Effect of Equatorial Long Waves on the North Equatorial Countercurrent

Abstract

An analytical, reduced-gravity, equatorial long wave model is employed to study the effect of equatorial long waves on the North Equatorial Countercurrent (NECC). This model is first forced by an abruptly switched-on-off zonal wind stress which is distributed uniformly in the zonal direction. As the wind is turned on, an eastward current with a maximum speed at 5¢XN and a trough of thermocline at 3¢XN are generated in the western basin. These features coincide with the observed distribution of NECC. The initial intensification of this eastward current is due to the forced first meridional mode Rossby wave. The duration of intensification is related to the wind fetch and Rossby wave speed. After the forced Rossby wave has passed, the eastward current is decelerated or accelerated by a series of reflected Rossby waves generated at the eastern boundary. As the steady state is reached, the easterly wind is then turned off. The forced first meridional mode Rossby wave, now generated by wind relaxation, decelerates the eastward current and even turns the current westward, due to overshooting.
Replacing the uniform wind with a linear wind, we find that the general features remain unchanged but the NECC now has larger amplitude and smaller zonal domain. The present linear wind has similar zonal distribution to the trade wind on the tropical Atlantic Ocean.
Finally, the linear wind stress distribution, with a representative of time variations observed along the equator, is applied to force the model. The resultant ocean response is compared with the velocity measurement at 6 °N, 28 ° W. General agreement is found. The equatorial long waves, especially the forced Rossby waves, have significant impact upon the NECC.

Read 905 times