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| Titel |
The relationship between African easterly waves and equatorial waves and the influence from the Southern Hemisphere |
| VerfasserIn |
John Methven, Yang Guiying, Kevin Hodges, Steve Woolnough |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250154143
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| Publikation (Nr.) |
 EGU/EGU2017-19208.pdf |
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| Zusammenfassung |
| There is strong intraseasonal and interannual variability in African
easterly waves (AEWs). AEWs are crucial to precipitation across West
Africa, but also generate positive vorticity centres that sometimes
develop into tropical storms which can in turn spin-up into hurricanes
in the easterlies across the North Atlantic. In this paper we show
that there are connections between African easterly waves (AEWs),
equatorial Rossby (R1 and R2) waves and westward-moving mixed Rossby
gravity (WMRG) waves and that the conditions for propagation of
equatorial waves may have a major influence on AEW and hence tropical
cyclone variability.
Two analysis approaches are taken using ERA-Interim data from
1979-2010: i) positive vorticity centres within AEWs are tracked at
600 hPa over West Africa to the Atlantic region and ii) the
re-analysis data is filtered using a broad frequency and zonal
wavenumber band and the filtered meridional wind is projected onto the
horizontal structure functions derived from equatorial wave
theory.
The tracked vorticity centres are part of AEWs and are found to move
along with features in the meridional wind projecting onto R1 and R2
waves. In contrast, the structures projecting onto WMRG waves move
westwards at a faster rate. The projection is calculated independently
on each pressure level to create composite cross-sections of each wave
mode in the zonal-height plane, shown relative to the 600 hPa
vorticity centres. The R2 waves tilt in the sense necessary for
baroclinic growth and amplify from east to west, indicating that R2
horizontal structure captures the baroclinic wave component of AEWs.
The composites show that the R2 structures have a wavelength matching
the spacing between vorticity centres, while R1 and WMRG waves are
longer. Intriguingly, the WMRG component has very strong
cross-equatorial flow immediately to the east of positive vorticity
centres developing on the AEJ. Although the WMRG propagates faster to
the west and gets ahead of the original vorticity centre, the next AEW
vorticity centre to the east develops with cross-equatorial flow in
the same phase. This flow brings moist air from the southern
hemisphere at low levels on the eastern flank of the vorticity centre,
while there is an upper tropospheric "return flow" into the southern
hemisphere above. Thus, there is a strong cross-equatorial component to
the developing tropical storm outflow.
WMRG waves may aid the initiation and development of AEW vorticity
centres. Over West Africa, regressions show that the eastward group
propagation of a WMRG packet precedes the genesis of vorticity centres
on the AEJ. In years with stronger AEW activity, the upper
tropospheric easterlies are stronger at the equator and extend further
into the southern hemisphere. It is shown that stronger easterlies
provide a waveguide for SH westward-moving Rossby waves in the upper
troposphere to penetrate into the tropics, exciting equatorial WMRG
waves and hence stronger AEW activity via the lower tropospheric
cross-equatorial flow associated with WMRG waves. |
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