The impact of the Atlantic cold tongue on West African monsoon onset in regional model simulations for 1998-2002

Abstract Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

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Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

Journal of Climate ◽

10.1175/jcli4218.1 ◽

2007 ◽

Vol 20 (15) ◽

pp. 4014-4032 ◽

Cited By ~ 27

Author(s):

Benjamin Sultan ◽

Serge Janicot ◽

Philippe Drobinski

Keyword(s):

Diurnal Cycle ◽

Deep Convection ◽

West African Monsoon ◽

Monsoon Onset ◽

West African ◽

Onset Date ◽

The West ◽

African Monsoon ◽

Nocturnal Jet ◽

Heat Low

Abstract This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date. The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet. Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

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Impact of West African Monsoon convective transport and lightning NOx production upon the upper tropospheric composition: a multi-model study

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-2245-2010 ◽

2010 ◽

Vol 10 (2) ◽

pp. 2245-2302 ◽

Cited By ~ 5

Author(s):

B. Barret ◽

J. E. Williams ◽

I. Bouarar ◽

X. Yang ◽

B. Josse ◽

...

Keyword(s):

West African Monsoon ◽

Convective Transport ◽

West African ◽

Lightning Activity ◽

African Monsoon ◽

Model Study ◽

Multidisciplinary Analysis ◽

Low Concentrations ◽

The Tropics ◽

The Impact

Abstract. Within the African Monsoon Multidisciplinary Analysis (AMMA), we investigate the impact of nitrogen oxides produced by lightning (LiNOx) and convective transport during the West African Monsoon (WAM) upon the composition of the upper troposphere (UT) in the tropics. For this purpose, we have performed simulations with 4 state-of-the-art chemistry transport models involved within AMMA, namely MOCAGE, TM4, LMDz-INCA and p-TOMCAT. The model intercomparison is complemented with an evaluation of the simulations based on both spaceborne and airborne observations. The baseline simulations show important differences between the UT CO and O3 distributions simulated by each of the 4 models when compared to measurements of the African latitudinal transect from the MOZAIC program and to distributions measured by the Aura/MLS spaceborne sensor. We show that such model discrepancies can be explained by differences in the convective transport parameterizations and, more particularly, the altitude reached by convective updrafts (ranging between ~200–125 hPa). Concerning UT O3, the majority of models exhibit low concentrations compared to both MOZAIC and MLS observations south of the equator, with good agreement in the Northern Hemisphere. Sensitivity studies are performed to quantify the effect of deep convective transport and the influence of LiNOx production on the UT composition. These clearly indicate that the CO maxima and the elevated O3 concentrations south of the equator are due to convective uplift of air masses impacted by Southern African biomass burning, in agreement with previous studies. Moreover, during the WAM, LiNOx from Africa are responsible for the highest UT O3 enhancements (10–20 ppbv) over the tropical Atlantic between 10° S–20° N. Differences between models are primarily due to the performance of the parameterizations used to simulate lightning activity which are evaluated using spaceborne observations of flash frequency. Combined with comparisons of in-situ NO measurements we show that the models producing the highest amounts of LiNOx over Africa during the WAM (INCA and p-TOMCAT) capture observed NO profiles with the best accuracy, although they both overestimate lightning activity over the Sahel.

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The Impact of Aeolus wind observations on the West African Monsoon

10.5194/egusphere-egu21-9449 ◽

2021 ◽

Author(s):

Maurus Borne ◽

Peter Knippertz ◽

Martin Weissmann ◽

Michael Rennie ◽

Alexander Cress

Keyword(s):

Weather Prediction ◽

West African Monsoon ◽

West African ◽

Boreal Summer ◽

Horizontal Line ◽

The West ◽

Great Opportunity ◽

African Monsoon ◽

The Difference ◽

The Impact

Tropical Africa is characterized by the world-wide largest degree of mesoscale convective organisation. During boreal summer, the wet phase of the West African Monsoon (WAM), the midlevel African easterly jet (AEJ) over the Sahel allows for the formation of synoptic-scale African easterly waves (AEWs) with a maximum intensity close to the West African coast. AEWs interact with convection and its mesoscale organization through modifications in humidity, temperature and vertical wind shear, and often serve as initial disturbances for tropical cyclogenesis. In addition, rainfall can be modulated by other types of tropical waves such as Kelvin or mixed Rossby gravity waves. Upper-tropospheric conditions are dominated by the Tropical Easterly Jet (TEJ), whose variability appears to be connected to convective activity. Overall, our quantitative understanding of the WAM system is still limited. The observational network over the region is sparse and rainfall forecasts with current Numerical Weather Prediction models are hardly better than climatology.The Aeolus satellite launched in 2018 offers a great opportunity to further investigate the WAM with an unprecedented density of free-tropospheric wind data. Assimilating Aeolus wind observations in denial experiments using the current operational system of the European Centre for Medium-Range Weather Forecasts (ECMWF) shows that the main circulation features of the WAM are greatly impacted: the AEJ and the TEJ are systematically weaker and stronger respectively by~1m/s in the analysis fields including Aeolus data. As a consequence AEWs also show a weakening in the propagation amplitude. We are currently investigating the contributions of the HLOS (horizontal line-of-sight) Rayleigh and Mie wind observations to these observed differences. Mie observations (i.e., those related to backscatter from hydrometeors and aerosol particles) seems to contribute strongly to the difference in the AEJ, which lies within a convectively active region with a high aerosol load. On the other hand, the difference seen in the TEJ appears to originate mostly in the Rayleigh (i.e., clear air) observations. Surprisingly, the ascending and descending HLOS observations contribute differently to the data impact, possibly revealing a remaining bias or model problems with the diurnal cycle. Future work will include systematic comparisons between the operational systems of DWD and ECMWF to understand the influence of different data assimilation approaches as well as the impact on forecasts.

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