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In moving south, , parcel gains positive / (anticlockwise) relative vorticity, i

Figure 5.19 (a) Diagram to show how in a Rossby wave the need to conserve potential vorticity (f + Q/D leads to a parcel of water oscillating about a line of latitude $ while alternately gaining and losing relative vorticity For details, see text.

(b) The path taken by a current or airstream affected by a Rossby wave. Note that the flow pattern is characterized by anticyclonic and cyclonic eddies, and that the wave-form moves westward relative to the current or airstream.

dotiwise I anticyclonic) eddy dotiwise I anticyclonic) eddy

Figure 5.19 (a) Diagram to show how in a Rossby wave the need to conserve potential vorticity (f + Q/D leads to a parcel of water oscillating about a line of latitude $ while alternately gaining and losing relative vorticity For details, see text.

(b) The path taken by a current or airstream affected by a Rossby wave. Note that the flow pattern is characterized by anticyclonic and cyclonic eddies, and that the wave-form moves westward relative to the current or airstream.

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atmosphere may reach 100 m s_l and so Rossby waves in an airstream may move eastward relative to the Earth, while still moving westward relative to the airstream. If the eastward motion of air in the airstream is approximately equal to the westward motion of the wave-form, stationary Rossby waves result. In the ocean, flow velocities rarely reach 1 m s_l and so even in eastward-flowing currents. Rossby waves nearly always move westward relative to the Earth. Indeed, the Antarctic Circumpolar Current is the only current in which Rossby waves are carried eastward.

The way in which Kelvin and Rossby waves affect ocean circulation depends on the latitude. At middle and high latitudes, information about a change in the wind stress propagates mainly westwards, by means of Rossby waves, so the ocean near western boundaries is affected by events in mid-ocean to a much greater extent than the ocean near eastern boundaries. By contrast, at low latitudes information can travel westwards by Rossby waves or eastwards by Kelvin waves in the equatorial wave guide. In addition, because of the equatorial wave guide, the upper ocean in low latitudes can respond to changing winds much faster than is possible away from the Equator. This is partly because the equatorial wave guide supports both Rossby and Kelvin waves, and partly because Rossby waves travel fastest there. For example, a Rossby wave can take as little as three months to travel west across the equatorial Pacific, whereas it could take ten years to cross the Pacific at 30° Nor 30° S.

It would not be appropriate to go further into the details of either Rossby or Kelvin waves in this Volume. However, one of the most intriguing aspects of these waves is that when an equatorial Kelvin wave reaches the eastern boundary, it not only splits and travel polewards along the coast (as described in Section 5.3.1. for the tropical Atlantic), but may also be partially reflected as a Rossby wave. This can be seen in the computergenerated diagrams shown in Figure 5.20.

Figure 5.20 Computer-generated diagrams showing the pi ogress from mid-Pacific to the South American coast, of an internal equatorial Kelvin wave. The contour numbers may be regarded as either the depression of the thermocline in metres or the accompanying rise in sea-level in centimetres. The diagrams show the situation at successive monthly intervals. In (c). the equatorial Kelvin wave has split into two poleward-travelling coastal Kelvin waves. Note that the coastal boundary has the effect of increasing the amplitude of the disturbance. The equatorial Kelvin wave has also just been partially reflected as an equatorial Rossby wave, as can be seen by the circular contours which result from the rotatory motion associated with the wave. Because the two eddies are on either side of the Equator, both are anticyclonic and lead to topographic highs (H), although the northerly one is clockwise and the southerly one anticlockwise (cf. Figure 5.19(b)). (In (b) and (c), the small-scale waves in the contours are artefacts of the model.)

Figure 5.20 Computer-generated diagrams showing the pi ogress from mid-Pacific to the South American coast, of an internal equatorial Kelvin wave. The contour numbers may be regarded as either the depression of the thermocline in metres or the accompanying rise in sea-level in centimetres. The diagrams show the situation at successive monthly intervals. In (c). the equatorial Kelvin wave has split into two poleward-travelling coastal Kelvin waves. Note that the coastal boundary has the effect of increasing the amplitude of the disturbance. The equatorial Kelvin wave has also just been partially reflected as an equatorial Rossby wave, as can be seen by the circular contours which result from the rotatory motion associated with the wave. Because the two eddies are on either side of the Equator, both are anticyclonic and lead to topographic highs (H), although the northerly one is clockwise and the southerly one anticlockwise (cf. Figure 5.19(b)). (In (b) and (c), the small-scale waves in the contours are artefacts of the model.)

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