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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