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Provide some predictability to the tropical atmosphere beyond the diurnal cycle.
Equatorial waves modulate deep convection inside the ITCZ ans the SCPZ (South Convergence Pacific Zone)
Which relevance for forecasters ?
Chap 4. Equatorial trapped waves
and tropical large-scale oscillations
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Chap 4. Equatorial trapped waves
and tropical large-scale oscillations
4.1 The equatorial trapped waves
4.2 The tropical large-scale oscillations
4.2.1 The Madden-Julian Oscillation (MJO)
4.2.2 The Quasi-Biennial Oscillation (QBO)
4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting
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4.1 The equatorial trapped waves
This course stress only on waves coupled with deep convection (indeed, it exist some waves which don’t modulate convection)
They initiate between 12°N/12°S and they vanish beyond 20° (whence the name ‘trapped waves’).
A valid proxy for deep convection is the Outgoing Longwave Radiation (OLR);
The OLR behave about as the Infrared Red radiation and so, anomalies of OLR are negative when deep convection occur.
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Figure showing the OLR variance for all equatorial wave (included MJO) beyond the diurnal cycle
Reminder : the variance show the modulation of deep convection through waves
• Maximum of variance ‣ in the summer hemisphere ‣ especially over Indian Ocean
and West Pacific
4.1 The equatorial trapped waves : OLR variance for all equatorial waves
Source : Wheeler et Kiladis, 99
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4.1 The equatorial trapped waves : The Kelvin wave
Modulate deep convection between 7°N/7°S
Explain 10% of the total of OLR variance along the Equator, especially from february to august
Period : 15-20 days
Speed phase = + 15 to 20 m/s
Source : Wheeler et Kiladis, 99
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Modulate deep convection between 7 and 15° of latitude
Explain 7% of the total of OLR variance at 10° of latitude from the Indian Ocean to the West Pacific (ITCZ an SCPZ affected), particularly from november to march
Period = 15-20 m/s
Speed phase = -5 m/S
4.1 The equatorial trapped waves : Equatorial Rossby (ER)
Source : Wheeler et Kiladis, 99
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Modulate deep convection between 3 and 10° of latitude
Explain 4% of the total of OLR variance at 7.5° of latitude near dateline (ITCZ and SCPZ affected), particularly from september to november
Period =4-5 days
Speed phase = -23m/s
4.1 The equatorial trapped waves : Mixed Rossby-Gravity (MRG)
Source : Wheeler et Kiladis, 99
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Which moves eastwards (Eastwards Inertial Gravity, EIG) ; EIG explain 4% of the total of OLR variance at 7.5° near date line
Which moves westwards (westwards Inertial Gravity, WIG) : WIG1 explain 6% of the total of OLR variance at the Equator over eastern hemisphere
Which moves westwards : WIG2 explain 2% of the total of OLR variance at 5° of latitude over eastern hemisphere
4.1 The equatorial trapped waves:
The gravity waves
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Chap 4. Equatorial trapped waves
and tropical large-scale oscillations
4.1 The equatorial trapped waves
4.2 The tropical large-scale oscillations
4.2.1 The Madden-Julian Oscillation (MJO)
4.2.2 The Quasi-Biennial Oscillation (QBO)
4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting
general contents
Chap. 4 Equatorial trapped waves
and tropical large-scale oscillations
4.1 The equatorial trapped waves
4.2 The tropical large-scale oscillations
4.2.1 The Madden-Julian Oscillation (MJO)
4.2.2 The Quasi-Biennial Oscillation (QBO)
4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting
sommaire
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4.2.1 The Madden-Julian Oscillation (MJO)
It is firstly mentionned by Madden and Julian in 1971 as being a fluctuation of zonal wind at surface of 2-3 m/s and a fluctuation of mean sea-level pressure (0.7 hPa) over Canton Island (West Equatorial Pacific)
Finally, this fluctuaction is an intraseasonal oscillation with a period of 40 to 50 days, called ‘Madden-Julian Oscillation’, which modulate deep convection in tropics from Indian Ocean to Western Pacific.
Main features of the MJO :
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4.2.1 : The MJO cycle
Westerly speed Phase
5 m/s (Equatorial Africa)
5 m/s (Indian Ocean)
5 m/s (Indonésia)
5m/s (Western Pacific)
5 m/s (dateline)
10 to 15 m/s (eastern Pacific)
10 to 15 m/s (America)
10 to 15 m/s (Atlantic)
time :days
1 5
6 10
11 15
16 20
21 25
26 30
31 35
36 40
Italic = inactive MJO
Source : Madden et Julian,1971.
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4.2.1 The OLR variance linked to MJO
The MJO explain 10 to 15% of the total of OLR variance at 10° of latitude (latitude of the maximum)
The MJO behaves like an equatorially-trapped wave : no signal beyond 20°; the MJO could be a mixture between an Equatorial Rossby (ER=) wave and a Kelvin wave
Seasonal variability of the MJO : maximum in january and february
Source : Wheeler et Kiladis, 99
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4.2.1 The MJO : 3D conceptual model
Top figure show active MJO at 90°E :Over India andCentral Indian Ocean
The deep convection phase is coupledwith westerly anomalies (+ 3 m/s) and fall of MSLPat surface and upper easterlies (- 6 m/s)
Bottom figure (10 days after the top figure) : enhanced convection over 150°E (West Pacific) and suppressed convection over Indian monsoon (90°E)
Source : Rui et Wang, 1990
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Chap 4. Equatorial trapped waves
and tropical large-scale oscillations
4.1 The equatorial trapped waves
4.2 The tropical large-scale oscillations
4.2.1 The Madden-Julian Oscillation (MJO)
4.2.2 The Quasi-Biennial Oscillation (QBO)
4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting
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4.2.2 The Quasi-Biennial Oscillation (QBO) : Main Features
Described as an approximately 26-month alternation between easterlies ans westerlies in the equatorial stratosphere (between 23 and 30 km).
The amplitude is as large as 20 m/s between 10 and 40 hPa and decrease towards adjacent layers and higher latitudes.
Source : d’après Coy, 1979, 1980
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4.2.2 The Quasi-Biennial Oscillation (QBO) : Main Features
Impact of the QBO on tropical storm intensity and
frequency
Hypothesis of the QBO : Equatorial region is favourable for vertical propagation of equatorial gravity waves energy (group velocity) from mid-troposphere towards low stratosphere; then combined action of Kelvin/MRG waves propagate energy towards mid-stratosphere where we observe the QBO peak intensity.
general contents
Chap 4. Equatorial trapped waves
and tropical large-scale oscillations
4.1 The equatorial trapped waves
4.2 The tropical large-scale oscillations
4.2.1 The Madden-Julian Oscillation (MJO)
4.2.2 The Quasi-Biennial Oscillation (QBO)
4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting
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4.3 A review of ‘synoptic to intraseasonnal’ tropical waves with
relevance to forecasting
• In Australia, real time filtering of MJO OLR has already been implemented in an semi-operationnal setting, and has shown some success for both monitoring and forecasting beyond the medium range (i.e. beyond several days)
• And real time filtering of others waves is also used, especially for monitoring, not very useful for forecasting (ER, Kelvin, MRG)
• Finally, we have animation OLR for MJO, ER,
Kelvin, MRG on this web site :
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And if you want to know more about waves you can visit the UFR (Unity Teaching-Research
Department) website : :http://intra-ufr.enm.meteo.fr/pages/ufr/ressources/
ressour_rech/biblio/biblio_index.htm
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OLR variance linked to EIG wave
Source : Wheeler et Kiladis, 99
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OLR variance linked to WIG1 wave
Source : Wheeler et Kiladis, 99
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OLR variance linked to WIG2 wave
Source : Wheeler et Kiladis, 99
References
- Coy, L., 1979 : ‘An unusually large westerly amplitude of the quasi-biennial oscillation’. J. Atmos. Sci., Vol.36, p.174-176.
- Coy, L., 1980 : ‘Corrigendum’. J. Atmos. Sci., Vol.37, p.912-913
-Madden, R. A. et P. R. Julian, 1971 : Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific. Journal of the Atmospheric Sciences, Vol.28, p. 702-708
- Rui, H., Wang, B., 1990 : ‘Development characteristics and dynamic strcuture of tropical intraseasonal convcetion anomalies’. J. Atmos. Sci., Vol.47, p.357-379
- Wheeler, M., Kiladis, G., N., 1999: ‘Convectively coupled equatorial waves : analysis of clouds and temperature int the wavenumber-frequency domain’. J. Atmos. Sci., Vol.56, p.374-399
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