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Mechanisms controlling ENSO: A simple hybrid coupled model study

Mechanisms controlling ENSO: A simple hybrid coupled model study. Cheng-Wei Chang 1 * and Jia-Yuh Yu 2 1. Institute of Geography, Chinese Culture University, Taipei, Taiwan, cwchang@atmos.pccu.edu.tw , +886-2-28610511 ext25705

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Mechanisms controlling ENSO: A simple hybrid coupled model study

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  1. Mechanisms controlling ENSO: A simple hybrid coupled model study Cheng-Wei Chang1* and Jia-Yuh Yu2 1. Institute of Geography, Chinese Culture University, Taipei, Taiwan, cwchang@atmos.pccu.edu.tw, +886-2-28610511 ext25705 2. Department of Atmospheric Sciences, Chinese Culture University

  2. Contents • 1. Introduction • 2. Data Sources • 3. Mechanisms: 3-1、Westerly Wind Bursts 3-2、Non-homogeneous Air-sea Feedback • 4.Concluding Remarks

  3. Introduction • During the cold epoch (1960s and 1970s), the ENSO scenario (viewed as SSTA) starts in the east and propagates westward along the equator. • During the warm epoch (1980s and 1990s), eastward propagating SSTA associated with westerly wind burst in the central Pacific strengthen ENSO ( Rasmusson and Carpenter 1982; Zhang and Busalacchi 1999).

  4. The surface winds over the tropical Pacific Ocean show energetic large-scale variability on timescales ranging from a few days to decades (Luther et al. 1983; Luther and Harrison 1984; Harrison and Luther 1990). • The instability involves feedbacks between SST, which affects the atmospheric circulation, and the dynamics of the ocean circulation must adjust to the changes in wind.

  5. Air-sea interaction over deep convection region The vertically-integrated moist static energy Quasi-equilibrium convective constraints assumption (Betts and Miller, 1986)(Yu and Neelin,1997)

  6. Over deep convection region Long time average Gross moist stability (M) (Yu and Neelin,1997)

  7. Data Sources • NCEP/NCAR re-analysis grid data of atmosphere • Reconstruction OI SST • Domain:90°N～90°S/0°E～0°W • Horizontal resolution: 2.5°×2.5° • Study period: 1949~2000

  8. Simple Hybrid Coupled Model • Ocean Component – Cane-Zebiak (CZ) model with Niller-Kraus vertical mixing scheme • Atmospheric Component – Empirical atmospheric model based on SVD projections of the first 7 modes • Domain: 0°E～0°W/30°N～30°S • Horizontal resolution: 2°×1°

  9. The Obs. ENSO

  10. ENSO started in the eastern basin

  11. The Obs. ENSO

  12. ENSO started in the eastern and central basin

  13. The OBS. westerly wind bursts • Hartten(1996) 1000-hPa zonal winds anomaly exceeded 5 m/s with a zonal extent over 10° and lasting 10days

  14. Hybrid coupled model Feb. ½~Apr. ½ westerly wind bursts(Max 10m/s) 15°N 120°E 180°E 15°S

  15. WWB effect Belamarl,2003 Feb. ½~Apr. ½ WWB can excite the ENSO-like pattern

  16. The hybrid couple model simulates ENSO Feb. ½~Apr. ½ • Ideal exp. +WWB

  17. Stand run Stand run+ WWB Feb. ½~Apr. ½ • What cause WWB? • how to maintain it in long time?

  18. Gross moist stability (M) • climatology

  19. Hybrid coupled model non-homogeneous air-sea feedback 15°N 120°E 180°E 15°S

  20. Stand run Stand run+ WWB Stand run+ non-hom non-homogeneous air-sea feedback

  21. Air-sea interaction increases interannual period 1.5years 1years 1.5years ~1years

  22. Stand run Stand run+ non-hom Half period ~1 year Half period ~1.5 years It makes the WWBs effectively self-sustained in the tropical region

  23. ENSO decadal change

  24. The decadal change of WWB • WWB’ strength and period in warm epoch are stronger and longer than in cold epoch

  25. The decadal change of M • climatology • warm epoch

  26. Stand run Stand run+warm Stand run+non-hom Decadal effect

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