar air between 9 and 18 degrees F (5 and 10 degrees C). Because of a larger landmass, with the majority of the highest mountains and land–sea boundaries in the Northern Hemisphere, planetary waves form more strongly in the Northern Hemisphere. Once the wave dissipates, the polar air begins to cool. In the Southern Hemisphere, landforms also produce planetary waves, although they are weaker there because there are fewer tall mountain ranges and vast open ocean surrounding Antarctica. The warming of the Arctic stratosphere suppresses ozone destruction. Ozone exists in the lower level of the stratosphere and is caused by sunlight splitting the oxygen molecules at cooler temperatures, with less ozone destruction at warmer temperatures.

The Himalayas and other land features create the planetary atmospheric waves that serve to decrease the formation of an ozone hole at the northern pole and therefore limit solar ultraviolet radiation exposure in the Arctic. Climate change could open ozone holes in the Arctic; in the spring of 1997, weak planetary waves created conditions that formed a small ozone hole over the Arctic. The chemistry of ozone destruction requires very cold air temperatures in the stratosphere, and because of planetary wave action, the Arctic stratosphere stays warmer than the Antarctic stratosphere.

In contrast, researchers announced in 1992 that El Niño weather changes and a large number of planetary waves in the atmosphere had caused shrinking of the Antarctic ozone hole, with the ozone hole in September 2002 at half the size it was in 2000. Large-scale weather patterns (similar to a semipermanent area of high pressure) generate more frequent and stronger planetary waves. If the waves are more frequent and stronger as they move from the surface to the upper atmosphere, they warm the upper air. Because ozone breaks down more easily with colder temperatures, the warmer the upper air around the “polar vortex,” or rotating column of winds that reach into the upper atmosphere where the protective ozone layer is, the less ozone is depleted.

Researchers working in Esrange, Sweden, studied the main features of planetary waves and variability of the semidiurnal tide, with planetary wave periods observed by meteor radar. They focused observation on 5, 8–10, 16, and 23 day planetary waves by meteor radar measurements in the mesosphere and lower thermosphere. In the winter, when the planetary waves are significantly amplified, a very strong periodic variability of the semidiurnal tide is also observed. This result indicates that the most probable mechanism responsible for the periodic tidal variability during winter is in situ nonlinear coupling between tides and planetary waves. They established a correlation between the planetary wave and semidiurnal tide and secondary waves with frequency, phase, and vertical wavenumber (wavelength) correlation.

The influence of planetary waves on global system dynamics with airflow and temperature distribution include the indirect effect of upper air patterns on lower air patterns through feedback, linking all layers of the atmosphere. Planetary waves (also called Rossby waves) form in the ocean and affect ocean circulation over longer periods of time—from one to 10 years.