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CHAPTER 1: INTRODUCTION
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1.1 Aims of the Thesis
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1.2 The Sahel: geographic and climatic setting
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1.3 The broader context: drought and famine in the
Sahel
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1.4 Twentieth century rainfall variability and climate
change in the Sahel
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1.5 Atmospheric dust and Sahelian drought
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1.6 Chapter summary
Top
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CHAPTER 2: CLIMATE CHANGE IN
THE SAHEL: THE SCIENTIFIC CONTEXT
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2.1 Introduction
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2.2 Overview
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2.3 Historical and palaeoclimatic context
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2.4 The land surface and Sahelian climate change
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2.5 The role of the oceans and global temperature
patterns
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2.5.1 Sahel rainfall and sea surface temperatures
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2.5.2 Other ocean-atmosphere interactions
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2.5.3 The thermohaline circulation
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2.5.4 The impact of sulphate aerosols
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2.5.5 The effect of other aerosol types on global
temperature patterns
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2.5.6 Sahel rainfall and twentieth century global
and hemispheric air temperatures
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2.6 Atmospheric circulation and drought in the Sahel
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2.7 Summary of potential rainfall modulation mechanisms:
the relative importance of "internal" and "external"
processes
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2.8 The importance of North African dust emissions
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2.9 General climatology of North African dust
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2.9.1 Magnitude of North African dust emissions
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2.9.2 Source regions
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2.9.3 Seasonality of dust production
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2.9.4 Westward dust transport over the Atlantic
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2.10 Recent increases in dust concentrations
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2.11 Reasons for changes in dust production
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2.11.1 Land-surface properties and dust production
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2.11.2 Dust emission and wind
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2.12 Dust aerosol characteristics and radiative forcing
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2.12.1 Dust-radiation interactions
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2.12.2 Size distribution of North African dust aerosols
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2.12.3 optical depth as an indicator of aerosol concentration
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2.13 Thermal impact of tropospheric dust
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2.13.1 Dependence of impact of tropospheric dust on
the underlying surface
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2.13.2 Thermal impact of dust over the ocean
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2.13.3 Thermal impact of dust over northwestern Africa
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2.14 Dynamical impact of North African dust
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2.14.1 The Saharan Air Layer (SAL)
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2.14.2 Dust and convection
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2.14.3 Consequences for the regional circulation
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2.15 Summary
Top
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CHAPTER 3: DATA AND GENERAL
METHODOLOGY (summary)
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3.1 Introduction
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3.2 Observational data
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3.3 Geophysical Fluid Dynamics Laboratory model data
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3.4 Hadley Centre model data
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3.5 Reanalysis data
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3.6 Remote sensing of atmospheric dust and the Infra-red
Difference Dust Index
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3.6.1 Physical considerations
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3.6.2 The Infra-red Difference Dust Index (IDDI):
technical background
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3.6.3 Creation of the final IDDI climatology
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3.6.4 Validation and interpretation of the IDDI
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3.7 Methodology: correlation analyses and significance
testing
Top
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CHAPTER 4: MODELLING SAHEL CLIMATE
ON MILLENNIAL TIMESCALES
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4.1 Introduction
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4.2 The historical and palaeoclimatic context
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4.2.1 The importance of an historical context
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4.2.2 Palaeodata and historical records
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4.2.3 Problems of interpretation of historical records
and palaeodata
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4.3 Use of long-timescale GCM simulations
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4.4 Data and methodology
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4.4.1 Processing of rainfall data
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4.4.2 Analysis of rainfall variability
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4.4.3 Analysis of rainfall-temperature relationships
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4.5 Results from observed data
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4.5.1 Changes in persistence of Sahelian rainfall
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4.5.2 Associations between observed Sahel rainfall
and global SST patterns
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4.6 GFDL model results
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4.6.1 Mean rainfall
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4.6.2 Rainfall seasonality
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4.6.3 Modelled wet and dry episodes
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4.6.4 A model generated multi-decadal scale dry episode
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4.6.5 Interannual variability and persistence
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4.6.6 Correlation of Sahel rainfall with local air
temperatures
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4.6.7 IHTC-rainfall correlations
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4.6.8 Summary of GDFL results
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4.7 HadCM2 results
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4.7.1 Rainfall variability
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4.7.2 Rainfall seasonality
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4.7.3 Comparison of modelled and observed twentieth
century rainfall
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4.7.4 Modelled twenty first century rainfall
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4.7.5 Modelled Sahel rainfall and local air temperatures
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4.7.6 IHTC rainfall correlations
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4.7.7 Summary of HadCM2 results
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4.8 Summary
Top
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CHAPTER 5: DUST VARIABILITY OVER
NORTHERN AFRICA
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5.1 Introduction
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5.2 Scales of analysis
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5.3 Study areas
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5.4 The issue of land degradation
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5.5 Identification of dust source regions
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5.5.1 The north-central Sahel
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5.5.2 The west Sahara and western Sahel
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5.5.3 The east Sahel-Sahara
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5.5.4 Other Sahelo-Saharan sources
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5.5.5 Seasonal patterns of dust production and non-Sahelo-Saharan
sources
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5.5.6 Character of the source regions
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5.6 Sahel and Sahara IDDI indices: a comparison
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5.6.1 Mean zonal variations of IDDI
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5.6.2 Locating the meridional maximum of dust production
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5.6.3 Commonality of Sahelian and Saharan dust loadings
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5.7 Interannual variability of the IDDI
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5.7.1 Annual IDDI anomalies
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5.7.2 Wet season IDDI anomalies
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5.7.3 Early dry season (November-December) IDDI anomalies
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5.7.4 Late dry season (January-April) IDDI anomalies
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5.7.5 Individual monthly IDDI anomalies
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5.8 Summary
Top
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CHAPTER 6: CLIMATIC IMPACTS
ON DUST PRODUCTION
(see summary of findings)
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6.1 Introduction
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6.2 Deflation and land-surface properties
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6.3 Data and methodology
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6.3.1 Dust-rainfall relationships
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6.3.2 Dust-rainfall correlation analyses
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6.3.3 Field intercomparisons between IDDI and climatological
data
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6.3.4 Climatological indices and IDDI values
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6.3.5 Easterly wave activity and IDDI values
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6.4 Results I: IDDI-rainfall field intercomparisons
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6.4.1 Annual anomalies
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6.4.2 Wet season anomalies
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6.4.3 Early dry season anomalies
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6.4.4 Late dry season anomalies
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6.5 Results II IDDI-rainfall correlations
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6.5.1 Lag 1-year relationships
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6.5.2 Lag 2-year relationships
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6.6 Results III: Dust production and sea-level pressure
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6.6.1 General description
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6.6.2 SLP anomalies associated with high dust loadings
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6.6.3 SLP anomalies associated with low dust loadings
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6.6.4 Summary and discussion of IDDI-SLP field relationships
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6.7 Results IV: Local IDDI values and pressure gradients
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6.7.1 IDDI values and meridional pressure variations
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6.7.2 IDDI values and zonal pressure variations
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231
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6.8 Results V: Relationships between IDDI and 1000
hPa wind
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6.8.1 Correspondence of IDDI values with surface wind
values
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235
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6.8.2 Dust loadings and wind anomalies
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238
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6.9 Results VI: Dust production and easterly wave
activity
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6.10 Summary
Top
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CHAPTER 7: THE IMPACT OF
ATMOSPHERIC DUST ON CLIMATE (see summary of
findings)
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247
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7.1 Introduction
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247
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7.2 Hypotheses to be tested and aims of the analysis
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248
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7.2.1 Dust-temperature relationships
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7.2.2 Dust-vertical motion relationships
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250
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7.2.3 Dust impacts on the West African Monsoon
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7.3 Methodology and data processing
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7.3.1 In situ correlations with temperature
and vertical velocity
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7.3.2 IDDI relationships with the monsoonal air flow
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253
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7.4 Results I: IDDI-temperature correlations
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255
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7.4.1 Statistical distribution of correlations
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255
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7.4.2 Interpretation of significance of individual
correlations
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262
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7.4.3 Vertical distribution of correlations
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7.4.4 Diurnal variation of correlations
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265
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7.4.5 Geographical and seasonal variations and altitudes
of transport
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7.4.6 Summary of IDDI-temperature relationships
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274
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7.5 Results II: IDDI-vertical motion correlations
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7.5.1 Statistical distribution of correlations
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7.5.2 1000 hPa fields
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7.5.3 850 hPa fields
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7.5.4 700 hPa fields
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7.5.5 600 hPa fields
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7.5.6 200 hPa fields
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285
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7.5.7 Direction of the causal relationship between
dust and vertical velocity
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286
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7.6 Results III: Meridional wind indices and dust
loadings
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288
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7.7 Summary
Top
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293
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CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS
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297
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8.1 Introduction
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297
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8.2 Climatic and environmental change in the Sahel
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298
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8.3 General conclusions regarding the Infra-red Difference
Dust Index
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300
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8.4 The IDDI as diagnostic of climate and land-surface
change
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301
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8.5 Dust impacts on climate
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306
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8.6. The dust cycle and dust-climate feedback processes
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309
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8.7. Recommendations for future work
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8.7.1 Data improvement
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8.7.2 Modelling activities
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8.7.3. Empirical analysis
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8.8. Concluding Statements
Top
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REFERENCES
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315
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APPENDIX I: COMPOSITE FIELDS
OF IDDI AND SLP ANOMALIES
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APPENDIX II: COMPOSITE FIELDS OF IDDI AND 1000 hPa
WIND ANOMALIES
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APPENDIX III: FIELDS OF LOCAL IDDI-TEMPERATURE CORRELATIONS
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APPENDIX IV: FIELDS OF LOCAL IDDI-VERTICAL VELOCITY CORRELATIONS
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