Although there have been numerous studies documenting the processes/environments that lead to the intensification of African easterly waves (AEWs), only a few of these studies investigated the effect of those processes or the environment on the predictability of AEWs. Here, the large-scale modulation of AEW intensity predictability is evaluated using the 51-member ECMWF ensemble prediction system (EPS) during an active AEW period (July–September 2011–13). Forecasts are stratified based on the 72-h AEW intensity standard deviation (SD) to evaluate hypotheses for how different processes contribute to large forecast SD. While large and small SD forecasts are associated with similar baroclinic and barotropic energy conversions, forecasts with large SD are characterized by higher relative humidity values downstream of the AEW trough. These areas of higher humidity are also associated with higher precipitation and precipitation SD, suggesting that uncertainty associated with diabatic processes could be linked with large AEW intensity SD. Although water vapor is a strong function of longitude and phase of convectively coupled equatorial waves, the cases with large and small SD are characterized by similar longitude and wave phase, suggesting that AEWs occurring in certain locations or convectively coupled equatorial wave phases are not more or less predictable.
- NSF-PAR ID:
- 10398909
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 150
- Issue:
- 8
- ISSN:
- 0027-0644
- Page Range / eLocation ID:
- 2055 to 2072
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.
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The characteristics of African easterly waves (AEWs) and their relationship with synoptic‐scale plumes of Saharan mineral dust (SMD) are examined using 37 summers (July–September, 1980–2016) of 3‐D fields from the Modern‐Era Retrospective analysis for Research and Applications, version 2 (MERRA‐2). The AEWs and SMD plumes are obtained by projecting the kinematic, thermodynamic and dust fields onto the 2–6‐day time‐filtered aerosol optical depth (AOD) near the west coast of North Africa (15°N, 15°W). Results show that the kinematic fields of the AEWs have significant structure north of the African easterly jet (AEJ). For the SMD, signals occur in two regions relative to the AEW over Africa: in the northerlies ahead of the AEW trough in the Sahel (10–20°N), and in the southerlies behind the AEW trough in the Sahara (20–27°N). The SMD signals are emitted on the flanks of a low surface‐pressure system associated with the northern circulation, mixed vertically in ascent regions below 600 hPa and removed by dry and wet deposition processes. Over the eastern Atlantic Ocean, the kinematic fields of the AEWs weaken, which modifies the propagation of the SMD signals. Consequently, the SMD signals from the two sources merge in the Saharan air layer (SAL) over the Atlantic.
The direct radiative effects of SMD on the AEWs are also investigated over Africa.
The perturbation diabatic heating rate is combined with the perturbation temperature field to infer the structure of the diabatic generation term in the energetics of the AEWs. The implied energetics show that an SMD‐induced generation region of eddy potential energy occurs at mid‐levels (850–650 hPa) north of the AEJ, which replenishes energy that is converted by baroclinic processes.
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The dynamics of African easterly waves (AEWs) are investigated from the perspective of potential vorticity (PV) using data from global reanalysis projects. To a leading order, AEW evolution is governed by four processes: advection of the wave-scale PV by background flow, advection of background PV by the AEW, diabatic forcing due to wave-scale moist convection, and coupling between the wave and background diabatic forcing. Moist convection contributes significantly to the growth of AEWs in the midtroposphere, and to both growth and propagation of AEWs near the surface. The former is associated with stratiform clouds while the latter with deep convection. Moist convection helps maintain a more upright AEW PV column against the background shear, which makes the wave structure conducive for tropical cyclogenesis. It is also argued that—contrary to the hypothesis in some prior studies—the canonical diabatic Rossby wave model is likely not applicable to AEWs.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/MWR-D-21-0321.1
