			Home About us Contact

African Easterly Jet (african + easterly_jet)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

Simulation of the intraseasonal and the interannual variability of rainfall over West Africa with RegCM3 during the monsoon period

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2010
M. B. Sylla
Abstract Intraseasonal and interannual variability of rainfall is simulated using the International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM3) over West Africa. The intraseasonal variability of rainfall showing three distinct phases and the monsoon jump is well reproduced in the simulation. In addition, the regional model shows that while the monsoon rainbelt moves to the Sahel, the African Easterly Jet (AEJ) undergoes a northward migration and a weakening from June to August, when the core is at its northernmost location. This coexists with the appearance and the strengthening of the Tropical Easterly Jet (TEJ), the development and increased activity of the African Easterly Waves (AEWs), and the intensification and northward shift of the ascent between the AEJ and the TEJ core levels and axis. Similarly, the simulated interannual variability of rainfall over West Africa, the Guinea region, and the Sahel, as well as the variability of atmospheric features during contrasting wet and dry years, is also well captured. In fact, in the simulation during dry years the AEWs activity is decreased while the AEJ is strengthened and migrates southward, the TEJ becomes weaker, and the ascent between the levels of the AEJ and the TEJ decreases. The simulated rainfall variability and the behavior of the related features during the rainy season and during contrasting wet and dry years are in line with previous studies that used observations and reanalysis. We conclude that this model performance is of sufficient quality for application to the study of climate processes and mechanisms over West Africa. Copyright © 2009 Royal Meteorological Society [source]

Pre-rainy season moisture build-up and storm precipitation delivery in the West African Sahel

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2008
J. Bayo Omotosho
Abstract The salient differences between the years of above and below normal precipitation, particularly within the long period of 1972,1990 with persistently decreasing Sahelian rainfall, are investigated for Kano, a Nigerian station within the Sahel. Daily rainfall data from 1916 to 2000, storm records from 1951 to 2000 and radiosonde data for three dry and three wet years are used in this study. Results confirm previous findings that the African Easterly Jet (AEJ) located in the 700,600 mb layer is stronger during the dry than in wet years. Significantly, however, it is shown that during the wet years, there is stronger and deeper early season (April,June) build-up of moisture below the AEJ. Furthermore, throughout the period from April to August, the middle troposphere was almost always drier than normal during the dry years and moist than normal in the wet years. Consequent upon these, the storms, which deliver almost all the rainfall in the Sahel, produce at least 150% more precipitation during the wet than in the dry years, though the June to September or annual total number of storms differs by only about 30%. Finally, during the dry years, the onset of rainfall is found to be generally very late compared to the long-term mean, with shorter length of the rainy season. Copyright © 2007 Royal Meteorological Society [source]

A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2001
S.E. Nicholson
Abstract This article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important ,local' factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a ,wet mode' and a ,dry mode' in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958,1967 (representing the wet mode) and 1968,1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter-tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so). The results of this study suggest that the key factor controlling the occurrence of the ,wet Sahel' mode versus the ,dry' mode is the presence of deep, well-developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well-known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the ,wet Sahel' and ,dry Sahel' modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well-known dipole and no-dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea-surface temperatures (SSTs) in the Gulf of Guinea. Copyright © 2001 Royal Meteorological Society [source]

Modulation of the African easterly jet by a mesoscale convective system

ATMOSPHERIC SCIENCE LETTERS, Issue 3 2010
Zhuo Wang
Abstract The modulation of the African easterly jet (AEJ) by a mesoscale convective system (MCS) is examined in a numerical simulation. An AEJ with a strong and confined core is simulated before the formation of the MCS north of the AEJ axis, and the jet is ,split' with two separate cores after the passage of the MCS. Our diagnosis suggests that the MCS may be triggered by a wave propagating south of the AEJ axis. A momentum budget analysis indicates that the meridional circulation associated with the MCS weakens the jet to its south and forms the secondary jet to its north. Copyright © 2010 Royal Meteorological Society [source]