The Mean Troposphere of West Africa

Aspliden, C. I.; Adefolalu, D. O.

doi:10.1175/1520-0450(1976)015<0705:tmtowa>2.0.co;2

Cited by 28 publications

(13 citation statements)

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“…The semi-annual cycle is an unmistakable feature in precipitation at Libreville, where local rainfall peaks in April-May and October-November (Asplinden and Adefolalu 1976). It is also visible in RH and θ e in the boundary layer at Abidjan, even though the second peak of rainfall is not evident.…”

Section: (B) Meridional Windmentioning

confidence: 99%

The seasonal cycle in the lower troposphere over West Africa from sounding observations

Zhang

Woodworth

2006

Quart J Royal Meteoro Soc

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SUMMARYIn situ sounding observations from selected sites in West and Central Africa were used to document the seasonal cycle in the lower troposphere associated with the annual migration of precipitation. The main purpose of this exercise is to provide a description of the seasonal cycle free of the errors and biases of numerical models, which can be substantial in this region. Many features in the circulation typically associated with the West African Monsoon (WAM) exist through almost the entire seasonal cycle in the region. The low-level easterly jet, known as the African Easterly Jet during the monsoon season, remains prominent and closely related to the rain band in other phases of the seasonal cycle, even when the rain band is located near the equator. The surface southerly monsoon onshore flow penetrates through the rain band in the entire season cycle. The depth of the southerly surface monsoon flow undergoes a seasonal variation, and is greatest during the peak season of the monsoon. A unique feature is a low-level (∼700 hPa) northerly flow penetrating through the rain band in various phases of the seasonal cycle. Risking possible biases of land-bound sounding data, these observations are synthesized into a schematic describing the salient features in the low-level circulation associated with the rain band in different phases of the seasonal cycle. This synthesized circulation pattern in West Africa is in contrast to the traditional conceptual circulation pattern associated with the marine ITCZ and other monsoon systems. The results from this study call for more research attention to the low-level meridional circulation for a full understanding of the dynamics of the WAM. This study also advocates the study of the WAM to be conducted in a broad context of the seasonal cycle in the region and in comparison to the marine ITCZ.

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Section: (B) Meridional Windmentioning

confidence: 99%

The seasonal cycle in the lower troposphere over West Africa from sounding observations

Zhang

Woodworth

2006

Quart J Royal Meteoro Soc

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“…The model was initialized globally with zero winds and a vertical temperature profile based on the August mean atmosphere for West Africa at 5 • E taken from Aspliden and Adefolalu (1976). The initial period is thus one of geostrophic and physical process adjustment.…”

Section: Modelling Approachmentioning

confidence: 99%

The impact of moist processes on the African easterly jet–African easterly wave system

Cornforth

Hoskins

Thorncroft

2009

Quart J Royal Meteoro Soc

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ABSTRACT:The African Easterly Jet-Easterly Wave (AEJ-AEW) system was explored in an idealised model. Prescribed zonally symmetric surface temperature and moisture profiles determine the AEJ which becomes established through meridional contrasts in dry and moist convection.As in previous studies, a realistic AEJ developed with only dry convection. Including moist processes, increased its development rate, but reduced its speed and meridional extent. AEWs grew through barotropic-baroclinic conversions. Negative meridional potential vorticity (PV) gradients arose in the zonally symmetric state through the intrusion of the low-PV Saharan boundary layer. Since moist processes strengthened this significantly through diabatically generated PV in the Intertropical Convergence Zone, moist AEWs were three times stronger. Larger barotropic conversions and faster AEJ development increased the moist wave growth-rate. Jet-level and northerly low-level amplitudes grew, but in the moist case the low-level amplitudes weakened as the AEW interacted with convection, consistent with their absence from observations during the peak monsoon. Striking dependencies between the AEJ, AEW and rainfall existed. Two time-scales governed their evolution, depending on the transfer coefficients: (1) the AEJ's replenishment rate influenced by heat fluxes, and (2) the wave growth-rate, by damping, and the slower jet development rate.Moist AEWs were characterized by intermittent growth/decay, with growth preceded by increased mean rainfall and later, weakening AEJs. These dependencies established an internal 8-10-day variability, consistent with intra-seasonal observations of 9-day rainy sequences. This internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist AEW life cycle.

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“…These slowly-evolving cloud formations are very persistent, but give little rainfall. On the neighbouring land mass the result is almost complete disappearance of rainfall, and also a cooling of the lower atmospheric layers as a result of the persistence of this cloud cover which intercepts solar radiation (Aspliden & Adefolalu 1976). In the African equatorial zone today the coolest part of the year is that dominated by stratiform cloud (Maley 1987(Maley , 1989.…”

Section: Climatic and Ecological Conditions Relating To The Developmementioning

confidence: 99%

The African rain forest – main characteristics of changes in vegetation and climate from the Upper Cretaceous to the Quaternary

Maley

1996

Proc., Sect. B Biol. sci.

303

473

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SynopsisThis chapter sets out to give a historical overview of the African rain forest from its origins, towards the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms which were then becoming established. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa which in turn led to the extension of tropical forest to various sites on the shores of the Tethys Sea. There were probably at this time common taxa and similar vegetation patterns stretching from the Gulf of Guinea to the Tethys Sea.Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo–Zaïre basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, with a periodicity of about 100 000 years determined by the orbital variations of the earth around the sun, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. The most arid periods resulted in the fragmentation of the forest cover, and the forest biotopes and their biodiversity were preserved in a series of refugia. The lowering of temperatures also resulted in the extension of montane flora to low altitudes, with migration of montane flora and fauna between main mountain ranges. These compounded phenomena of isolation and migration, probably involving genie exchange, must have resulted in numerous speciation phenomena. Subsequently, such montane flora or fauna became isolated on mountain areas during periods of maximum warming, in the last instance in the course of the Holocene, when a vast forest cover became re-established from Guinea westwards, and to the East as far as the Lake Victoria area. The phases of maximum fragmentation, which appear to have been connected with only the coldest periods – in the last instance during the second part of isotopic stages 6 (fromc.160 to 130 000 years) and 2 (fromc.24 to 12000 years BP) – relate to less than 10% of the last 800 000 years, and the phases of maximum forest extension would likewise appear to be less than 10% of the period. The remaining 80–90% of the time relates to ‘intermediate situations’ which varied from period to period, and these intermediate extension situations seem to have been the norm over the larger part of the Quaternary, rather than the present situation which is closer to a situation of maximum extension.

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The Mean Troposphere of West Africa

Cited by 28 publications

References 1 publication

The seasonal cycle in the lower troposphere over West Africa from sounding observations

The seasonal cycle in the lower troposphere over West Africa from sounding observations

The impact of moist processes on the African easterly jet–African easterly wave system

The African rain forest – main characteristics of changes in vegetation and climate from the Upper Cretaceous to the Quaternary

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