9.11: Rossby and Kelvin Waves

Rossby waves, often called “planetary waves,” are caused by the interaction of the gradient in the magnitude of the Coriolis effect with latitude (CC12), and the geostrophic flow of water around a sea surface depression or with the flow of air around a depression or elevation of atmospheric pressure (Chaps. 9, 10, CC13). Thus, these waves occur both in the ocean and in the atmosphere. Their generation is a somewhat complex process, but in the ocean, they have wavelengths of hundreds to thousands of kilometers and wave heights of just a few centimeters. Because they are so long and low, their presence in the oceans was not confirmed until satellites were able to measure sea surface height very precisely across wide areas of ocean within short periods of time.

Rossby waves move only from east to west. However, they can appear to move from west to east if they are carried by a much faster-moving air or water-mass movement. For example, satellite images show atmospheric Rossby waves as clouds following the large-scale meanders of the jet stream. Though weather systems generally move from west to east with the main flow of air masses in the atmosphere, Rossby waves embedded in this flow travels from east to west. Rossby waves move at speeds that vary with latitude, but the speeds of oceanic Rossby waves are on the order of only a few kilometers per day. Thus, for example, a single wave can take months or even years to cross the Pacific Ocean at mid latitudes.

There are two types of Kelvin waves: coastal and equatorial. Coastal Kelvin waves have sufficiently long periods and slow speeds that they are deflected into and constrained to move along a boundary by the Coriolis effect. Coastal Kelvin waves thus flow along the coastal margin in a counterclockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere. At the equator, where the Coriolis effect is zero, there is a special case where Kelvin waves flow directly along the equator from west to east. Equatorial Kelvin waves travel about three times as fast as Rossby waves, so they can cross the Pacific Ocean in about 70 days.

Rossby and Kelvin waves and their effects are poorly understood, but they are believed to affect phytoplankton distribution in the open oceans. They are also believed to affect weather—dramatically in some instances—by creating periodic but irregular variations in the location of ocean currents, such as the Gulf Stream, and in surface water temperatures (and thus atmospheric energy) in various ocean regions. Kelvin and Rossby waves are also associated with the development and relaxation of the El Niño/Southern Oscillation (ENSO; Chap. 7).