|Rossby waves: what are they?|
words of introduction
Rossby waves, also known as planetary waves as they owe their origin to the shape and rotation of the earth, are one of the most intriguing natural phenomena. They are easily observed in the atmosphere (i.e. as large-scale meanders of the mid-latitude jet stream), but their existence in the oceans, first theoricised by Carl-Gustav Rossby in the 1930s, has been only indirectly confirmed before the advent of satellite oceanography. Why is it so difficult to spot them? To answer such a question, have a look at the schematic of a "first-mode baroclinic" Rossby wave to the right.
It is the big difference in the horizontal and vertical scale of these waves which makes them so difficult to observe. Also, very often they may not be so nicely periodical as our sketch would imply, but may instead take the form of "solitary waves" (i.e. a single 'bump' or trough). In any case, their horizontal scale is of the order of hundreds of km, while the amplitude of the oscillation at the sea surface is just a few centimeters, practically impossible to measure with in-situ techniques.
Another important characteristic
is that they always travel from East to West, following the parallels.
And they do not go fast - the speed varies with latitude and increases
equatorward, but is of the orded of just a few cm/s (or a few km/day,
if you prefer). This means that at mid-latitudes (say, 30 degrees N or
S) one such wave may take several months - or even years - to cross the
Pacific Ocean. Yes, you read well: in some cases they may cross an
entire oceanic basin, being originated close to the eastern boundaries
and being (as a first approximation) non-dispersive. Here
you can read a few notes on the underlying theory and the generation mechanism.
The importance of being a Rossby wave.....
One might think that these almost invisible waves are uninfluential - this is wrong! They have major effects on the large-scale ocean circulation, and thus on weather and climate. What Rossby waves do is summarised in the figure on the left.
Perhaps the most important effect of these waves is on western boundary currents, such as the Gulf Stream. Rossby waves can intensify the currents, as well as push them off their usual course. If we keep in mind that those currents transport huge quantities of heat, we can easily understand that even a minor shift in the position of the current can dramatically affect weather over large areas of the globe. In the North Pacific, for instance, a Rossby wave, after the 10 years or so that it takes to cross the basin, can push the Kuroshio Current northwards and affect weather on the North America continent. This might have happened already in 1993, the culprit Rossby wave being an effect of the 1982-83 El Niño - see the paper by Jacobs et al in the 4th august 1994 issue of Nature.
How to observe Rossby waves in satellite data
Piece of cake! (well, not quite...). First, you need an instrument capable to measure their signature on the surface. The radar altimeter is perfectly suited to do that as it is capable of measuring the height of the sea surface to a precision of few centimetres (see the pages of the main instruments of that kind flown so far: the NASA/CNES TOPEX/POSEIDON mission and the altimeters on board ESA's ERS satellites). Given that Rossby waves travel almost zonally, it is possible to observe them by taking zonal (west-east) sections of Sea Surface Height Anomalies from each orbital cycle and piling them up into a longitude-time plot (also known as a Hovmöller diagram). In that plot, the waves appear quite clearly as diagonal (i.e. going from bottom right to top left) alignments of crests and troughs. By measuring the slope of the alignments we can estimate the speed of propagation of the waves.
So, we can somehow see the waves with satellites: this is just where the fun starts... or at least where many questions arise. Is it possible to study the waves globally? What waves parameters (wavelength, period, etc...) can we reasonably estimate? Which techniques should we use? Do we see the waves in other satellite datasets, for instance Sea Surface Temperature? How do our observations compare with theory and models of Rossby waves? To know how we tackle these and similar Rossby-wave related problems, please go back to our main page....
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