Africa

wind-generated disturbance

Gulf of Guinea passage of Kelvin wave

Figure 5.19 (at Schematic diagram showing a disturbance of the upper ocean caused by an abrupt change irt the overlying wind field (b) Such a disturbance may he generated in the western Atlantic and travel eastwards as an eouatoiiai Kelvin wave: at the eastern boundary, this splits info two coastal Kelvin waves, which contribute to seasonal upwellmg in the Gulf of Guinea fcf. Figure 5.9).

As discussed in Section 5.1.1, the equatorial thermocline slopes up from the west and is nearesi to the surface tat the eastern coast. The passage of the Kelvin wave only bring!* cooler, sub-tliermocline water to the surface where the thermocline is already fairly shallow.

in the Indian Ocean, it is the equatorial wave guide that permits such swift responses to changes in wind direction. At the start of the South-West Monsoon, westerly winds in the western part of the ocean cause water to move awu> from the African coast, lifting the ihermoeimc. The resulting upward bulge in the thermocline travels east as a Kelvin wave (Figure 5. IX), accompanied by a strong eastward (low of surface water, until it reaches Sumatra, thus bringing about a sea-surface slope up to the east, and a slope in the thennocline down to the cast. Because eastward flow does not start simultaneously all the way along the Equator, but propagates along the Equator as a wave, at certain times of year there are areas of both eastward ami westward How on the Equator (Figure 5,12tat and (d)).

When the winds over the Indian Ocean change from westerlies to northerlies/ttorth-easterlies at the end of the South-West Monsoon, the upper ocean begins to "rearrange itself" and in less than a week the sea-siirtace slope up to the east is beginning to decline. This change is being brought about not by means of Kelvin waves, but by Rossby or planetary waves.

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