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Regime shifts in the West African Monsoon System and Seasonal Rainfall Predictions

Regime shifts in the West African Monsoon System and Seasonal Rainfall Predictions . Wassila M. Thiaw, Vadlamani B. Kumar, and Nicholas Novella Climate Prediction Center National Centers for Environmental Predictions. Sahel (12N-18N; 18W-20E) Jul-Sep Precipitation Index 1948-2011. 1995-04.

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Regime shifts in the West African Monsoon System and Seasonal Rainfall Predictions

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  1. Regime shifts in the West African Monsoon System and Seasonal Rainfall Predictions Wassila M. Thiaw, Vadlamani B. Kumar, and Nicholas Novella Climate Prediction Center National Centers for Environmental Predictions

  2. Sahel (12N-18N; 18W-20E) Jul-Sep Precipitation Index 1948-2011 1995-04 1972-92 2005-11 1950-67

  3. Northern Sub-Sahara Africa Jul-Sep Precipitation Difference 2005-10 minus 1972-92 2005-10 minus 1950-67 Marginal deficit Rainfall increase

  4. Western Sahel (12N-18N; 10W-18W) Jul-Sep Precipitation 1948-2011 2010: Wettest year since 1948

  5. Correlation Global SST and Sahel Rainfall

  6. Correlation Global SST and Western Sahel Rainfall

  7. Global OLR and SST Difference - August 2005-10 minus 1981-2000 OLR SST

  8. Continental Heat Low and African Easterly and Westerly Jets T 2005-10 T Difference 2005-10 minus 1981-2000 U Zonal Wind at 600 hPa Acceleration of AEJ in area of strong meridional T gradient

  9. NCEP Climate Forecast System (CFS) Predictions Saha et al., 2006 Saha et al., 2010 Schemm et al., pc

  10. Observed Rainfall Difference 2005-10 minus 1981-2000

  11. CFS Rainfall Difference 2005-10 minus 1981-2000 June IC April IC CFS V1 CFS V1 CFS V2 CFS V2 CFS 382 The high resolution CFS T382 captured the rainfall recovery remarkably well.

  12. Tropical North Atlantic SST Anomaly (10-30N; 18W-30W) and Western Sahel JAS Rainfall Obs, CFS T382 April IC JAS Observed rainfall MJJ Observed SST Predicted rainfall Predicted SST CFS V1 MJJ SST and JAS Rainfall April IC CFS V1 July SST and JAS Rainfall June IC CFS V2 MJJ SST and JAS Rainfall April IC CFS V2 July SST and JAS Rainfall June IC

  13. Forecast Skill for JAS Rainfall April IC June IC CFS V1 CFS V1 CFS V2 CFS V2 CFS T382 Area Average Skill 12-18N; 10-18W

  14. Summary • There is evidence of a regime shift in the West African monsoon system associated with changes in the global climate. • While the southern oceans were important in modulating western Sahel rainfall in the period 1951-80, the warming in the tropical Atlantic played a more prominent role in the rainfall recovery. • Tropical north Atlantic warming began in the early 70s. The upward rainfall trend leading to the most recent humid period lagged this warming by approximately 10 years. The era of largest rainfall increase coincided with a period of rapid SST increase. • The changing trends in Sahel rainfall make seasonal rainfall predictions challenging. Of the 3 versions of the CFS, the high spatial resolution version T382 has a better representation of the WAM and captured best the tropical north Atlantic warming and the rainfall increase in the western Sahel. • The operational CFS V2 captured the Atlantic warming, but exhibited jumpiness in rainfall predictions. The next step is to conduct a thorough diagnostics to understand the biases and see if we can extract useful information that could be used to improve predictions.

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