The causes of extreme rainfall in East Africa

The causes of extreme rainfall in East Africa: insights from observed data and GCMs Emily Black, Julia Slingo and Ken Sperber

Why this area? • Good rainfall data • High rainfall during the autumn season • Prone to floods Area of interest

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?

The Seasonal cycle in East African rainfall South-west monsoon Long rains Short rains

El Nino and East African rainfall Equatorial region Southern region

Pacific Ocean: • Warming in east and central Pacific Ocean Indian Ocean • Warming in the west • Cooling in the east Perturbed zonal SST gradient

Large-scale climate phenomena and East African rainfall El Nino ? ? High short rains in East Africa Indian Ocean processes ? Indian Ocean Dipole (Webster et al 1999; Saji et al (1999)) • 1961 and 1997 both dipole years • Peaks in October/November

Rainfall DMI Question: What is it about extreme but not moderate events that triggers high rainfall?

Wind composites – NCEP reanalysis Climatology Extreme Moderate

Reversed SST gradient for several months Less transport of moisture away from the African continent Easterly zonal wind anomalies in the northern central Indian Ocean Weakening of the climatological moderately strong westerlies High rainfall in East Africa

Simulation of this relationship in atmosphere-only and coupled GCMs – the feasibility of model studies 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 model ECHAM – 4 forced by observed SST 1979 – 1997 (T 42, 19 vertical levels) • Coupled model SINTEX coupled model 200 year run (T 42, 19 levels)

Rainfall seasonal cycle in atmosphere only/coupled models Observation ECHAM 4 SINTEX

SST during rainy years Observed ECHAM 4 SINTEX

Comparison between model and observed winds (extreme IOZM years) NCEP reanalysis ECHAM-4 SINTEX

Relationship between rainfall and the DMI Observed SINTEX ECHAM-4

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 nonlinear with the possibility that the coupled system enters different regimes depending on the strength of the SST boundary forcing