Abstract/Summary

A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby-gravity (WMRG) waves and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broad-band filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave which tilts eastwards below the African Easterly Jet and westwards above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centres within AEWs and equatorial WMRG waves. Although the waves do not phase-lock, positive vorticity centres amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southwards flow) and east of this in the lower troposphere (northwards flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northwards flow. In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3-4 days) and ii) they enable westward-moving RWs to propagate into the tropical wave guide from the SH. RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastwards group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism.