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contributor authorYang, Gui-Ying
contributor authorMethven, John
contributor authorWoolnough, Steve
contributor authorHodges, Kevin
contributor authorHoskins, Brian
date accessioned2019-09-19T10:07:22Z
date available2019-09-19T10:07:22Z
date copyright3/7/2018 12:00:00 AM
date issued2018
identifier otherjas-d-17-0184.1.pdf
identifier urihttp://yetl.yabesh.ir/yetl1/handle/yetl/4261770
description abstractAbstractA 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 broadband-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 that tilts eastward below the African easterly jet and westward above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centers within AEWs and equatorial WMRG waves. Although the waves do not phase lock, positive vorticity centers amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southward flow) and east of this in the lower troposphere (northward 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 northward 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 waveguide from the SH. The RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastward group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism.
publisherAmerican Meteorological Society
titleLinking African Easterly Wave Activity with Equatorial Waves and the Influence of Rossby Waves from the Southern Hemisphere
typeJournal Paper
journal volume75
journal issue6
journal titleJournal of the Atmospheric Sciences
identifier doi10.1175/JAS-D-17-0184.1
journal fristpage1783
journal lastpage1809
treeJournal of the Atmospheric Sciences:;2018:;volume 075:;issue 006
contenttypeFulltext


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