1. © Crown copyright Met Office Forecasting the onset of the African rainy seasons Michael Vellinga, Alberto Arribas and Richard Graham S2S Conference, Washington, Feb 2014

2. © Crown copyright Met Office Background In numerous regions timing of rainfall events is at least as important to users as rainfall amounts: Onset and cessation of rainy season Occurrence of dry/wet spells Strong user demand for subseasonal rainfall forecasts for a range of lead times: months-weeks-days

3. What climate information do African decision makers need? © Crown copyright Met Office Highest priority: predictions of rainy season onset, cessation and dry spell risk (temporal distribution of rainfall) Predictions needed well ahead of the season Consultation with 9 climate service providers in Africa (across 8 countries)

4. West African Monsoon onset

5. © Crown copyright Met Office Approach 1. Develop rainy season onset fc, 1-3 months’ lead 2. Use multi-variate approach to define onset, to give more reliable forecasts Combine rainfall, OLR and dynamical onset indicators 3. Formulate onset indicators in a way that minimises sensitivity to model rainfall bias Developed for sub-Saharan Africa (Vellinga, Clim. Dyn. 2012), presented at RCOFs across Africa and Asia Forecast skill analysed in multiple systems (Met Office and ENSEMBLES database)

6. Examples of onset indicators

7. © Crown copyright Met Office Example: Precipitation-based method (isochrones) Rainfall accumulation Time  Average onset Late onset

8. © Crown copyright Met Office Observed mean evolution: 20 th isochrone for OND (from 1 st Aug.) Connect points of ‘equal arrival time’ Colours indicate time of local arrival of 20% of average season total rainfall GPCP average for 18 th Sep - 1 st Jan (1996-2009/10)

9. © Crown copyright Met Office GloSea4 mean evolution: 20 th isochrone for OND (from 1 st Aug.) Colours indicate time of local arrival of 20% of average season total rainfall GloSea4 average for 18 th Sep - 1 st Jan (1996-2009/10)

10. © Crown copyright Met Office GloSea4 Forecast for 2011 Arrival of 20 th isochrone Probability of early arrival:Probability of late arrival:

11. © Crown copyright Met Office New ways of displaying information for decision making

12. © Crown copyright Met Office GloSea4 forecast skill ROC scores 20 th isochrone for 1 August hindcasts Early arrival:Late arrival:

13. © Crown copyright Met Office Conclusions and future work Skilfull forecast of subseasonal rainfall characteristics [onset] are possible in spite of mean rainfall biases Needs to be judicious in defining fc variable: combine fc skill with user relevance. Iterative process between ‘modeller’ and users Need to translate ‘tercile maps’ into PDF for onset dates Provision of seamless forecasts from days to months ahead: Mechanisms and sources of fc skill are different (MJO, midlatitude intrusions, etc). Skill largely unexplored

14. Thank you