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Section 3.4 Introduction to the West African Monsoon

Section 3.4 Introduction to the West African Monsoon. The largest regional deficit of rainfall observed during the last century. Wet period. 20%. 10%. 0. -10%. -20%. Dry Period. Uncertainties about the future. Climate Variability Impacts: Water Agriculture Health Demographics

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Section 3.4 Introduction to the West African Monsoon

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  1. Section 3.4 Introduction to the West African Monsoon

  2. The largest regional deficit of rainfall observed during the last century Wet period 20% 10% 0 -10% -20% Dry Period

  3. Uncertainties about the future

  4. Climate Variability Impacts: Water Agriculture Health Demographics Security

  5. Introduction to the West African Monsoon Annual Cycle of Rainfall and associated Water Vapour Transport Thorncroft et al (2011) QJRMS, 137, 129-147

  6. The Coupled Monsoon System Key features of the WAM Climate System during Boreal summer Heat Low SAL AEJ ITCZ Cold Tongue

  7. Meridional Circulations North-South Section along the Greenwich Meridian AEJ 50oC 90oC θ θe θe θ 20oC 60oC

  8. Meridional Circulations Shallow Meridional Circulation (SMC) over ocean, especially in Spring AEJ 50oC 90oC θ θe θe θ 20oC 60oC

  9. Annual Cycle of Mean Rainband Data: GPCP (Global Precipitation Climatological Project). Resolution: pentad on a 2.5o grid. Averaged from 10oW to 10oE over 23 years (1979-2001). c.f. Gu and Adler (2004)

  10. Data • Observations, reanalysis and operational analysis data including: • pentad 2.5o GPCP • Reynolds SST 1o, weekly and daily • Reanalysis from the ECMWF: daily 2.5o ERA40 • The period of study is 1979-2001

  11. Relationship between rainfall and surface conditions Relationship between SKT and surface meridional wind SKT and rainfall Warming over the continent due to the surface solar heating. Rapid cooling of the ocean surface south of the equator between April and June  rapid rise in MSLP: Acceleration of southerly winds across the equator. c.f. Okumara and Xie (2004) MSLP and VWND

  12. Relationship between rainfall and surface conditions Equivalent potential temperature • Peak rainfall always lies south of thetae peak • Gradient in thetae still important • Location of heat low important for poleward extent

  13. Total Column Moisture Flux Convergence

  14. Total Column Moisture Flux Convergence Peak in moisture flux convergence linked to heat low shallow meridional circulation – acts to moisten the column and extend the rainfall polewards (c.f. Sultan and Janicot (2000,2003), Hagos and Cook (2008))

  15. Total Column Moisture Flux Convergence Peak in moisture flux convergence over ocean

  16. Total Column Moisture Flux Convergence Rapid shift and increase in moisture flux convergence towards coast between April and May

  17. Total Column Moisture Flux Convergence Rapid reduction in moisture flux convergence during June – linked to end of coastal rains

  18. Total Column Moisture Flux Convergence Rapid increase in moisture flux convergence beginning of July linked to Sahelian rainfall onset

  19. Meridional Moisture Fluxes Mid-levels (850-500mb) Impact of Heat Low SMC Low-levels (sfc-850mb)

  20. Meridional Moisture Fluxes Mid-levels (850-500mb) Equatorward moisture flux at mid-levels enhances moisture flux convergence in rainy zone : enhances rainfall there? Polewards of this there is dry advection: inhibits rainfall there? Low-levels (sfc-850mb)

  21. Meridional Moisture Fluxes Mid-levels (850-500mb) Low-levels (sfc-850mb) Marked increase in cross-equatorial moisture fluxes during April-May Linked to cold tongue development and coastal onset

  22. Schematic evolution ITCZ 1. Ocean phase (Feb-April): -Main rainband is broad with peak values just poleward of the Equator (~1oN ). The rainfall is located mostly over the warmest water (>28oC) with little over the land. -At the end of this period the cold tongue starts to develop, resulting in a broad region of SSTs close to the equator falling below 28oC. - Does the heat low SMC impact the surface winds? SMC HL SST

  23. Schematic evolution • 2. Coastal phase (May-mid-June): • -Cold tongue development associated with a rise in equatorial surface pressure, and an acceleration of southerlies and associated moisture flux towards the coast. • -Marked moisture flux convergence, just equatorward of the land (~4oN) is associated with the highest rainfall of the annual cycle, and the first rainy season for coastal regions of West Africa. • c.f. Zheng, Eltahir and Emanuel (1999) • Okumara and Xie (2004) • Gu and Adler (2004) • Caniaux et al (2009) • - Peak rainfall is located over warmest water ITCZ SMC HL SST

  24. Schematic evolution • 3. Transitional Phase (End of June) • June represents a period where the environment becomes less favorable for convection in the coastal region. This is consistent with coastal upwelling and a reduction of SSTs there. • Intense coastal rainfall can only be transient? • Why doesn’t it rain more in June? • Does this weakening promote the perception of a “jump” often discussed in the literature? ITCZ SMC HL SST

  25. Schematic evolution 4. Sahelian Phase (July-August): - Between June and July the peak in moisture flux convergence reaches 10oN and increases rapidly consistent with the observed Sahelian rainfall onset. - In July and August moisture flux divergence is present over the coastal region consistent with continued suppression of rainfall there. c.f. Sultan and Janicot (2000.2003) Sijikumar et al (2006) Ramel et al (2006) Hagos and Cook (2007) ITCZ SMC HL SST

  26. ERA40 vs NCEP1 rainfall Wet bias in Spring? Dry bias in Sahel in Summer Dry bias in Spring?

  27. 2005 Large variation in the coastal onset. Earliest cold tongue development in Spring 2005 – earliest coastal onset. Strongest HL in Spring 2007 during the oceanic regime  possible role in delaying the coastal onset via subsidence 20 11 2006 3 21 2007 29 23

  28. Concluding remarks on Annual Cycle • At some level the coastal onset seems easier to understand than the Sahelian onset – with peak rainfall following the peak in SSTs • What processes determine the nature and variability of the cold tongue (role of heat low, sub-surface ocean structure, Atlantic ocean variability, radiation)? • Why is cold tongue development more rapid in the Atlantic than in the Pacific? • Can climate models represent these coupled processes? • Need more in situ observations in the tropical East Atlantic! • Need more work on nature and causes of variability of coastal rains (next)

  29. Annual Cycle of Synoptic Weather Systems TD-filtered OLR (AEW-activity) Peaks in summer We know little about the nature and causes of AEW-variability Kelvin-filtered OLR Peaks in Spring Key synoptic system for pre-coastal phase and possibly the coastal phase

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