Regional rainfall variability over
Central Africa:
What is influencing it?
Aims and Motivation:
• Investigate the overriding mechanisms responsible rainfall over central Africa.
• Understand the cause of interannual/interdecadal variability during the rainy
season(s). Also, to asses the impacts of Teleconnections over the region.
• Investigate the sensitivity of the Central Africa region to future climate and land
use change through the use of differing scenario’s in a regional climate model.
• Increased understanding of the overriding mechanisms responsible for variations
in rainfall for forecasters.
Background:
•
Central Africa roughly covers 2.6% of the Earth’s surface with an estimated
population of around 120 million people, which is expected to rise to and
estimated 250 million by 2050.
• The countries of central Africa rely economically upon rain-fed agriculture for the
majority of their livelihood(mainly subsidence based).
• Land use change, particularly from deforestation may play a vital role in the
future climate of the region dictated by increasing population pressures.
Processes of influence:
•MCS/Squall lines
•The Inter-tropical Convergence Zone (ITCZ)
•Sea Surface Temperatures (SSTs)
•Regional African Jets:
1. African Easterly Jet-North (AEJ-N)
2. African Easterly Jet-South (AEJ-S)
3. Tropical Easterly Jet (TEJ)
4. Westerly African Jet (WAJ)
Role of the vegetation?
15°N
WAJ
TEJ
5°N
EQ
5°S
AEJ-S
10°S
15°S
Teleconnections?
• EL-Nino Southern Oscillation (ENSO)
• Large-scale circulations (Hadley and Walker circulations)
Dry
AEJ-N
10°N
August
Wet
15°N
10°N
5°N
AEJ-N
EQ
5°S
10°S
TEJ
15°S
AEJ-S
January
15°
N
0° EQ
15°S
Variability in the
rainbelt for August
From Nicholson,
2008 and 2009.
Rainbelt:
Variability in rainfall linked to
variability in Intensity and
position.
Methodology:
Data Sources:
•Rain gauge network
•NCEP-NCAR reanalysis
•ERA-40 reanalysis
Region A
Rainfall (mm)
300
200
100
0
Region B
Regionalisation:
•Based upon seasonal rainfall cycle
Rainfall (mm)
300
100
0
300
Region D
Rainfall (mm)
Rainfall (mm)
Region C
Region A
3000
200
100
0
300
200
100
0
Region E
2000
Region F
Mpika station
1996
1993
1990
1987
1981
1984
Kawa Station
200
100
0
300
rainfall (mm)
Year
Kapa Station
1978
1975
1972
1969
1966
1963
1960
1957
1954
1951
0
Rainfall (mm)
300
1000
1948
Annual rainfall (mm)
Composite Analysis:
•Time series
•5-driest years of the wet season(s)
•5-wettest years of the wet season(s)
•Cross validation approach
200
200
100
0
Wet
Example - Region B: MAM
Composites
OLR
SST
Dry
Wet
Dry
Tropical SSTs
200mb
Vectors
Region B: SON Composites
Wet
Dry
SSTs
Tropical SSTs
Wet
Dry
SLP
In short: A complex relationship exists where a combination of differing
regional and global scale processes can influence the rainfall over central
Africa.
It is thus imperative to better understand how these processes work and
how future climate and regional land use change in central Africa impact
them and rainfall.
Preferable area for large-scale, organised, deep convection
AEJ-S
AEJ-N
500mb
Preferable area for
shallow convection
Seasonal rainfall (mm)
TEJ
200mb
WAJ
1000mb
20ᵒS
EQ
20ᵒN
Schematic depicting a favourable jet configuration for enhanced rainfall over
central Africa (note: Example shown for when the AEJ-S is present).
Preferable area for large-scale, organised, deep convection
TEJ
200mb
AEJ-S
AEJ-N
500mb
Preferable area for
shallow convection
Seasonal rainfall (mm)
?
WAJ
1000mb
20ᵒS
EQ
20ᵒN
Schematic depicting an unfavourable jet configuration for enhanced rainfall over
central Africa (note: Example shown for when the AEJ-S is present).
? – Highly localised enhanced rainfall?
Modelling:
 HadAM3
 PRECIS
 HadAM3– for resolution test (regional-scale variability)
Uses of models:
 Control run comparison with observation and reanalysis data. Test
hypothesis’s on what controls variability in the region.
 Sensitivity studies – land cover /future climate experiments – What role
does the land surface play?
NCEP minus PRECIS model example: JJA
NCEP minus PRECIS model example: JJA
NCEP vs. ERA40
A comparison of reanalysis data over central Africa.
Region B MAM:
ERA
NCEP
Region B:
ERA
NCEP
Region B MAM:
ERA
NCEP
Region B:
ERA
NCEP
Region B:
ERA
NCEP
Future work:
• HadAM3 run - Climatology run, how well does
it compare theories
• TAMSAT analysis/validation of region
• Land cover change sensitivity experiments
• Climate change sensitivity experiments.