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Chap 4. Equatorial trapped waves and tropical large-scale oscillations

Chap 4. Equatorial trapped waves and tropical large-scale oscillations. 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 ?.

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Chap 4. Equatorial trapped waves and tropical large-scale oscillations

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  1. Chap 4.Equatorial trapped waves and tropical large-scale oscillations • 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 ?

  2. 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 Areview of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting

  3. 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.

  4. 4.1 The equatorial trapped waves : OLR variance for all equatorial waves 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 Source : Wheeler et Kiladis, 99 • Maximum of variance ‣ in the summer hemisphere • ‣ especially over Indian Ocean • and West Pacific

  5. 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

  6. 4.1 The equatorial trapped waves :Equatorial Rossby (ER) • 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 Source : Wheeler et Kiladis, 99

  7. 4.1 The equatorial trapped waves :Mixed Rossby-Gravity (MRG) • 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 Source : Wheeler et Kiladis, 99

  8. 4.1 The equatorial trapped waves:The gravity waves • 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

  9. 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 Areview of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting

  10. Chap. 4Equatorial 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 Areview of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting sommaire

  11. 4.2.1 The Madden-Julian Oscillation (MJO) Main features of the 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.

  12. Source : Madden et Julian,1971. 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 • 10 • 15 • 20 • 25 • 26 30 • 35 • 3640 4.2.1 : The MJO cycle Italic = inactive MJO

  13. 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

  14. 4.2.1 The MJO : 3D conceptual model Source : Rui et Wang, 1990 Top figure show active MJO at 90°E : Over India and Central Indian Ocean The deep convection phase is coupled with westerly anomalies (+ 3 m/s) and fall of MSLP at 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)

  15. 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 Areview of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting

  16. 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

  17. 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.

  18. 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 Areview of ‘synoptic to intraseasonnal’ tropical waves with relevance to forecasting

  19. 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 :

  20. 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

  21. OLR variance linked to EIG wave Source : Wheeler et Kiladis, 99

  22. OLR variancelinked to WIG1 wave Source : Wheeler et Kiladis, 99

  23. OLR variancelinked to WIG2 wave Source : Wheeler et Kiladis, 99

  24. 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 References

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