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The causes of extreme rainfall in East Africa: insights from atmospheric and coupled GCMs Emily Black, Julia Slingo and Ken Sperber Questions: 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? Data and Models used: •Observational data GISST SST (monthly means), NCEP reanalysis wind •Atmosphere only model ECHAM – 4 forced by observed SST 1979 – 1997 (T42, 19 vertical levels) •Coupled model SINTEX coupled model 200 year run (T42, 19 levels) South-west monsoon Long rains Short rains Long rains Short rains Relationship between the IOZM and East African rainfall in observational/reanalysis data Relationship between the IOZM and rainfall Rainfall during IOZM years Wind field Zonal wind anomalies Vector wind field Easterly zonal wind anomalies in the northern central Indian Ocean Weakening of the climatological moderately strong westerlies Reversed SST gradient for several months Less transport of moisture away from the African continent ??? High short rains in East Africa Rainfall seasonal cycle in atmosphere only/coupled models Observation ECHAM 4 SINTEX SST during rainy years Observed ECHAM 4 SINTEX Time series of DMI and rainfall Relationship between rainfall and the DMI Observed SINTEX ECHAM-4 Extreme and Moderate IOZM years SINTEX Observed Extreme Moderate Definition: Extreme years: DMI greater than 1 s.d. from the mean for three contiguous months between August and November Moderate years: DMI greater than average but less than 1 s.d. from the mean for three contiguous months between August and November Rainfall composites Extreme Observed ECHAM 4 SINTEX Moderate Wind composites – NCEP reanalysis Climatology Extreme Moderate Wind composites – ECHAM 4 Climatology Extreme Moderate Wind composites – SINTEX Climatology Extreme Moderate Summary and Conclusions •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