1. Deficiencies in seasonal rainfall simulated by CMIP5 climate models
Richard Allan | University of Reading | Karlsruhe Oct 17
PhD work by Caroline Dunning with co-supervisor Emily Black (University of Reading)

2. Introduction AMIP simulations CRU Observations
Contrasting Africa rainfall trends 
Is timing of seasonal rains captured by models and what are the projections?
Impact relevance e.g. Adejuwon and Odekunle (2006) J. Clim; Caminade et al. (2014) PNAS
Method to diagnose onset & cessation using harmonic analysis + cumulative rainfall (Liebmann et al J. Clim.; Dunning et al JGR)
Evaluation of CMIP5 simulations (Dunning et al ERL) and evaluation of future projections in seasonality of rainfall
AMIP simulations CRU Observations
JJA trends
Maidment et al GRL
Annual
Biannual
Adejuwon and Odekunle, 2006 J. Clim – when the LDS is too early, long or intense, yields can be reduced
Caminade et al. (2014) PNAS - Malaria
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

3. CMIP5 simulated rainfall
Coherent seasonal progression of wet season
Coupled models don’t capture biannual regime (Ghana, Cote d’Ivoire)
Dunning et al ERL
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

4. CMIP5 simulated timings
OBS
CMIP
AMIP
BCC CSM1
MIROC4h
Coastal southern west Africa: late 2nd rains in models simulations
Most coupled simulations fail to capture biannual regime
Except MIROC4h, BCC CSM1
Links to ITCZ bias e.g. Monerie et al., 2017 Clim. Dyn; Roehrig et al. (2013) J Clim; Nnamchi et al. (2015) Nature Comms
Early onset in Sahel (also issues with ERA Interim depiction e.g. Hill et al JGR)
Dunning et al. (2017) ERL
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

5. Deficient CMIP5 bi-annual regime
A small region on the southern west African Coastline exhibits a biannual rainfall regime; two wet seasons are separated by the Little Dry Season/August break e.g. Chineke et al. (2010) Clim. Ch.
Atmosphere-only simulations capture seasonal cycle, coupled simulations don’t
May relate to errors in the Gulf of Guinea sea surface temperature (SST) & deficient representation of SST/rainfall relationship: See also Okumura & Xie (2004) J. Clim., Cabos et al. (2017) Clim. Dyn.
SST seasonal cycle
Dunning et al ERL
OBS/AMIP
CMIP5 5
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

6. Projections in onset/cessation
Projected future increase in rain per wet day over west Africa (below)
Projected onset/cessation dates over southern west Africa (left)
OBS AMIP hist+RCP8.5 RCP4.5
(RCP4.5, RCP8.5)
(RCP4.5, RCP8.5)
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

7. Current & Projected Change in Onset
Later onset projected over west Africa (RCP8.5 ~1 day/decade)
Projected change in cessation date not significant
Observed trends for earlier onset: internal variability/ dataset inhomogeneity?
Changes in total seasonal rain not significant: fewer rainy days, more rainfall/rainy day.
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

8. Conclusions
Methodology: identify onset/cessation of one/two wet seasons across Africa using harmonics and cumulative rainfall (Dunning et al JGR)
AMIP/CMIP representation of the seasonal cycle:
Atmosphere-only and coupled simulations capture gross observed patterns of seasonal progression/timing
Coupled simulations fail to capture the biannual regime over coastal West Africa; seasonal SST bias implicated (Dunning et al ERL)
Future projections:
Projected later onset, shorter wet season over parts of west Africa
Ongoing investigation: Sahara heat low, ITCZ width, Angola low, South Atlantic Anticyclone e.g. Cabos et al. (2017) Clim. Dyn.?
How is current & future monsoon progression modulated by cloud-radiation-circulation interactions? Are cloud-precipitation biases linked e.g. Hannack et al. (2017) J. Clim
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

9. Presenter | Organisation
Project Meeting October 2017
Presenter | Organisation

10. OLD Conclusions
Trends & variability in African rainfall strongly linked to internal climate variability (Maidment et al. (2015) GRL)
Methodology to diagnose the onset and cessation dates of wet seasons over Africa: Dunning et al. (2016) JGR
Spurious variability in ERA Interim & some satellite products
Future work: apply methodology to climate model simulations, investigate mechanisms and develop impact relevant metrics
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

11. Diagnosing wet season onset/cessation
Timing/duration characteristics of wet/dry seasons over Africa of importance for agriculture and impacts
Develop methodology to diagnose onset & cessation dates of wet seasons over Africa
Harmonic analysis to diagnose single/double rainy season regimes
Cumulative rainfall to estimate onset/cessation (modified based on Liebmann et al. 2012)
Dunning et al. (2016) JGR
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

12. Example progression of onset/ cessation of wet season(s) - GPCP
Considered GPCP 1DD, TRMM 3B42, ARCv2, TARCATv2, CHIRPS, ERA Interim in subsequent analysis (Dunning et al. (2016) JGR)
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

13. Future Projections… Sahel??
Onset and cessation are projected to get later over West Africa.
Total seasonal rainfall is projected to remain constant, with decreases in the number of rainy days, and increases in rainfall per rainy day.
Project Meeting October 2017
Richard Allan | University of Reading
@rpallanuk

14. Presenter | Organisation
Project Meeting October 2017
Presenter | Organisation

15. Presenter | Organisation
Project Meeting October 2017
Presenter | Organisation