Sharon E. Nicholson

The nature of rainfall variability over Africa on time scales of decades to millenia

This paper begins with an overview of the African rainfall regime, noting in particular the contrast among various regions . of the continent, followed by a description of the nature of climatic i.e., rainfall variability over Africa on time scales of decades and centuries. The decadal scale is examined using modern data covering the twentieth century. The century scale is examined using historical reconstructions of climate, based on a combination of geologic, geographic and historical . information e.g., lake chronologies, landscape descriptions, archives and diaries . The presentation includes some results of an analysis of a new historical semi-quantitative data set for Africa covering the last two centuries. It was produced using a combination of historical information, nineteenth century rainfall records, and statistical relationships among various sectors of Africa. Presented here are reconstructions of lake level fluctuations for numerous lakes of eastern and southern Africa. This overview of climatic fluctuations is utilized to uncover inherent spatial and temporal characteristics of the rainfall variability. The dominance over time of various spatial modes is emphasized and the questions of synchroneity of the hemispheres and the abruptness of change are considered. The contrast between the two hemispheres is also surveyed, notably the different time scales of variability and potential causal factors in the variability. One of the most important contrasts is the multi-decadal persistence of anomalies over most of northern Africa. This has implications for the causes of long-term fluctuations, even those historical and paleo-time scales. q 2000 Elsevier Science B.V. All rights reserved.

Desertification, Drought, and Surface Vegetation: An Example from the West African Sahel

Many assumptions have been made about the nature and character of desertification in West Africa. This paper examines the history of this issue, reviews the current state of our knowledge concerning the meteorological aspects of desertification, and presents the results of a select group of analyses related to this question. The common notion of desertification is of an advancing “desert,” a generally irreversible anthropogenic process. This process has been linked to increased surface albedo, increased dust generation, and reduced productivity of the land. This study demonstrates that there has been no progressive change of either the Saharan boundary or vegetation cover in the Sahel during the last 16 years, nor has there been a systematic reduction of “productivity” as assessed by the water-use efficiency of the vegetation cover. While it also showed little change in surface albedo during the years analyzed, this study suggests that a change in albedo of up to 0.10% since the 1950s is conceivable.

THE RELATIONSHIP OF THE EL NIÑO–SOUTHERN OSCILLATION TO AFRICAN RAINFALL

This is a comprehensive study of the rainfall response over Africa to ENSO episodes in the Pacific. The harmonic method utilized by Ropelewski and Halpert is applied to 90 regionally averaged rainfall time series for the period 1901‐1990. The analysis was a composite of 20 episodes within this period. Seasons of maximum positive anomalies and maximum negative anomalies in the composite were identified. The method identifies 15 multiregion sectors where ENSO appears to modulate rainfall. The strongest signals are in eastern equatorial and south-eastern Africa. A continental-scale signal is also apparent. The magnitude, seasonal timing and duration, and consistency of the rainfall response to ENSO vary among the sectors and from episode to episode. The rainfall response is clearly seasonally specific. In general, the onset of the ENSO signal in rainfall commences far to the south and propagates latitudinally northward. For this reason, the equatorial regions are out-of-phase with the continental pattern. The ENSO mechanism is probably responsible for many of the well-established rainfall teleconnections over the continent, including the strong tendency for opposite anomalies in equatorial and southern Africa. There is a strong tendency for positive anomalies to occur during the first half of the ENSO cycle, negative during the second half. This corresponds to ‘cold’ and ‘warm’ phases in the adjacent Atlantic and Indian Oceans; continentally, rainfall tends to be enhanced during the cold phase, reduced during the warm phase. The northward propagation is most pronounced during the cold phase; a similar propagation and phase shift occurs at this time in the Atlantic. The rainfall anomalies of the warm phase are nearly constant in phase, as are the SST anomalies in the Indian Ocean. This suggests that, in general, the Atlantic Ocean controls rainfall during the cold phase, the Indian Ocean during the warm phase.

A comparison of the vegetation response to rainfall in the Sahel and East Africa, using normalized difference vegetation index from NOAA AVHRR

AbstractThis article presents the results of a study of the relationship between rainfall and Normalized Difference Vegetation Index (NDVI) in East Africa and the Sahel. Monthly data for the years 1982 to 1985 have been analyzed. We have evaluated NDVI-rainfall relationships by vegetation type, using the major formations described by White (1983). In the article, a comparison of the differential response of vegetation growth to rainfall in the two study regions is emphasized.The most important conclusions of our research are as follows:(1)The spatial patterns of annually-integrated NDVI closely reflect mean annual rainfall.(2)There is a good relationship between rainfall variations and NDVI on seasonal and interannual time scales for areas where mean annual rainfall ranges from approximately 200 to 1200 mm.(3)In most cases, NDVI is best correlated with the rainfall total for the concurrent plus two antecedent months; the correlation is better in the Sahel than in East Africa.(4)The ratios of NDVI to rainfall are considerably higher in East Africa than in the Sahel.(5)Mean annually-integrated NDVI is linearly related to mean annual rainfall in the Sahel. In East Africa the relationship is approximately log-linear; above some threshold value of rainfall, NDVI values level off and vary minimally with rainfall. Two possible explanations of this last conclusion are suggested: above this threshold, rainfall is no longer the limiting factor in vegetation growth and/or NDVI is not a good indicator of vegetation growth. The latter is a likely possibility since NDVI directly reflects photosynthetic activity and becomes a poor indicator of biomass (i.e., growth) as high canopy densities are reached. The NDVI-rainfall relationship for East Africa is markedly similar to the relationship between NDVI and Leaf Area Index demonstrated by Sellers (1985) and Asrar et al. (1984).

Land surface processes and Sahel climate

This paper examines the question of land surface-atmosphere interactions in the West African Sahel and their role in the interannual variability of rainfall. In the Sahel, mean rainfall decreased by 25–40% between 1931–1960 and 1968–1997; every year in the 1950s was wet, and nearly every year since 1970 has been anomalously dry. Thus the intensity and multiyear persistence of drought conditions are unusual and perhaps unique features of Sahel climate. This article presents arguments for the role of land surface feedback in producing these features and reviews research relevant to land surface processes in the region, such as results from the 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX)-Sahel experiment and recent studies on aerosols and on the issue of desertification in the region, a factor implicated by some as a cause of the changes in rainfall. Included also is a summary of evidence of feedback on meteorological processes, presented from both model results and observations. The reviewed studies demonstrate numerous ways in which the state of the land surface can influence interactions with the atmosphere. Surface hydrology essentially acts to delay and prolong the effects of meteorological drought. Each evaporative component of the surface water balance has its own timescale, with the presence of vegetation affecting the process both by delaying and prolonging the return of soil moisture to the atmosphere but at the same time accelerating the process through the evaporation of canopy-intercepted water. Hence the vegetation structure, including rooting depth, can modulate the land-atmosphere interaction. Such processes take on particular significance in the Sahel, where there is a high degree of recycling of atmospheric moisture and where the meteorological processes from the scale of boundary layer development to mesoscale disturbance generation are strongly influenced by moisture. Simple models of these feedback processes and their various timescales have demonstrated that the net feedback to the atmosphere is positive for both wet and dry surface anomalies. Hence the role of the surface is to reinforce meteorologically induced changes. Recovery from the dry state is slower than from the wet state, suggesting that dry conditions would tend to persist longer, as is actually observed in the Sahel. These simple models suggest that the surface hydrology locks the system into a drought mode that persists for several years, until the system randomly slips into a persistent wet mode. The hypothesis that desertification in the Sahel might likewise be responsible for the persistent drought is found to be untenable. Rather than a progressive encroachment of the desert onto the savanna, the vegetation cover responds dramatically to interannual fluctuations in rainfall. There is little evidence of large-scale denudation of soils, increase in surface albedo, or reduction of the productivity of the land, although degradation has probably occurred in some areas. There has, however, been a steady buildup of dust in the region over the last half a century. Significant radiative effects of the dust have been demonstrated; therefore the dust has probably influenced large-scale climate. The buildup is probably mainly a result of changes in the land surface that accompanied the shift to drier conditions, but it may have been exacerbated by anthropogenic factors. Complex general circulation models nearly universally underscore the importance of feedback processes in the region. Although it has not been unequivocally demonstrated that the rainfall regime of the Sahel is modulated by surface processes, there is recent observational evidence that this is case.

Validation of TRMM and Other Rainfall Estimates with a High-Density Gauge Dataset for West Africa. Part II: Validation of TRMM Rainfall Products

Gauge data from a West African network of 920 stations are used to assess Tropical Rainfall Measuring Mission (TRMM) satellite and blended rainfall products for 1998. In this study, mean fields, scattergrams, and latitudinal transects for the months of May‐September and for the 5-month season are presented. Error statistics are also calculated. This study demonstrates that both the TRMM-adjusted Geostationary Observational Environmental Satellite precipitation index (AGPI) and TRMM-merged rainfall products show excellent agreement with gauge data over West Africa on monthly-to-seasonal timescales and 2.5 83 2.58 latitude/longitude space scales. The root-mean-square error of both is on the order of 0.6 mm day 21 at seasonal resolution and 1 mm day21 at monthly resolution. The bias of the AGPI is only 0.2 mm day21, whereas the TRMM-merged product shows no bias over West Africa. Performance at 1.0 83 1.08 latitude/longitude resolution is also excellent at the seasonal scale and good for the monthly scale. A comparison with standard rainfall products that predate TRMM shows that AGPI and the TRMM-merged product perform as well as, or better than, those products. The AGPI shows marked improvement when compared with the GPI, in reducing the bias and in the scatter of the estimates. The TRMM satellite-only products from the precipitation radar and the TRMM Microwave Imager do not perform well over West Africa. Both tend to overestimate gauge measurements.

African environmental and climatic changes and the general atmospheric circulation in late pleistocene and holocene

The paper describes the environmental and climatic changes which took place in Africa from the late Pleistocene through the Holocene and the general atmospheric circulation patterns which likely correspond to them. Three major periods are considered: (1) a period of aridity and dune building c. 20,000-12,000 B.P. in which the Sahara advanced considerably southward; (2) a moist, lacustrine period c. 10,000-8,000 B.P.; and (3) a second moist, lacustrine period toward c. 6,500-4,500 B.P. in which the entire Sahara desert contracted considerably. The prevailing atmospheric circulation patterns are theorized on the basis of corresponding changes of surface boundary conditions-primarily changing thermal character—and known dynamic behavior of the atmosphere.

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

The West African Sahel is well known for the severe droughts that ravaged the region in the 1970s and 1980s. Meteorological research on the region has flourished during the last decade as a result of several major field experiments. This paper provides an overview of the results that have ensued. A major focus has been on the West African monsoon, a phenomenon that links all of West Africa. The characteristics and revised picture of the West African monsoon are emphasized. Other topics include the interannual variability of rainfall, the atmospheric circulation systems that govern interannual variability, characteristics of precipitation and convection, wave activity, large-scale factors in variability (including sea-surface temperatures), and land-atmosphere relationships. New paradigms for the monsoon and associated ITCZ and for interannual variability have emerged. These emphasize features in the upper atmosphere, as well as the Saharan Heat Low. Feedback mechanisms have also been emphasized, especially the coupling of convection with atmospheric dynamics and with land surface characteristics. New results also include the contrast between the premonsoon and peak monsoon seasons, two preferred modes of interannual variability (a latitudinal displacement of the tropical rainbelt versus changes in its intensity), and the critical importance of the Tropical Easterly Jet.

A Study of the Dynamic Factors Influencing the Rainfall Variability in the West African Sahel

Abstract This study examines selected dynamical factors associated with wet and dry years in the West African Sahel. The approach is to evaluate the temperature, wind, and moisture fields and the dynamic instabilities for a 4-yr“wet” composite (1958–61) and a 4-yr “dry” composite (1982–85). The analysis, limited to the months of June through September, is carried out using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-Year Reanalysis Project dataset. Two upper-air datasets are also evaluated to confirm trends apparent in the NCEP–NCAR data. This study confirms some of the results of earlier studies, such as the weaker African easterly jet (AEJ) and stronger tropical easterly jet (TEJ) during wet years, but suggests a different interpretation of the wet–dry contrasts. In the Sahel, the most important characteristic of the AEJ appears to be its latitudinal location rather than its intensity. This governs the instability mechanisms. The AEJ is displ...

The Spatial Coherence of African Rainfall Anomalies: Interhemispheric Teleconnections

Abstract The spatial patterns of rainfall variability over the African continent are examined using a regionally averaged data set comprising the records of 1087 stations. Typical rainfall departure patterns, or anomaly types, are derived using a linear correlation method to assess the similarity of annual departure maps for the years 1901 to 1973. For the region north of the equator, most years fall into one of six types, four of which show a strong opposition between equatorial and subtropical latitudes and two of which show departures of the same sign over the whole region. For the continent as a whole, six types are also derived. These reflect the patterns derived using northern Africa alone, with conditions in the southern subtropics resembling those in subtropical latitudes north of the equator. Continentally, two principal spatial patterns of rainfall variation are evident: anomalies of the same sign over most of Africa and anomalies of the opposite sign in equatorial and subtropical latitudes. Thu...