1. 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.twcwchang@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 Quasi-equilibrium convective constraints assumption (Betts and Miller, 1986) (Yu and Neelin,1997) The vertically-integrated moist static energy

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

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

23. The Obs. ENSO

24. ENSO started in the eastern and central basin

37. 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

38. Feb. ½~Apr. ½ 120°E180°E 15°N 15°S westerly wind bursts (Max 10m/s) Hybrid coupled model

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

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

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

42. Gross moist stability (M) climatology

43. Hybrid coupled model non-homogeneous air-sea feedback 120°E180°E 15°N 15°S

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

45. 1years 1.5years ~1years Air-sea interaction increases interannual period 1.5years

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

47. ENSO decadal change

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

49. The decadal change of M warm epoch climatology

50. Decadal effect Stand runStand run+warm Stand run+non-hom

51. Concluding Remarks The westerly wind burst occurring in the western/central Pacific is able to excite or strengthen the El Nino event. Non-homogeneous air-sea feedback (strong in western Pacific while weaker elsewhere) can produce a series of El Nino events with longer period (~3 years) compared to homogeneous case (~2 years). Decadal change of the observed ENSOs (i.e., longer period and stronger strength in warm epoch) might be due to the change of air-sea feedback.

52. Thanks for listening

56. Gross moist stability (M)

57. Conclusion. Westerly Wind Bursts modulate the development of the ENSO.

58. Conclusion. WWBs modulated by the Non-homogeneous air- sea feedback over deep convection region. To increase the air-sea feedback strength over deep convection region, which makes the WWBs effectively self-sustained in the tropical region.

59. Conclusion. The decadal oscillation modulates ENSOs by intensifying air-sea feedback over western pacific ocean in warm epoch

61. Similar to Obs. WPC Obs.