Coastal currents in low latitudes (with application to the Somali and El Niño currents)

Near the equator, the directly wind-driven coastal jet along the western boundary of an ocean basin differs from its counterpart along the eastern boundary in several respects. (1) The western boundary current is more intense by an order of magnitude, because it flows in the direction of the pressure force associated with density gradients whereas the eastern boundary current is opposed by this pressure force. (2) In the west the flow at the depth of the thermocline is still in the direction of the wind, but in the east the pressure force drives an undercurrent that is practically as strong as the surface current. (3) Variability on time scales from a week to months is much higher in the east than the west. (4) On time scales longer than a month long Rossby waves disperse the eastern boundary current westcard, but the western current remains a coastal jet. (5) A relaxation of the wind causes a prompt reversal in the direction of the flow in the east but in the western region, where the current flows 'downhill', there is only a gradual deceleration. The relevance of these results to the Somali Current in the western Indian Ocean and El Niño Current in the eastern equatorial Pacific Ocean is discussed.