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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.

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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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4.3 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