1. Evolution of our understanding of weather systems over West Africa J.P Lafore CNRM-GAME (Météo-France) Toulouse With help and/or slide contributions from: Nicole Asencio, Christelle Barthe, Guilaine Canut, Fabrice Chauvin, Fleur Couvreux, Cyrille Flamant, Andreas Fink, Françoise Guichard, Serge Janicot, Nick Hall, Peter Knippertz, Christophe Lavaysse, Stephanie Leroux, Doug Parker, Bernhard Pospichal, Rémy Roca, Romain Roehrig, J-Luc Redelsperger, Chris Thorncroft… Evaluating climate processes in West-Africa: New perspectives from AMMA A joint meeting of the RMetS and the EMS London, UK, July 21 October 2009

2. Outline 1.WAM Conceptual Model before AMMA (2002) 2.Progresses performed last 7 years i)Each component Monsoon layer, PBL Heat Low, monsoon burst Dynamics: Waves (AEWs), jets, intrusions… Clouds and convection ii)Couplings: aerosols, continental & oceanic surfaces iii)A larger 4D view (spatially and temporally) 3.Conclusion / further works

3. Key features of the WAM (1) AEJ Cold Tongue SAL ITCZ Heat Low Key features of the West African Monsoon Climate System during Boreal summer Chris Thorncroft

4. Key weather systems (2) Key weather systems in the West African and Tropical Atlantic regions AEWs MCSs SAL TC Chris Thorncroft

5. 1. Saharan Heat Low (HL)  2 convergent fluxes Northerly wind (Harmattan) ITD Monsoon Flux 2. Baroclinicity  African Easterly Jet 600-700 hPa  AEWs (instable) 3. Convection: favorable conditions ~AEJ: CAPE+Shear+Dry Air 4. Upper Trop: Anticyclonic Divergent Flux  acceleration TEJ+SubT Jet 5. Dry intrusions from midlatitudes + Subsidence above the HL © Météo-France Subsidence WAM Conceptual model

6. Turbulence in the PBL and monsoon layer: Dry tongue SOP Flights Active phase Transition Onset * Night-time average ° Daytime average 2006 Annual evolution of the water vapour at the ground in Niamey ATR Aircraft turbulent measurements SOP Flights below PBL top D -1.5 σ Dry tongues: - WVMR < -1,5 σ - w < 0 - θ > 0 D ~ 2-3 km ~ 400 m Pospichal et al. 2009 qv Canut et al. 2009

7. Canut et al. 2008 θ (K)‏ ww Turbulence in the PBL and monsoon layer: Dry tongue wq Documentation of dry tongues Strong contribution to exchanges between the monsoon flow and the SAL Different PBL regimes before and after the onset Correlations  Not parameterized in models! Canut et al. 2009 Coherent with Zhang et al 2006 study (few soundings!) + strong diurnal cycle

8. Integrated Water Vapor (IWV) evolution in 2006 0-10°E Monsoon surges: Seasonal Cycle Couvreux et al. 2007 A MJ J AS O ND 15°N

9. Composite of pre-onset surges Couvreux et al. 2009 Horizontal scale of ~1000 km [0-10E] ITD -- P (0.5 hPa) --ITD (v) Surges of monsoon  Heat Low precedes the monsoon burst Surges occur all during the monsoon season Stronger and coupled with convection after onset Not always coupled with AEWs HL Surge during the monsoon: 23-27 July 2006 Northward penetration of moisture Associated with intense convective events (Algeria) resulting in a HL weakening Barthe et al. 2009 Cuesta et al. 2009

10. Detection - Climatologie Seasonal evolution of the HL location Occurrence probability Heat Low Laveysse et al. 2009a, b Intra-seasonal Variability from ERA-40 2 band-periods: 3-10 & 10-30 days Objective criteria Low Level Atmospheric Thickness 925-700 hPa (LLAT)  2 regions of Ventilation Cold – moist Air advection

11. African Easterly Waves: some news ideas (since GATE!) Hall et al. 2006 Thorncroft et al. 2008 Sensitivity to the heating location: impact on AEWs activity at Day 10 in the rectangular area. A new idea: AEWs are a response to the convection activity at the AEJ entrance (Soudan-Tchad) if the AEJ 3D structure is favourable.

12. The strongest waves are mainly associated with jets that are strong in the south and west. The surface area covered by the jet and the PV gradient reversal seem to have a stronger influence than their intensity. African Easterly Waves and AEJ structure? Leroux, Hall and Kiladis 2009 Strong response Weak response

13. Phase I: Initiation (initial value problem) Phase II: Baroclinic growth Phase III: West coast developments  Q1(z) profile is crucial A conceptual model for AEW life-cycles (Berry and Thorncroft 2005) African easterly waves - Scale interactions

14. Heat Low intraseasonal variability  2 regimes 1004 1025 Sea Level Pressure (hPa) + Wind at 925hPa Θ 850 (°C) + Wind at 850hPa HLWHLE Chauvin, Roehrig and Lafore 2009

15. Composite of the HL mode of variability Composites of θ 850 Difference between HLW and HLE sequences Comes from midlatitudes Penetrates over Eastern Sahel Impacts Chauvin, Roehrig and Lafore 2009

16. Link with mid-latitudes -2d Composites of the geopotential height at 200hPa Difference between HLW and HLE sequences Southeastward propagation of a Rossby wave disturbance, along the polar and subtropical jet axis. The wave-like pattern “energy” is maximum 2 days before the HL phases maximum : HL intraseasonal variability driven by the mid-latitudes ? Chauvin, Roehrig and Lafore 2009

17. Impact on the West African Monsoon +4 Cold air surges of Vizy and Cook (2009) Propagation ~600km/day As AEWs W/m² Convective activity Satellite OLR Lag 0 Chauvin, Roehrig and Lafore 2009 Similar results by Lavaysse et al 2009b Composite study (Klaus 10-30 days) Min WAHL Max WAHL 7 days

18. Mean trajectory of Extratropical Dry-Air Intrusions into the WAM Long-lived MCS (> 12 hr) are enhanced over the Sahel box AEJ TEJ STJ Pol J Extratropical Dry-Air Intrusions into the WAM Midtroposphere An Important Factor for the Convective Activity over the Sahel Roca et al 2005

19. Conclusion AMMA  an unique documentation of the WAM We already learnt a lot since 2002 with AMMA  About key processes: PBL, monsoon, HL, AEWs, cold tongue…  About couplings: aerosols, surface (ocean and continental)  More global view: Interactions with Extratropics: see also Knippertz & Fink results for winter and spring Connections with the East Africa With the Atlantic: Gulf of Guinea, cold tongue…  Intraseasonal variability: see Janicot’s talk for t >10 days Further works  Build a complete “dynamical conceptual model”  Case studies, climatologies, hierarchy of models (resolution, idealized…) Applications  Predictability of the WAM  Improve the simulation of the WAM system See AMMA Special Issue in QJRMS (2010 beginning) ~30 papers