On the Dynamics of the East African Jet. II: Jet Transients

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  • 1 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

A barotropic, primitive equation model on an equatorial beta plane is used to investigate the transient behavior of the East African jet. Both analytic and numerical solutions provide insight into the jet response to a diurnal fluctuation in the friction coefficient over land and to temporal variations in the upstream (eastward) and southern boundary forcings.

Results indicate that the diurnal variation in the strength of the surface drag over land can account for the observed increase in the speed and westward shift of the jet core during the night. The observed large variations in the meridional wind just offshore and in the zonal wind field are not explained by the theory.

In contrast to the diurnal variations in the finestructure of the jet, time-dependent variations in the upstream and southern boundary forcings can produce changes in the large-scale features of the jet. For either type of transient perturbation, the change in the jet speed can be significant and may explain the observed jet surges. In the case of southern. boundary forcing, this result demonstrates that eastward propagating, middle-latitude disturbances can have a significant effect on the flow at the equator in the presence of an impermeable western boundary.

Abstract

A barotropic, primitive equation model on an equatorial beta plane is used to investigate the transient behavior of the East African jet. Both analytic and numerical solutions provide insight into the jet response to a diurnal fluctuation in the friction coefficient over land and to temporal variations in the upstream (eastward) and southern boundary forcings.

Results indicate that the diurnal variation in the strength of the surface drag over land can account for the observed increase in the speed and westward shift of the jet core during the night. The observed large variations in the meridional wind just offshore and in the zonal wind field are not explained by the theory.

In contrast to the diurnal variations in the finestructure of the jet, time-dependent variations in the upstream and southern boundary forcings can produce changes in the large-scale features of the jet. For either type of transient perturbation, the change in the jet speed can be significant and may explain the observed jet surges. In the case of southern. boundary forcing, this result demonstrates that eastward propagating, middle-latitude disturbances can have a significant effect on the flow at the equator in the presence of an impermeable western boundary.

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