The causes of extreme rainfall in East Africa: insights from observed data and GCMs

Emily Black, Julia Slingo and Ken Sperber

Area of interest

Why this area?

•Good rainfall data

•High rainfall during the autumn season

•Prone to floods

Talk Outline

•Relationship between the short rains in equatorial East Africa and circulation in the Indian Ocean basin (observations and reanalysis data)

•Simulation of this relationship in atmosphere-only and coupled GCMs – the feasibility of model studies

•Summary and conclusions

Talk Outline

•Relationship between the short rains in equatorial East Africa and circulation in the Indian Ocean basin (observations and reanalysis data)

•Simulation of this relationship in atmosphere-only and coupled GCMs – the feasibility of model studies

•Summary and conclusions

•What SST patterns are associated with high rainfall in East Africa?

•How might these SST patterns drive East African rainfall?

South-west monsoon

Long rains Short rains

Long rains Short rains

The Seasonal cycle in East African rainfall

Equatorial region

Southern region

El Nino and East African rainfall

Indian Ocean

•Warming in the west

•Cooling in the eastPerturbed zonal SST gradient

Pacific Ocean:

•Warming in east and central Pacific Ocean

El Nino

High short rains in East Africa

Indian Ocean processes ?

?

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Indian Ocean Dipole (Webster et al 1999; Saji et al (1999))

•1961 and 1997 both dipole years

•Peaks in October/November

Large-scale climate phenomena and East African rainfall

Question:

What is it about extreme but not moderate events that triggers high rainfall?

DMI

Rai

nfal

l

Wind composites – NCEP reanalysis

Extreme

Moderate

Climatology

Reversed SST gradient for several months

Easterly zonal wind anomalies in the northern central Indian Ocean

Weakening of the climatological moderately strong westerlies

Less transport of moisture away from the African continent

High rainfall in East Africa

1) How well are the short rains in East Africa represented by coupled and atmosphere only GCMs?

2) How does the IOZM interact with East African rainfall in a coupled / atmosphere only GCM? Is this realistic?

3) What insights into the mechanisms controlling East African rainfall can be gained using a coupled GCM?

Models used:

•Atmosphere only modelECHAM – 4 forced by observed SST 1979 – 1997 (T42, 19 vertical levels)

•Coupled modelSINTEX coupled model 200 year run (T42, 19 levels)

Simulation of this relationship in atmosphere-only and coupled GCMs – the feasibility of model studies

Rainfall seasonal cycle in atmosphere only/coupled models

Observation ECHAM 4 SINTEX

SST during rainy years

Observed

SINTEX

ECHAM 4

Comparison between model and observed winds (extreme IOZM years)

NCEP reanalysis

ECHAM-4 SINTEX

Relationship between rainfall and the DMI

Observed

ECHAM-4SINTEX

Summary and conclusions

•a dynamic scenario that explains the excessive rainfall in East Africa during IOZM years has been presented

•the relationship between the IOZM and East African rainfall in the SINTEX coupled model is similar in some ways to the relationship implied by reanalysis and observational data

•the relationship between the IOZM and East African rainfall is not well simulated by an atmosphere only models

•in both coupled model simulations and observations the relationship between the IOZM and East African rainfall is non-linear with the possibility that the coupled system enters different regimes depending on the strength of the SST boundary forcing