scholarly journals The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

2020 ◽  
Vol 41 (1) ◽  
pp. 328-347
Author(s):  
Sharon E. Nicholson ◽  
Douglas Klotter
Keyword(s):  
Tropical Easterly Jet ◽  
Sahel Rainfall

scholarly journals Sahel Rainfall–Tropical Easterly Jet Relationship on Synoptic to Intraseasonal Time Scales

2019 ◽  
Vol 147 (5) ◽  
pp. 1733-1752 ◽  
Author(s):  
Alexander Lemburg ◽  
Jürgen Bader ◽  
Martin Claussen
Keyword(s):  
West Africa ◽  
Time Scales ◽  
West African Monsoon ◽  
Reanalysis Data ◽  
Limiting Factor ◽  
Convective Activity ◽  
Affect Intensity ◽  
Tropical Easterly Jet ◽  
Sahel Rainfall ◽  
Upper Level

Abstract The tropical easterly jet (TEJ) is a characteristic upper-level feature of the West African monsoon (WAM) circulation. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. In contrast, the relationship between Sahel rainfall and the regional TEJ on synoptic to intraseasonal time scales is unclear. Therefore, this relationship is investigated by means of multiple statistical analyses using temporally highly resolved measurement and reanalysis data. It is shown that average correlations between convective activity and regional TEJ intensity remain below 0.3 for all synoptic to intraseasonal time scales. Especially on the synoptic time scale, the TEJ significantly lags anomalies in convective activity by one or two days, which indicates that convection anomalies are more likely to drive changes in the regional TEJ than vice versa. To further shed light on the role of the TEJ for rainfall over West Africa, a previously proposed effect of TEJ-induced upper-level divergence on the development of mesoscale convective systems (MCSs) is examined more closely. An analysis of nearly 300 Sahelian MCSs shows that their initiation is generally not associated with significant TEJ anomalies or jet-induced upper-level divergence. Furthermore, no statistically significant evidence is found that preexisting TEJ-related upper-level divergence anomalies affect intensity, size, and lifetime of MCSs. A limiting factor of this study is the focus on TEJ-induced upper-level divergence. Therefore, a possible effect of the TEJ on Sahel rainfall via other mechanisms cannot be ruled out and should be subject to future studies.

scholarly journals Upper Tropospheric Water Vapor Transport from Indian to Sahelian Regions

Atmosphere ◽  
2018 ◽  
Vol 9 (10) ◽  
pp. 403 ◽  
Author(s):  
Abdoulaye Sy ◽  
Bouya Diop ◽  
Joël Van Baelen ◽  
Christophe Duroure ◽  
Yahya Gour ◽  
...  
Keyword(s):  
Water Vapor ◽  
Indian Monsoon ◽  
Water Vapor Transport ◽  
Substantial Part ◽  
Monsoon Region ◽  
Direct Role ◽  
Air Masses ◽  
Upper Troposphere ◽  
Tropical Easterly Jet

We present a study of upper tropospheric westward transport of air masses coming from the Indian monsoon zone over the period 1998–2008. The objective is to characterize upper tropospheric transport of water vapor from the Indian to Sahelian regions, and to improve the understanding of the dynamical mechanisms that govern water vapor variations in West Africa and the interconnections between India and the Sahel, focusing on the direct role of the Indian monsoon region on Sahel tropospheric water vapor and precipitation. The calculations of forward trajectories with LACYTRAJ (LACY TRAJectory code) and humidity fluxes show that a substantial part (40 to 70% at 300 hPa) of trajectories coming from the upper troposphere of the monsoon region crossed the Sahelian region in a few days (3–14 days), and water vapor fluxes connecting these two regions are established when the Indian monsoon begins at latitudes higher than 15° N in its south–north migration. The intensity and orientation of water vapor fluxes are related to the tropical easterly jet, but they are from the east above the high convection zones. Between 1998 and 2008, these fluxes between the 500–300 hPa pressure levels are associated with precipitation in Sahel only if they are from the east and with an intensity exceeding 8 kg·(m·s)−1.

scholarly journals Interdecadal Changes in the Leading Ocean Forcing of Sahelian Rainfall Interannual Variability: Atmospheric Dynamics and Role of Multidecadal SST Background

2018 ◽  
Vol 31 (17) ◽  
pp. 6687-6710 ◽  
Author(s):  
Roberto Suárez-Moreno ◽  
Belén Rodríguez-Fonseca ◽  
Jesús A. Barroso ◽  
Andreas H. Fink
Keyword(s):  
Southern Oscillation ◽  
Rainfall Variability ◽  
Summer Rainfall ◽  
Rain Gauge ◽  
Sst Variability ◽  
Cold Events ◽  
Sahel Rainfall ◽  
West African Sahel ◽  
First Time

The atmospheric response to global sea surface temperatures is the leading cause of rainfall variability in the West African Sahel. On interannual periodicities, El Niño–Southern Oscillation, the Atlantic equatorial mode, and Mediterranean warm/cold events primarily drive variations of summer rainfall over the Sahel. Nevertheless, the rainfall response to these modes of interannual SST variability has been suggested to be unstable throughout the observational record. This study explores changes in the leading patterns of covariability between Sahel rainfall and SSTs, analyzing the dynamical mechanisms at work to explain the nonstationary relationship between anomalies in these two fields. A new network of rain gauge stations across West Africa is used for the first time to investigate these instabilities during the period 1921–2010. A hypothesis is raised that the underlying SST background seems to favor some interannual teleconnections and inhibit others in terms of the cross-equatorial SST gradients and associated impacts on the location of the intertropical convergence zone. Results of this study are relevant for improving the seasonal predictability of summer rainfall in the Sahel.

scholarly journals Wave Activity on the Tropical Easterly Jet

2007 ◽  
Vol 64 (7) ◽  
pp. 2756-2763 ◽  
Author(s):  
S. E. Nicholson ◽  
A. I. Barcilon ◽  
M. Challa ◽  
J. Baum
Keyword(s):  
West Africa ◽  
Asian Monsoon ◽  
Model Simulation ◽  
Reanalysis Data ◽  
West African ◽  
Planetary Scale ◽  
African Easterly Jet ◽  
Wave Disturbances ◽  
Tropical Easterly Jet ◽  
Wave Development

Abstract This article examines the question of the existence of waves on the tropical easterly jet (TEJ) over West Africa. The TEJ is a well-known feature of the Asian monsoon and waves on the jet have been implicated in various weather phenomena. Its role in West African meteorology has received little attention. A model simulation of wet and dry years over West Africa predicted wave development on the TEJ. NCEP reanalysis data confirmed the existence of these waves, using case studies in the dry year 1983 and the wet year 1950. Both the simulated and observed waves are of planetary scale, with a period of 5–6 days. Potential vorticity (PV) theory suggests that they develop via interactions between the surface and the TEJ. Overall, the results suggest that interactions between the TEJ and African easterly jet play an important role in the development of wave disturbances over West Africa.

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