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INTRASEASONAL Oscillations—the Heartbeat of 14-100 Day Weather Variability

INTRASEASONAL Oscillations—the Heartbeat of 14-100 Day Weather Variability. Paul E. Roundy. Cold Air Outbreak Timeline. Active convective anomaly amplified in Indian ocean (mid February 2003)

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INTRASEASONAL Oscillations—the Heartbeat of 14-100 Day Weather Variability

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  1. INTRASEASONAL Oscillations—the Heartbeat of 14-100 Day Weather Variability Paul E. Roundy

  2. Cold Air Outbreak Timeline • Active convective anomaly amplified in Indian ocean (mid February 2003) • Anomaly approached maritime continent (around February 20)—heating deflected pacific jet stream pattern

  3. Cold Air Outbreak • Large edge wave formed (late February), propagated poleward • Cold air stream enhanced by strengthened west-coast ridge

  4. Convective Anomaly Pattern26 February, 2003 From BMRC, Matthew Wheeler, Australia

  5. Cold Air Outbreak, February 27, 2003 From NCEP Reanalysis

  6. The Large-Scale ConvectiveDisturbance • Tropical Intraseasonal or Madden-Julian Oscillation

  7. What IS the MJO? • Large-scale disturbance of deep convection and winds that controls up to half of the variance of tropical convection in some regions • Brief history

  8. Simplified Madden-Julian Oscillation Composite OLR from A.J. Matthews, 2000.

  9. MJO Statistics • Eastward propagation, 4 +/- 2 ms-1. Also has standing wave behavior • 30-60 day period • Wavenumber 1-4 (planetary scale) • Interacts with midlatitudes, but some of this is nonlinear and hard to quantify

  10. Schematic of Mature MJO

  11. How Does It Propagate? • Is a matter of debate, but, probably involves • interactions with equatorial waves • Kelvin wave • Equatorial Rossby wave • Feedbacks from convection • Sea surface temperatures—air-sea interaction • Land interactions

  12. Kelvin Wave • Equatorial zonal wind disturbance L L H H

  13. Kelvin Wave • Propagation mechanism: H L Pressure Fall Pressure Fall Pressure Rise Dry wave propagates eastward at greater than 40 ms-1

  14. Convective Kelvin Wave z Convection removes Some of the accumulating mass, slows propagation H L x Propagation speed: less than 20 ms-1

  15. Equatorial Rossby Wave H L Pressure Rises Pressure Falls Pressure Rises L H

  16. Wave Cooperation • Kelvin and Rossby waves linked by convection, land, and air-sea interaction combine to produce the observed disturbance.

  17. Decay Region Formation Region

  18. Active Convection

  19. Enhanced Easterlies Active Convection

  20. Deflected Jet Stream Active Convection Energy Build-up

  21. Cold air outbreak enhancement Active Convection

  22. Effect of Background • Intraseasonal oscillations are modified by convective coupling, so they must be modified by • The annual cycle • Interannual processes like ENSO

  23. Prediction of MJO • Global weather models predict it with some skill to about 7 or 8 days • Filtering methods allow prediction up to 20 days (Wheeler and Weikmann, 2001) • Statistical schemes may allow prediction for more than 40 or 50 day lead times

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