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Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation

Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation. Hai Lin, Gilbert Brunet Meteorological Research Division, Environment Canada Jacques Derome McGill University TTISS, Monterey, September 14, 2009. Outlines. Observed MJO – NAO connection

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Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation

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  1. Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation Hai Lin, Gilbert Brunet Meteorological Research Division, Environment Canada Jacques Derome McGill University TTISS, Monterey, September 14, 2009

  2. Outlines • Observed MJO – NAO connection Lin et al. 2009 (J. Climate) • Intraseasonal variability in a dry GCM Lin et al. 2007 (J. Atmos. Sci.)

  3. NAO and MJO connection • NAO: dominant large scale pattern in the extratropics with significant influence on weather from eastern North America to Europe • MJO: dominant tropical intraseasonal mode, coupled with convections and variability in diabatic heating • One-way impact, or two-way interaction? • A possible mechanism for both the NAO and MJO

  4. Data and methodology Definition of the NAO: 2nd REOF of monthly Z500 NAO index: projection of pentad Z500 anomaly onto this pattern Period: 1979-2003 Extended winter, November to April (36 pentads each winter)

  5. Data and methodology Definition of the MJO: combined EOF of OLR, u200 and u850 in the band of 15°S – 15°N (Wheeler and Hendon, 2004) NAO index: RMM1 and RMM2 Period: 1979-2003 Extended winter, November to April (36 pentads)

  6. Composites of tropical Precipitation rate. Winter half year November-April Xie and Arkin pentad data, 1979-2003

  7. Pentads in MJO phases Extended winter from 1979 to 2004

  8. Lagged composites of the NAO index

  9. Lagged composites of the NAO index

  10. Lagged probability of the NAO indexPositive: upper tercile; Negative: low tercile

  11. Tropical influence

  12. Wave activity flux and 200mb streamfunction anomaly

  13. Extratropical influence Lagged regression of 200mb U to NAO index

  14. U200 composites Extratropical influence Lagged regression of 200mb U to NAO index

  15. Tropical intraseasonal variability (TIV) in a dry GCM

  16. Model and experiment • Primitive equation AGCM (Hall 2000). • T31, 10 levels • Time-independent forcing to maintain the winter climate (1969/70-98/99)  all variabilities come from internal dynamics • No moisture equation, no interactive convection • 3660 days of integration

  17. Model Result Zonal propagation 10S-10N Unfiltered data 20-100 day band-pass Stronger in eastern Hemisphere

  18. TIV in the dry model • Kelvin wave structure • Phase speed: ~15 m/s (slower than free Kelvin wave, similar to convective coupled Kelvin wave, but there is no convection)

  19. What causes the TIV in the dry model? • 3-D mean flow instability (Frederiksen and Frederiksen 1997) • Tropical-extratropical interactions (all wave energy generated in the extratropics) Moisture and convection related mechanisms are excluded Possible mechanisms

  20. ISO in a dry model 250 hPa PV’ and wave activity flux Linked to tropical eastward propagation in the eastern Hemisphere  Global propagation of low-frequency wave activity

  21. Summary • Two-way interaction between the MJO and the NAO • Increase of NAO amplitude 5~15 days after the MJO-related convection anomaly reaches western Pacific • Certain MJO phases are preceded by strong NAOs • TIV generated in a dry GCM • Tropical-extratropical interactions are likely responsible for the model TIV

  22. Implication to the MJO • A possible mechanism for the MJO: triggering, initialization • Contribution of moisture and tropical convection: spatial structure, phase speed

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