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.