A review of drought concepts

Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003

. This paper presents a comprehensive review and analysis of the available literature and information on droughts to build a continental, regional and country level perspective on geospatial and temporal variation of droughts in Africa. The study is based on the review and analysis of droughts occurred during 1900–2013, as well as evidence available from past centuries based on studies on the lake sediment analysis, tree-ring chronologies and written and oral histories and future predictions from the global climate change models. Most of the studies based on instrumental records indicate that droughts have become more frequent, intense and widespread during the last 50 years. The extreme droughts of 1972–1973, 1983–1984 and 1991–1992 were continental in nature and stand unique in the available records. Additionally, many severe and prolonged droughts were recorded in the recent past such as the 1999–2002 drought in northwest Africa, 1970s and 1980s droughts in western Africa (Sahel), 2010–2011 drought in eastern Africa (Horn of Africa) and 2001–2003 drought in southern and southeastern Africa, to name a few. The available (though limited) evidence before the 20th century confirms the occurrence of several extreme and multi-year droughts during each century, with the most prolonged and intense droughts that occurred in Sahel and equatorial eastern Africa. The complex and highly variant nature of many physical mechanisms such as El Nino–Southern Oscillation (ENSO), sea surface temperature (SST) and land–atmosphere feedback adds to the daunting challenge of drought monitoring and forecasting. The future predictions of droughts based on global climate models indicate increased droughts and aridity at the continental scale but large differences exist due to model limitations and complexity of the processes especially for Sahel and northern Africa. However, the available evidence from the past clearly shows that the African continent is likely to face extreme and widespread droughts in future. This evident challenge is likely to aggravate due to slow progress in drought risk management, increased population and demand for water and degradation of land and environment. Thus, there is a clear need for increased and integrated efforts in drought mitigation to reduce the negative impacts of droughts anticipated in the future.

/pdf/a-review-of-droughts-on-the-african-continent-a-geospatial-4fj42g8jwj.pdf

A review of droughts on the African continent: a geospatial and long-term perspective

This study analyzes areal changes and regional climate variations in global semi-arid regions over 61 years (1948–2008) and investigates the dynamics of global semi-arid climate change. The results reveal that the largest expansion of drylands has occurred in semi-arid regions since the early 1960s. This expansion of semi-arid regions accounts for more than half of the total dryland expansion. The area of semi-arid regions in the most recent 15 years studied (1990–2004) is 7 % larger than that during the first 15 years (1948–1962) of the study period; this expansion totaled 0.4 × 106 and 1.2 × 106 km2 within the American continents and in the Eastern Hemisphere, respectively. Although semi-arid expansion occurred in both regions, the shifting patterns of the expansion are different. Across the American continents, the newly formed semi-arid regions developed from arid regions, in which the climate became wetter. Conversely, in the continental Eastern Hemisphere, semi-arid regions replaced sub-humid/humid regions, in which the climate became drier. The climate change in drying semi-arid regions over East Asia is primarily dominated by a weakened East Asian summer monsoon, while the wetting of semi-arid regions over North America is primarily controlled by enhanced westerlies.

/pdf/global-semi-arid-climate-change-over-last-60-years-4zbkf2w2e3.pdf

Global semi-arid climate change over last 60 years

The arrival of the summer monsoon over West Africa has been documented by using daily gridded rainfall data and NCEP‐NCAR reanalyses during the period 1968‐90, and OLR data over the period 1979‐90. Two steps have been characterized through a composite approach: the preonset and the onset of the summer monsoon. The preonset stage corresponds to the arrival in the intertropical front (ITF) at 15 8N, that is, the confluence line between moist southwesterly monsoon winds and dry northeasterly Harmattan, bringing sufficient moisture for isolated convective systems to develop in the Sudano‐Sahelian zone while the intertropical convergence zone (ITCZ) is centered at 58N. The mean date for the preonset occurrence is 14 May and its standard deviation is 9.5 days during the period 1968‐90. This leads to a first clear increase of the positive rainfall slope corresponding to the beginning of the rainy season over this Sudano‐Sahelian area. The onset stage of the summer monsoon over West Africa is linked to an abrupt latitudinal shift of the ITCZ from a quasi-stationary location at 58N in May‐June to another quasi-stationary location at 108N in July‐August. The mean date for the onset occurrence is 24 June and its standard deviation is 8 days during the period 1968‐ 90. This leads to a second increase of the positive rainfall slope over the Sudano‐Sahelian zone signing the northernmost location of the ITCZ and the beginning of the monsoon season. This abrupt shift occurs mostly between 108W and 58E, where a meridional land‐sea contrast exists, and it is characterized by a temporary rainfall and convection decrease over West Africa. Preonset dates, onset dates, and summer rainfall amount over the Sahel are uncorrelated during the period 1968‐90. The atmospheric dynamics associated with the abrupt ITCZ shift has been investigated. Between the preonset and the onset stages, the heat low dynamics associated with the ITF controls the circulation in the low and midlevels. Its meridional circulation intensity is the highest at the beginning of the monsoon onset. This can lead to 1) increased convective inhibition in the ITCZ through intrusion of dry and subsiding air from the north, and 2) increased potential instability through a greater inland moisture advection and a higher monsoon depth induced by a stronger cyclonic circulation in the low levels, through higher vertical wind shear due to westerly monsoon wind and midlevel African easterly jet (AEJ) increases, through enhancement of the instability character of the AEJ, and through increased shortwave radiation received at the surface. During the monsoon onset, once the rainfall minimum occurred due to the convective inhibition, the accumulated potential instability breaks the convective inhibition, the inertial instability of the monsoon circulation is released, and the associated regionalscale circulation increases, leading to the abrupt shift of the ITCZ. Then the ITCZ moves north up to 108N, where thermodynamical conditions are favorable. It is suggested by the authors that the abrupt shift of the ITCZ, initiated by the amplification of the heat low dynamics, could be due to an interaction with the northern orography of the Atlas‐Ahaggar Mountains. Subsidence over and north of this orography, due to both the northern branches of the heat low and of the northern Hadleytype cell, contributes to enhance the high geopotentials north of these mountains and the associated northeasterly winds. This leads to the development of a leeward trough that reinforces the heat low dynamics, maintaining an active convective ITCZ through enhanced moist air advection from the ocean, increasing the northern Hadley circulation, which reinforces the high geopotentials and the interaction with the orography through a positive feedback. The fact that an abrupt shift of the ITCZ is only observed on the western part of West Africa may result from the enhancement of moisture advection, which comes from the west and has a stronger impact west of the Greenwich meridian. The northwest‐southeast orientation of the Atlas‐Ahaggar crest can induce the interaction with the heat low, first in the east where the mountains are nearer to the ITF than in the west, and second in the west. Another consequence of the possible orography-induced interaction with the atmospheric circulation is that the induced leeward trough, increasing the cyclonic vorticity in the heat low, may stimulate moisture convergence in the oceanic ITCZ near the western coast of West Africa.

https://hal.archives-ouvertes.fr/hal-00155510/document

The West African Monsoon Dynamics. Part II: The “Preonset” and “Onset” of the Summer Monsoon

The correlation between Sahel rainfall and El Nino–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200 hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Nino-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

Analysis of 149 raingauge series (1946–1988) shows a weak positive correlation between late summer rainfalls (January–March) in tropical southern Africa and the Southern Oscillation Index (SOI). The correlation coefficients have been unstable since World War II. They were close to zero before 1970 and significant thereafter. Before 1970, southern African late summer rainfalls were more specifically correlated with regional patterns of sea surface temperature (SST), mainly over the southwestern Indian Ocean. After 1970, teleconnections with near global SST anomaly patterns, i.e. over the central Pacific and Indian oceans, dominate the regional connections. The increase in the sensitivity of the southern African rainfall to the global SO-related circulation anomalies is simultaneous with the correlation between SOI and more extensive SST anomalies, particularly over the southern Indian Ocean. This feature is part of longer term (decadal), global SST variability, as inferred from statistical analyses. Numerical experiments, using the Meteo-France general circulation model ARPEGE-Climat, are performed to test the impact of the observed SST warming in the southern Indian and extratropical oceans during El Nino Southern Oscillation (ENSO) events on southern African rainfall. Simulated results show that ENSO events, which occurred in the relatively cold background of the pre-1970 period in the southern oceans, had a little effect on southern Africa climatic conditions and atmospheric circulation. By contrast, more recent ENSO events, with warmer SST over the southern oceans, lead to a climatic bipolar pattern between continental southern African and the western Indian Ocean, which is characterized by reduced (enhanced) deep convection and rainfall over the subcontinent (the western Indian Ocean). A weaker subtropical high-pressure belt in the southwestern Indian Ocean is also simulated, along with a reduced penetration of the moist southern Indian Ocean trade winds over the southern African plateau. These results are consistent with the strong droughts observed over all southern Africa during ENSO events since 1970.

Modification of the southern African rainfall variability/ENSO relationship since the late 1960s

The relationship between sea-surface temperature (SST) inter-annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950–1999 period. Exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer. Interestingly, these areas are also significantly associated with the far southwestern Pacific (Tasman Sea area). The patterns and time series of co-variability between the southern Atlantic and Indian Oceans are shown to correspond to SST modes previously described in the literature as ‘subtropical dipoles’, independently for the Atlantic and Indian Oceans. Composite analyses show that austral summers characterized by simultaneous warm (and to a lesser extent cold) SST anomalies in the southwestern (northern) part of both southern oceans are related to atmospheric anomalies mainly involving a southward shift and a strengthening of the subtropical high-pressure systems over both basins. These anomalies are embedded in a hemispheric signal associating two cores of positive pressure anomalies within the South Pacific anticyclone. The global picture appears to have a wave number 4 spatial structure. The associated low-level wind and latent heat-flux anomalies and the lags between atmospheric variables and SST anomalies are consistent with an atmospheric forcing on the ocean. Potential links of these patterns with large-scale modes of climate variability in the Southern Hemisphere are discussed. Copyright © 2003 Royal Meteorological Society.

Sea-surface temperature co-variability in the Southern Atlantic and Indian Oceans and its connections with the atmospheric circulation in the Southern Hemisphere

This article investigates through numerical ex- periments the controversial question of the impact of El Nino-Southern Oscillation (ENSO) phenomena on cli- mate according to large-scale and regional-scale inter- hemispheric thermal contrast. Eight experiments (two considering only inversed Atlantic thermal anomalies and six combining ENSO warm phase with large-scale inter- hemispheric contrast and Atlantic anomaly patterns) were performed with the Me« te« o-France atmospheric general circulation model. The definition of boundary conditions from observed composites and principal components is presented and preliminary results concerning the month of August, especially over West Africa and the equatorial Atlantic are discussed. Results are coherent with observa- tions and show that interhemispheric and regional scale sea-surface-temperature anomaly (SST) patterns could significantly modulate the impact of ENSO phenomena: the impact of warm-phase ENSO, relative to the atmo- spheric model intercomparison project (AMIP) climato- logy, seems stronger when embedded in global and regional SSTA patterns representative of the post-1970 conditions (i.e. with temperatures warmer (colder) than the long-term mean in the southern hemisphere (northern hemisphere)). Atlantic SSTAs may also play a significant role.

/pdf/global-atmospheric-response-to-specific-linear-combinations-39t9n8kosk.pdf

Global atmospheric response to specific linear combinations of the main SST modes. Part I: numerical experiments and preliminary results

Global atmospheric response to specific linear combinations of the main SST modes.

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