A Multiscale Analysis of the West African Monsoon Chris Thorncroft

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

50oC

20oC

θ θe

90oC

60oC

θe

AEJ

Meridional Circulations

Shallow Meridional Circulation (SMC) over ocean, especially in Spring

θ

50oC

20oC

θ θe

90oC

60oC

θe

AEJ

Meridional Circulations

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)

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 2.5o GPCP

– Reynolds SST 1o, weekly and daily

– Reanalysis from the ECMWF: daily 2.5o ERA40

• The period of study is 1979-2001

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

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))


Peak in moisture flux convergence over ocean


Rapid shift and increase in moisture flux convergence towards coast between April and May


Rapid reduction in moisture flux convergence during June – linked to end of coastal rains


Rapid increase in moisture flux convergence beginning of July linked to Sahelian rainfall onset

Meridional Moisture Fluxes

Mid-levels (850-500mb)

Low-levels (sfc-850mb)

Impact of Heat Low SMC




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?




Marked increase in cross-equatorial moisture fluxes during April-May

Linked to cold tongue development and coastal onset

Schematic evolution

SST

SMC

ITCZ1. 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?

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 (~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

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 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)

Schematic evolution

SST

SMC

ITCZ

HL

Wet bias in Spring?

Dry bias in Sahel in Summer

Dry bias in Spring?

ERA40 vs NCEP1rainfall

ERA40

NCEP1

can a strong heat low SMC suppress convection south of it?


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 2.5o grid. Averaged from 10oW to 10oE over 10 years (1998-2007).

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 2.5o grid. Averaged from 10oW to 10oE over 10 years (1998-2007).

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

tOC = 10 May

Period: 1998-2007

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 onsetStrong 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: 2005-2007

A comparison of 3 years: 2005-2007

05 05

06 06

0707

2005

2006

2007

11 20

21 3

23 29

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

MAY MARCH

APRIL FEBRUARY

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 OLRPeaks 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 m2) at 10N, 10W for June-September 1979-1993

Day 0Streamfunction (contours 1 X 105 m2 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: Genesis Two Theories for the Genesis of AEWs

925hPa

315K PV

AEJ satisfies the necessary conditions for barotropic andbaroclinic instability:Burpee (1972), Albignat and Reed, 1980).

Therefore we expect AEWs to arise from small random perturbations consistent with a “survival of thefittest” 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 40-50o 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!)


• The AEJ is too short!The jet is typically 40-50o 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?


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 30th July 2000 00z to 4th August 2000 18z.

•Convective outbursts in the first day of the sequence preceded the dynamical signal.


Day-4Streamfunction (contours 1 X 105 m2 s-1)


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.

Conceptual framework (ii) Baroclinic growth.

’ Max

700hPa Trough

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.



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, 1997-2007)

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

Importance of Guinea Highlands Region

Developing Non-Developing

Hopsch , Thorncroft and Tyle 2010

Composites of East Atlantic Developing and Non-Developing AEWs (1979-2001)

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

A Multiscale Analysis of the West African Monsoon Chris Thorncroft

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