a multiscale analysis of the west african monsoon chris thorncroft department of atmospheric and...
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![Page 1: A Multiscale Analysis of the West African Monsoon Chris Thorncroft Department of Atmospheric and Environmental Sciences University at Albany](https://reader036.vdocument.in/reader036/viewer/2022062807/56649cff5503460f949d10c8/html5/thumbnails/1.jpg)
A Multiscale Analysis of the West African Monsoon
Chris Thorncroft Department of Atmospheric and Environmental Sciences
University at Albany
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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
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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)
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The Coupled Monsoon System
Cold Tongue
SAL
ITCZ
Heat Low
Key features of the WAM Climate System during Boreal summer
AEJ
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North-South Section along the Greenwich Meridian
θ
50oC
20oC
θ θe
90oC
60oC
θe
AEJ
Meridional Circulations
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Shallow Meridional Circulation (SMC) over ocean, especially in Spring
θ
50oC
20oC
θ θe
90oC
60oC
θe
AEJ
Meridional Circulations
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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
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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
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• 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
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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
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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
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Total Column Moisture Flux Convergence
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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))
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Total Column Moisture Flux Convergence
Peak in moisture flux convergence over ocean
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Total Column Moisture Flux Convergence
Rapid shift and increase in moisture flux convergence towards coast between April and May
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Total Column Moisture Flux Convergence
Rapid reduction in moisture flux convergence during June – linked to end of coastal rains
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Total Column Moisture Flux Convergence
Rapid increase in moisture flux convergence beginning of July linked to Sahelian rainfall onset
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Meridional Moisture Fluxes
Mid-levels (850-500mb)
Low-levels (sfc-850mb)
Impact of Heat Low SMC
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Meridional Moisture Fluxes
Mid-levels (850-500mb)
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?
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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
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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
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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
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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
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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
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Wet bias in Spring?
Dry bias in Sahel in Summer
Dry bias in Spring?
ERA40 vs NCEP1rainfall
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ERA40
NCEP1
can a strong heat low SMC suppress convection south of it?
Total Column Moisture Flux Convergence
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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)
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Interannual Variability of Coastal Rainfall
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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.
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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.
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TRMM
CMAP
GPCP
Composite diagrams
tOC = 9 May
tOC = 10 May
tOC = 10 May
Period: 1998-2007
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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
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Relationship between the coastal onset and the SST cooling
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Relationship between the coastal onset and the Saharan Heat Low
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A comparison of 3 years: 2005-2007
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A comparison of 3 years: 2005-2007
05 05
06 06
0707
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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
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Interannual variability
Coastal rainfall intensity : 8.2 mm/d ; range: 9.1 mm/d
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Relationship between the coastal rainfall and the SST
MAY MARCH
APRIL FEBRUARY
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Relationship between the coastal onset and the Sahel onset
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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.
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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
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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)
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3. African Easterly Waves
Objectively diagnosed troughs (solid lines), African Easterly Jet (dashed), with PV (315K) and IR from METEOSAT (courtesy Gareth Berry)
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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)
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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.
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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.
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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.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?
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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)
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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.
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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
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OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-4Streamfunction (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
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OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-3Streamfunction (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
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OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-2Streamfunction (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
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OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993
Day-1Streamfunction (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
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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
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Idealised Modeling Study: Thorncroft, Hall and Kiladis (2008)
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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.
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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
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Phase II: Baroclinic Development - Scale Interactions
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’ Max
700hPa Trough
Conceptual framework (ii) Baroclinic growth.
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’ Max
700hPa Trough
Conceptual framework (ii) Baroclinic growth.
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Conceptual framework (ii) Baroclinic growth.
’ Max
700hPa Trough
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PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions
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PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions
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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.
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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!
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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!
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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
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Importance of Guinea Highlands Region
Developing Non-Developing
Hopsch , Thorncroft and Tyle 2010
Composites of East Atlantic Developing and Non-Developing AEWs (1979-2001)
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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?
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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)
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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….?
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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