Regional rainfall variability over Central Africa What is
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 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? Teleconnections? • EL-Nino Southern Oscillation (ENSO) • Large-scale circulations (Hadley and Walker circulations) Dry Wet
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: 300 150 100 50 250 Region B 200 150 100 50 0 1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 3000 2500 2000 1500 1000 500 0 Mpika station Year Kapa Station Kawa Station 200 150 100 50 300 Rainfall (mm) Region C 250 150 100 50 0 300 Region E 250 200 150 100 Region D 200 0 250 rainfall (mm) Region A 300 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 Annual rainfall (mm) 200 0 300 Regionalisation: • Based upon seasonal rainfall cycle Region A 250 Rainfall (mm) Data Sources: • Rain gauge network • NCEP-NCAR reanalysis • ERA-40 reanalysis 200 150 100 50 50 0 0 Region F
Wet Example - Region B: MAM Composites OLR SST Dry
Wet Dry Tropical SSTs 200 mb 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 500 mb Preferable area for shallow convection Seasonal rainfall (mm) TEJ 200 mb WAJ 1000 mb 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 200 mb AEJ-S AEJ-N 500 mb Preferable area for shallow convection Seasonal rainfall (mm) ? WAJ 1000 mb 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: Had. AM 3 PRECIS Had. AM 3– 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. ERA 40 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: • Had. AM 3 run - Climatology run, how well does it compare theories • TAMSAT analysis/validation of region • Land cover change sensitivity experiments • Climate change sensitivity experiments.
- Slides: 23