Post on 30-Jan-2016
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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:
10°N
5°N
EQ
5°S
10°S
15°N
15°S
August
AEJ-N
WAJTEJ
AEJ-S
10°N
5°N
EQ
5°S
10°S
15°N
15°S
January
AEJ-N
TEJ
AEJ-S
•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?
Teleconnections?• EL-Nino Southern Oscillation (ENSO)• Large-scale circulations (Hadley and Walker circulations)
Dry Wet
Rainbelt:
15°S
0° EQ
15°N
Variability in the rainbelt for August
Variability in rainfall linked to variability in Intensity and position.
From Nicholson, 2008 and 2009.
Methodology:Data Sources:•Rain gauge network•NCEP-NCAR reanalysis•ERA-40 reanalysis
Regionalisation:•Based upon seasonal rainfall cycle
Composite Analysis:•Time series•5-driest years of the wet season(s)•5-wettest years of the wet season(s)•Cross validation approach
Example - Region B: MAM CompositesWet Dry
OLR
SST
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.
20ᵒS 20ᵒNEQ
TEJ
AEJ-S AEJ-N
WAJ
Preferable area for large-scale, organised, deep convection
Preferable area for shallow convection
1000mb
200mb
Seas
onal
rain
fall
(mm
)
Schematic depicting a favourable jet configuration for enhanced rainfall over central Africa (note: Example shown for when the AEJ-S is present).
500mb
20ᵒS 20ᵒNEQ
TEJ
AEJ-S AEJ-N
WAJ
Preferable area for large-scale, organised, deep convection
Preferable area for shallow convection
1000mb
200mb
Seas
onal
rain
fall
(mm
)
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?
500mb
?
Modelling:HadAM3PRECISHadAM3– 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.