Rossby or planetary waves
Stories from Physics for 11-14 14-16
Gigantic meandering waves occur naturally in the atmospheres and oceans of planets. Rossby waves, also known as planetary waves, are long wavelength atmospheric and oceanic oscillations that usually propagate parallel to lines of longitude. The waves are driven by the Coriolis force (an apparent force that causes changes to the motion of objects within rotating systems) and pressure gradients, causing fluids travelling northwards from the equator to be deflected to the east and vice versa. The amplitude of Rossby waves is primarily in the horizontal direction because the restoring force for the oscillations is the Coriolis force.
- The waves have very long wavelengths: Rossby ocean waves have wavelengths of hundreds of kilometres and atmospheric waves thousands of kilometres. In contrast to such large wavelengths, oceanic Rossby waves typically have surface amplitudes of only around 10 cm and so are hard to detect by eye but can be observed by satellites. These small surface amplitudes can be deceptive – although there is only a 10 cm variation on the ocean surface, the same waves may cause oscillations as large as 90 m in the subsurface temperature gradient.
- The horizontal wave speed of Rossby waves depends on their latitude. Whilst waves close to the equator cross the Pacific Ocean in a few months, at mid-latitudes they can take 10-20 years to complete the crossing.
- Astronomers have recently observed that Rossby waves exist in the Sun. Researchers tracked bright points in the Sun’s corona to monitor the passage of the waves. Animations of Rossby waves on the Earth and on the Sun can be seen on the NASA website: nasa.gov/feature/goddard/2017/ waves-on-sun-give-nasa-new-insight-into-space-weather-forecasting