Further Case Studies of Tropical Atlantic Surface Atmospheric and Oceanic Patterns Associated with Sub-Saharan Drought

Lamb, Peter; Peppler, Randy A.

doi:10.1175/1520-0442(1992)005<0476:fcsota>2.0.co;2

Cited by 178 publications

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“…Lamb, 1978a,b;Folland et al, 1986Folland et al, , 1991Lamb and Peppler, 1992;Hastenrath et al, 1995;Ward, 1998;Giannini et al, 2003). Furthermore, Barnston et al (1996) demonstrated that the time-space behaviour of the SST field alone influences the seasonal rainfall in certain regions/seasons for Africa, both on inter-annual and inter-decadal time-scales.…”

Section: Variations In Global Sst Associated With June-september Ethimentioning

confidence: 99%

Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

Segele

Lamb

Leslie

2008

Intl Journal of Climatology

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143

171

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This study uses correlation, regression, and composite analyses for the period 1970-1999 to explore the relationships between the June-September rainfall in the Horn of Africa (especially Ethiopian) and large-scale regional atmospheric circulation patterns across Africa and the Atlantic and Indian Oceans, and global sea surface temperature (SST) anomalies. Abundant rainfall in the Horn of Africa is associated with enhanced westerlies across western and central Africa. These westerlies are produced by a stronger north-east directed mean sea level pressure (MSLP) gradient resulting from MSLP intensification over the Gulf of Guinea and deepening of the monsoon trough across the Arabian Peninsula. This is reflected by a strong correlation (−0.71) between 5-day (pentad) Ethiopian rainfall and the Gulf of Guinea minus the Arabian Peninsula MSLP difference. This correlation decreases to −0.39 when the seasonal cycles are removed from both time series. A wet Horn of Africa monsoon is also associated with deep moist air extending up to mid-troposphere and large water vapour transport convergence across much of Ethiopia, a strong Somali low-level jet, and a strong tropical easterly jet (TEJ). Although there are large changes in TEJ strength, the position of the jet axis shows little variation between wet and dry events. Associated with the TEJ, the strongest upper level divergence occurs at 100 hPa, where the raw/de-seasonalized zonal wind speed correlates negatively (−0.71/−0.23) with the corresponding Ethiopian rainfall at the pentad time-scale. Furthermore, SSTs over the equatorial Pacific, Indian, and southern Atlantic Oceans correlate strongly with contemporary Ethiopian summer rainfall. In general, Ethiopian rainfall is suppressed during El Niño and enhanced during La Niña. This identification and documentation of the regional atmospheric circulation patterns and global SST anomalies directly linked to rainfall variability over Ethiopia/Horn of Africa, are crucial for developing statistical prediction schemes and designing climate model simulations for the region on intra-seasonal to inter-annual time-scales.

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Section: Variations In Global Sst Associated With June-september Ethimentioning

confidence: 99%

Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

Segele

Lamb

Leslie

2008

Intl Journal of Climatology

Self Cite

143

171

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“…Lamb and Peppler, 1992;Nicholson et al, 2000;L'Hôte et al, 2002). For example, Lamb's index (Tarhule and Lamb, 2003;also see Figure 3(b)) has used the same 20 Sahelian stations since it was first developed in the mid-1970s; exactly half of those stations are west/east of the 6°W boundary used by Chappell and Agnew (2004), and only 13 station-years out of 1220 during 1941-2001 had missing data.…”

Section: Flaws In Chappell and Agnew (2004)mentioning

confidence: 99%

“…by standard deviation (SD)) using area-weighted averaging (e.g. Lamb and Peppler, 1992;Rowell et al, 1995;Nicholson et al, 2000). The normalization, combined with the area-weighted averaging, minimizes the impact of varying station networks on the regional time series in a region with a large precipitation gradient, like the Sahel (Jones and Hulme, 1996).…”

Section: Flaws In Chappell and Agnew (2004)mentioning

confidence: 99%

“…Hulme, 1992;Lamb and Peppler, 1992;Nicholson et al, 2000;L'Hôte et al, 2002), vegetation cover (Tucker et al, 1991), dust transport (Prospero and Lamb, 2003), and other agricultural and societal data (e.g. Nicholson et al 1998;Tarhule and Lamb 2003; and numerous reports in news media).…”

Section: Introductionmentioning

confidence: 99%

“…Charney, 1975;Nicholson, 2000), tropical Atlantic and global sea-surface temperature influences (e.g. Folland et al, 1986;Lamb and Peppler, 1992;Ward, 1998;Giannini et al, 2003), and atmospheric wave disturbances (Druyan and Hall, 1996), have been identified and investigated. It is safe to say that the decreasing rainfall and devastating droughts in the Sahel region during the last three decades of the 20th century are among the most undisputed and largest recent climate changes recognized by the climate research community.…”

Section: Introductionmentioning

confidence: 99%

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The recent Sahel drought is real

Dai

Lamb

Trenberth

et al. 2004

Intl Journal of Climatology

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340

207

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Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10-20°N and 18°W-20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology 24: 547-554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded.

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The Role of Orography in Determining the Sahelian Climate

Semazzi

Sun

1997

Int. J. Climatol.

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We have examined numerical modelling evidence based on the NASA GEOS‐1 general circulation model in order to investigate the role of orography in determining the West African summer climate anomalies. Orographic climatic control and the influence of sea‐surface temperature (SST) anomaly forcing were investigated by systematic suppression or inclusion of these effects in the numerical experiments. Our main finding is that the orography of Africa plays a major role in determining the climate of the Sahel and the coastal region of West Africa during the northern hemispheric summer season. These orographic effects are comprised of: (i) a quasi‐stationary orographic ridge–trough dipole generated by the passage of the low‐level prevailing easterlies over the Atlas–Ahaggar mountain complex over northern Africa, and (ii) an elongated zonal windward orographic ridge generated as the cross‐equatorial summer monsoons from the South Atlantic Ocean basin ascends over the elevated landmass of West Africa. Over the Sahel, the moist south‐westerly flow emanating from the Atlas–Ahaggar orographic trough over the Atlantic Ocean and the dry hot north‐easterlies originating from the corresponding orographic windward ridge tend to converge along a zonal axis thus reinforcing the Inter‐Tropical Convergence Zone. This results in wetter conditions over the Sahel than would have been the case in the absence of orography. The model simulations show that further to the south along the coastal region of West Africa, the zonal orographic ridge results in drier conditions than in the model runs when orography is suppressed. The net result is a permanent orographic‐induced rainfall dipole pattern over West Africa. Further model simulations were conducted in which the 1973 SST anomaly conditions were described with or without the orography of Africa. The 1973 SST anomaly pattern is characterized by positive anomalies in the Southern Hemisphere and negative anomalies in the Northern Hemisphere. The model results show a weaker orographic‐induced rainfall dipole pattern across West Africa in response to the 1973 SST anomaly pattern. The net result is wetter conditions along the coastal region and rainfall deficits over the Sahelian zonal strip. © 1997 by the Royal Meteorological Society.

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Further Case Studies of Tropical Atlantic Surface Atmospheric and Oceanic Patterns Associated with Sub-Saharan Drought

Cited by 178 publications

References 0 publications

Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

The recent Sahel drought is real

The Role of Orography in Determining the Sahelian Climate

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