By David Cromwell & Paolo Cipollini

Standing on a beach, watching the waves rolling in, is one of lifeís great pleasures. These are the ocean waves with which we are all familiar. But there is another type of ocean wave, invisible to the naked eye, which has considerable importance to ocean circulation and climate - Rossby waves.

Named after the Swedish-US meteorologist, Carl-Gustav Rossby (1898-1957), Rossby waves are a special class of planetary wave - waves that owe their existence to the rotation and shape of the Earth - and occur both in the atmosphere and the ocean. An atmospheric example is the meandering of the jet stream, first identified by Rossby himself and now exploited by airline pilots to reduce long-haul flight times. Oceanic Rossby waves are typically triggered by a disturbance such as a local storm, reversals in coastal currents, or reflection of other kinds of long-wavelength waves. They are more difficult to observe, but possibly even more important than waves in the atmosphere. Always travelling to the west, they are virtually the only means by which information, such as ocean temperature and density, can be transferred from the eastern ocean boundaries to the west.

Rossby waves can even be responsible for disturbing the strong western boundary currents. A Rossby wave was seen developing in the North Pacific, generated by the El Niño of 1982-83. After a decade, this wave shifted the Kuroshio Current in the northwest Pacific, affecting the climate of North America. It may even have been responsible for the dramatic weather events in North America in 1993, such as the Mississippi flooding.

At mid-latitudes in the ocean, Rossby waves have a surface signal with small amplitude - only around 10 cm high, with a long wavelength - some several hundred kilometres, and slow speed - just a few kilometres a day. Although sparse measurements at sea in the 1970s suggested the presence of oceanic Rossby waves, it was not until the advent of satellite technology that their true glory was revealed.

The launches of TOPEX/POSEIDON and the ERS satellites in the early 1990s have given new life to Rossby wave studies. In 1996 US scientists demonstrated that Rossby waves can be found in all the major ocean basins. They also showed that Rossby wave speed is around twice as fast as had been predicted. This means that the ocean responds twice as quickly to climate change.

SOC satellite oceanographers have been analysing data from ocean colour satellite-borne instruments, normally used for observing ocean-scale distributions of phytoplankton - tiny marine plantlife. Propagating signals, consistent with Rossby waves, can be seen, even though they are superimposed on the much stronger annual phytoplankton bloom. A comparison with altimeter data shows the waves propagate at comparable speed, while in other cases the relationship is more complex. One explanation is that biology could be more sensitive to slower-moving Rossby waves. If so, the influence of Rossby waves on phytoplankton growth would suggest an indirect effect on the removal of CO2 from the atmosphere and thus on the global carbon cycle.

These discoveries could dramatically improve the way we predict weather and climate change. Carl-Gustav would have been pleased to see the impact of his waves.

Dr Cipollini and Dr Cromwell are members of the Satellite Oceanography team at SOC. For more detail visit the website www.soc.soton.ac.uk/JRD/SAT/Rossby
 
 

(This short article has appeared on the issue 2 - Spring 2000 of OceanZone, the newsletters from SOC External affairs)