A Multiscale Analysis of the West African Monsoon Chris Thorncroft Department of Atmospheric and Environmental Sciences University at Albany

A Multiscale analysis of the West African Monsoon (1) Annual Cycle of Rainfall and associated Water Vapour Transport (2) Interannual Variability of the coastal rainfall in Spring (3) The African Easterly Wave Life-Cycle: (I) Genesis (II) Baroclinic Developments (III) West Coast Developments (4) Final Comments and Future Work

Annual cycle of Water Vapor Transport in the West African Monsoon region Chris Thorncroft, Hanh Nguyen (University at Albany) Chidong Zhang (RSMAS, Miami) Philippe Peyrille (MeteoFrance, Paris)

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

North-South Section along the Greenwich Meridian θ 50 o C 20 o C θ θe 90 o C 60 o C θe AEJ Meridional Circulations

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

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

Aims To document the climatological mean annual evolution of the water vapour transport and associated three-dimensional pattern of moisture convergence in the WAM and tropical Atlantic regions. (revisiting Cadet and Nnoli (1987)) To relate this to the regional circulations and low-level thermodynamic conditions, especially those linked to the Atlantic cold tongue and Saharan heat low. To improve understanding of the various phases of the annual cycle of WAM rainfall

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

Relationship between SKT and surface meridional wind SKT and rainfall MSLP and VWND 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) 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

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))

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

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

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

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

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

Meridional Moisture Fluxes Mid-levels ( mb) Low-levels (sfc-850mb) 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?

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

Schematic evolution SST SMC ITCZ 1. Ocean phase (Feb-April): -Main rainband is broad with peak values just poleward of the Equator (~1 o N ). The rainfall is located mostly over the warmest water (>28 o C) 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 28 o C. - Does the heat low SMC impact the surface winds? HL

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 (~4 o N) 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 Schematic evolution SST SMC ITCZ HL

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? Schematic evolution SST SMC ITCZ HL

4. Sahelian Phase (July-August): -Between June and July the peak in moisture flux convergence reaches 10 o N 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 ( ) Sijikumar et al (2006) Ramel et al (2006) Hagos and Cook (2007) Schematic evolution SST SMC ITCZ HL

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

ERA40 NCEP1 can a strong heat low SMC suppress convection south of it? Total Column Moisture Flux Convergence

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)

Interannual Variability of Coastal Rainfall

TRMM CMAP GPCP Resolution: pentad on a 2.5 o grid. Averaged from 10 o W to 10 o E over 10 years ( ). Similar patterns: - broad and weak in winter. - a rapid shift of the southern limit of the rainband in May. - most intense in spring. - a marked decline in June- July. - rapid shift to the Sahel in summer. - steady retreat following surface solar heating. Different intensities: - strong in winter for CMAP. - strong in spring for TRMM and CMAP. - weak in summer for CMAP.

TRMM CMAP GPCP Resolution: pentad on a 2.5 o grid. Averaged from 10 o W to 10 o E over 10 years ( ). Definition Coastal Onset defined in terms of the rapid reduction in rainfall over the equatorial region and the associated reduction in the width of the ocean rainfall.

TRMM CMAP GPCP Composite diagrams tOC = 9 May tOC = 10 May Period:

Interannual variability Coastal onset: 11 May range: 13 pentads Coastal length: 7 pentads range: 11 pentads Sahel onset: 6 July range: 7 pentads Hypotheses:Delayed cold tongue development delays coastal onset Strong heat low delays coastal onset

Relationship between the coastal onset and the SST cooling

Relationship between the coastal onset and the Saharan Heat Low

A comparison of 3 years:

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

Interannual variability Coastal rainfall intensity : 8.2 mm/d ; range: 9.1 mm/d

Relationship between the coastal rainfall and the SST MAYMARCH APRILFEBRUARY

Relationship between the coastal onset and the Sahel onset

Concluding remarks on Interannual Variability The West African coast is characterized by marked interannual variability in rainfall – both in terms of the onset (of the coastal phase) and amounts. Onset date is influenced strongly by the timing of the cold tongue development as well as the intensity of the heat low. Rainfall amounts are correlated with SSTs in the Pacific and SE Atlantic suggesting predictability with several months lead-time. Onset of the coastal phase is correlated with the Sahelian onset. Sahelian onset tends to occur roughly 2 months after the coastal onset.

Variability 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

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day 0 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue 3. African Easterly Waves Kiladis, Thorncroft, Hall (2006)

3. African Easterly Waves Objectively diagnosed troughs (solid lines), African Easterly Jet (dashed), with PV (315K) and IR from METEOSAT (courtesy Gareth Berry)

AEW life-cycle phases Phase I: Genesis (e.g. Thorncroft, Hall and Kiladis, 2008) Phase II: Baroclinic growth (e.g. Berry and Thorncroft, 2005) Phase III: Tropical Cyclogenesis (e.g. Hopsch, Thorncroft, Tyle, 2010)

I: AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism Phase I: GenesisTwo Theories for the Genesis of AEWs 925hPa  315K PV AEJ satisfies the necessary conditions for barotropic and baroclinic instability: Burpee (1972), Albignat and Reed, 1980). Therefore we expect AEWs to arise from small random perturbations consistent with a “survival of the fittest” view. Continues to be the consensus view.

I: AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism (evidence against!) Two Theories for the Genesis of AEWs The AEJ is too short! The jet is typically o long. It can only support two waves at one time. It is therefore not possible for AEWs to develop via a linear instability mechanism. The AEJ is only marginally unstable! Hall et al (2006) showed that in the presence of realistic boundary- layer damping the AEW growth rates are very small or zero. It is therefore not possible for AEWs to develop sufficiently fast to be important.

I: AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism (evidence against!) Two Theories for the Genesis of AEWs The AEJ is too short! The jet is typically o long. It can only support two waves at one time. It is therefore not possible for AEWs to develop via a linear instability mechanism. The AEJ is only marginally unstable! Hall et al (2006) showed that in the presence of realistic boundary-layer damping the AEW growth rates are very small. It is therefore not possible for AEWs to develop sufficiently fast to be important. So what can account for the existence of AEWs, their genesis and intermittancy?

Two Theories for the Genesis of AEWs II: AEWs are generated by finite amplitude forcing upstream of the region of observed AEW growth. Carlson (1969) suggested the importance of convection and upstream topography for the initiation of AEWs. Others pushed the linear instability hypothesis. More recent observational evidence has been provided by: Berry and Thorncroft (2005): case study of an intense AEW Kiladis et al (2006):composite analysis Mekonnen et al (2006):climatological view c.f. Farrel, B. (1987)

Satellite imagery METEOSAT-7 Water Vapour channel. Shown every 6 hours from 30 th July z to 4 th August z. Convective outbursts in the first day of the sequence preceded the dynamical signal.

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day 0 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-4 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-3 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-2 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-1 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day 0 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

Idealised Modeling Study: Thorncroft, Hall and Kiladis (2008)

Consequences Significance for weather prediction A significant convective outbreak in the Darfur region will favor the formation of a train of AEWs to the west over sub- Saharan Africa within a few days. For daily-to-medium range forecasts of AEWs, it is important to monitor, and ultimately predict, the nature of the upstream convection.

Consequences Significance for longer timescales In addition to considering the nature of mean AEJ, we should consider the nature and variability of finite amplitude convective heating precursors. It is likely that BOTH are important

Phase II: Baroclinic Development- Scale Interactions

 ’ Max 700hPa Trough Conceptual framework (ii) Baroclinic growth.

 ’ Max 700hPa Trough Conceptual framework (ii) Baroclinic growth.

 ’ Max 700hPa Trough

PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions

PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions

PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions From a PV-theta perspective, the heating rate profiles are crucial to know and understand.

PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions From a PV-theta perspective, the heating rate profiles are crucial to know and understand. Mesoscale-Microscale Interactions Ultimately these profiles are influenced by the nature of the microphysics!

AEWs often get a “boost” before they leave Africa; associated with mergers of PV from upstream and in situ generation. The Guinea Highlands region is one of the wettest regions of tropical North Africa. GPCP rainfall (mm/day) for Aug- Sep, ) Phase III West Coast Developments – Role of Guinea Highlands!

Average vorticity tracking statistics for June-July-August at 700hPa and 850hPa based on ERA40 using methodology of Thorncroft and Hodges (2001). Coherent cyclonic centers are tracked within the ITCZ at 700hPa and in the low-level baroclinic zone at 850hPa Importance of Guinea Highlands Region

Developing Non-Developing Hopsch, Thorncroft and Tyle 2010 Composites of East Atlantic Developing and Non-Developing AEWs ( )

4. Final Comments on Phase I: Genesis AEWs are forced by upstream finite amplitude precursors – including, most importantly, upstream convection, that is most commonly triggered by topography over Darfur, but sometimes the Ethiopian Highlands. Other forcing is possible including that associated with midlatitude troughs. What are the causes of intra-to-interannual variability of AEW-activity? What are the relative roles of variability in upstream precursors and variability in the AEJ?

4. Final Comments on Phase II: Baroclinic Developments AEWs interact with and develop in association with MCSs The PV-theta framework is ideal for studying scale interactions - but it remains a challenge for us to assess the diabatic heating profiles and associated PV structures at the mesoscale. Ongoing research is utilizing radar data from AMMA to shed light on thee mesoscale structures. High resolution modeling offers a useful tool to study these structures (c.f. YOTC)

4. Final Comments on Phase III: West Coast Developments AEWs tend to intensify at the West African coast. The nature of the resulting structures can impact the probability of tropical cyclogenesis downstream. What are the relative roles of upstream PV and in situ generated PV? Why do some intense AEWs weaken? What are the relative roles of the SSTs, Saharan air layer, dry air in the upper-troposphere, midlatitude troughs….?

4. Final Comments: Intraseasonal Variability There exists significant intraseasonal variability in AEW activity that is yet to be fully described and understood. Ideal timescale for studying interactions between weather and climate