The causes of extreme rainfall in East Africa: insights from observed data and GCMs Emily Black,...
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Transcript of The causes of extreme rainfall in East Africa: insights from observed data and GCMs Emily Black,...
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 ?
?
?
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