Tropical Intraseasonal Oscillation: Mechanisms, Evolution, and Seasonality

1. Structure, Evolution and Seasonality of Tropical Intraseasonal Oscillation: Tim Li Department of Meteorology and IPRC, University of Hawaii Acknowledgements: Bin Wang, X. Fu, H. Rui, X. Xie, C. Zhou S. Kemball-Cook, H. L. Drbohlav, X. Jiang

2. Outline • Review: MJO mechanisms • Major observational features • 3. A prototype model for MJO • 4. Mechanism for northward propagation • 5. Re-initiation in the western Indian Ocean • 6. Cause of seasonality of intraseasonal oscillations in tropical atmosphere and ocean

3. 1. Review • Mechanisms for eastward propagating MJO • Equatorial wave-Convection feedback (Wave-CISK): Lau and Peng 87,Miyahara 87,Takahashi 87, Chang and Lim 88, Hendon 88, Lau and Shen 88,Itoh 1989, Sui and Lau 89, Lim et al. 90, Dunkerton and Crum 91, Yoshizaki 91, Wang and Xue 92, Chao 95, Cho ‘00 • 2. Evaporation-wind feedback (WISHE): Emanuel 87, Neelin et al. 87, Wang 88, Yano and Emanuel 92, Emannuel 93, Xie et al. 93, Neelin and Yu 14, Yu and Neelin 94, Hayashi and Gold 97, Raymond and Torres 98, Raymond 2001 • 3. Frictional convergence feedback: Wang 1988, Wang and Chen 1989, Xie and Kubakawa 1990, Wang and Rui 1990, Blade and Hartmann 1993, Gaswami and Rao 94, Wang and Li 1994, Salby et al.1994, Hendon and Salby 1994, Jones and Weare 1996, Ohuchi and Yamasaki 1997, Li and Cho 1997, Maloney and Hartmann 1998, Moskowitz and Bretherton 2000, Lee et al. 2003 • Mechanisms for boreal summer ISO • Northward propagation: Webster 1983, Goswami and Shukla 1984, Wang and Xie 1997, Lau and Peng 1990, Kemball-Cook and Wang 2001, Lawrenec and Webster 2002, Hsu 2002, Jiang et al. 2003, Drbolav and Wang 2003 • Oscillation mechanism and westward propagation: Wang and Xie 1997 • Other Feedback Processes • Cloud-radiation interaction: Hu and Randal 94a,b, Slingo and Madden 91, Lee et al. ‘01 • Air-sea coupling: Hirst and Lau 90, Wang and Xie 98, Flateu et al. 97, Waliser et al. 99, Hendon 2000, Kemball-cook et al. ‘01, Fu et al. ’02 • Cumulus parameterization:

4. 2. Essential observed features • (Theoretical Implications) • Preferred planetary zonal scale and trapped meridional scale • Vertical structure: Baroclinic motion with an upward and westward tilt of vertical motion and humidity--- Roles of the PBL dynamics • Horizontal structure: Kelvin-Rossby wave couplet—Coupling of equatorial waves by convective heating and PBL frictional convergence • Amplification over the equatorial Indian Ocean and Western Pacific; decay over the maritime continent • Remarkable seasonal variations of propagation and intensity—Possible roles of the seasonal changes in the background circulation and SST (Humidity)

5. A Observed Horizontal Structure of MJO: Kelvin-Rossby wave couplet with BL friction leading convection A C C Hendon and Salby 1994

6. Observed Vertical Structure of MJO: BL Convergence leads Convection Sperber and Slingo 2003

8. Observed Propagation are complex and season-dependent Maddem-Julian Oscillation: Strong in boreal winter Equatorial eastward propagations (Madden-Julian 1972) Longitudinal variation (Knutson and Weickman 1986) Boreal summer ISO: Multi-mode behavior (Wang and Rui 1990) Northward propagation over Indian monsoon region (Yasunari 1980, Krishnamurti and Subrahmanyam 1982….) Off-equatorial westward propagation in western Pacific and South Asia (Murakami 1984: Lau and Chan 1986….) Equatorial eastward propagation is weak (Madden 1986, Hnedon and Salby 1994) Standing feature (Weickmann and Khalsa1990, Zhu and Wang 1993)

9. Can we explain the season-dependent, multi-mode propagation with a unified theoretical framework?

10. 3. A prototype model for MJO • Frictionally coupled • Kelvin-Rossby Wave Packet • Model: • Two-layer free troposphere Plus a well-mixed PBL • Aqua-Planet with specified SST (y) • Dynamical processes: • Equatorial Kelvin and Rossby waves • PBL frictional convergence • Interactive Convective (nonlinear or linear) heating (No wave-CISK) • Evaporation-wind feedback

11. What Essential Physics are need In a theoretical model

12. Governing Equations in the 2-1/2 layer model (1) (2) (3) (4) (5) I=q3/qcheating coefficient due to wave convergence B=qe/qcheating coefficient due to frictional convergence F=(qs-q0)/qcheating coefficient due to evaporation

13. Linear analysis: Growth rate and slow propagation of frictional coupled Kelvin-Rossby mode Slow eastward propagation Wavenumber selection

14. Horizontal structure of frictional coupled Kelvin-Rossby mode Unstable Normal mode (Model) Hendon and Salby 1994 (OBS)

15. Nonlinear Heating: Propagation along the equator Nonlinear heating makes a concentrated precipitation and a broad subsidence area

16. Nonlinear Heating Propagation of Kelvin-Rossby wave packet in uniform SST Precipitation/ Low-level winds (Every four days) Wang and Li 1994

17. Growth rate and Group velocity under nonlinear heating Sensitivity to underlying SST and BL depth Note the slow propagation and slow growth as SST higher than 29oC and the PBL top is below 900 hPa

18. Comparison of Frictional Feedback, Wave-CISK, and Wind-Evaporation Feedback (Under nonlinear heating)

19. Conclusions • MJO and boreal summer ISO can be explained by a unified prototype model: Equatorial Coupled Moist Waves by Friction (ECMWF) regulated by seasonal mean circulation and moist static energy distribution. • Frictional convergence feedback provides mechanisms for (a) long wave selection, (b) tilted vertical structure with stronger upper-level wind anomaly, (c) coupled Kelvin-Rossby wave structure, (4) slow eastward propagation, and (5) low-frequency amplification. • Basic-state vertical shears of zonal wind/meridional circulation and reduction of moist static energy over the maritime continent and central Pacific cause moist Rossby waves emanating from equatorial K-R wave packet, favoring northward propagation over Indian monsoon and WNP monsoon regions as well as off-equatorial westward propagation on intraseasonal time scales.

20. Mechanisms for Northward Propagation of the Boreal Summer Intraseasonal Oscillation Tim Li ( Department of Meteorology and IPRC University of Hawaii ) See Jiang, Li, and Wang 2004, JAS, 1022-1039

21. Outline • Introduction • BSISO structures revealed from ECHAM simulation and NCEP Reanalysis • Two internal atmospheric dynamics mechanisms • Eigenvalue analysis • Summary

22. Wang and Rui (1990) 1.Introduction: Madden and Julia(1971,1972) NE SE EN EN Yasunari (1979,1980) EE The cloud fluctuation with 40-day period shows northward movement over the Indian Ocean in summer Krishnamurti and Subrahmanyam (1982) A steady meridional propagation of a train of troughs and ridges Meridional length scale: 3000km Propagation speed: 0.75o lat per day • 3 categories: eastward (65%), independent northward (20%) and westward (15%) propagation.

23. Time–latitude section of daily precipitation rate estimates along 75–80E for Jun–Sep 1987 and 1988. Contour interval is 5 mm/day with the first contour at 5 mm/day. Time series of daily precipitation rate estimates averaged over 10–15N, 75–80E for Jun–Sep 1987 (mm day-1). From Lawrence and Webster (2001): Interannual Variations of the Intraseasonal Oscillation in the South Asian Summer Monsoon Region. JCL

24. Northward propagation mechanisms: A review • Atmosphere-land interactions: Webster(1983) • The key process: land surface heat fluxes into the boundary layer destabilizes the atmosphere ahead of convection and causes northward shift of the convective zone. • 2. Atmosphere-ocean interactions: Kemball-Cook and Wang (2002) • The negative latent heat flux anomaly ahead of convection leads to warmer SST and thus reduced static stability. This leads to the northward shift of convection. • 3. Rossby wave energy emanation: Wang and Xie (1997), Lawrence and Webster (2002) • The northward ‘‘propagation’’ is due to Rossby wave energy emanation from the equatorial eastward-moving ISO convection. In this scenario, the northward propagation is closely related to the eastward moving ISO.

25. Scientific questions: • What is the observed characteristic structure of the northward propagating ISO mode? • What is the mechanism for the northward propagation? Does it result from internal atmospheric dynamics or from air-sea interactions? • Why does the northward propagation appear over the monsoon regions?

26. 2. BSISO structures as revealed from ECHAM simulation and NCEP Reanalysis ECHAM AGCM 4.0 • Horizontal resolution: T30 • 19 vertical levels • Integrate for 15 years • Daily output • Climatological SST NCEP/NCAR Reanalysis • 1980-2001 daily averaged • Horizontal resolution: 2.5*2.5 degree • 12 vertical levels • u, v, w, q, olr

27. Rainfall evolution along 70~95oE (15~90 day filtered) 15 10 5 14 9 4 13 8 3 12 7 2 11 6 1

28. Composite rainfall evolution by model

29. Hovmoller Diagram of rainfall by ECHAM model (70~95oE) Average propagation speed (EQ~20oN) : 1.0 latitude degree/day.

30. Composite -OLR evolution (NCEP Reanalysis Data)

31. Composite -OLR evolution (NCEP Reanalysis Data) Average northward propagating speed: 0.93olatitude/day

32. ECHAM Model vorticity geopotential height vertical velocity divergence temperature specific humidity

33. NCEP vertical velocity vorticity geopotential height temperature divergence specific humidity

34. 3. Mechanisms Mechanism I: Role of vertical wind shear Vorticity

35. 21/2 Layer Model by Wang and Li (1993) Considering zonal symmetric case, 0 u1,v1, 1 2 u3,v3, 3 4 PBL uB,vB

36. N D1>0 - u1<0 w’ D->0 + D3<0 u3>0 N D1>0 D1>0 N + w’ w’ D->0 - D3<0 D3<0 N N

37. Easterly Shear Genesis of barotropic vorticity and divergence to the north of convection Northward shift of convective heating Boundary layer moisture convergence

38. Mechanism II: moisture advection-convection feedback Specific humidity

39. Summer Mean V (70~95oE) (1) Moisture advection by the mean flow (a) (b) (c) w’ w’ w’ q’ q’ q’ N N N

40. (2) Moisture advection by the anomalous meridional flow Summer Mean q (70~95oE, By NCEP) Li and Wang (1994) 20oN + _ EQ

41. 4. Eigenvalue problem Considering zonal symmetric case, 0 u1,v1, 1 2 u3,v3, 3 4 PBL uB,vB

42. Growth rate: Frequency: For given uT= -12 m/s Growth rate: 0.04  25 day Wave number 4  2500 km Phase speed: 0.6 m/s

44. Ten-year Wavenumber-frequency Analysis CMAP Rainfall Coupled Daily Mean

45. Summary The BSISO structure and evolution characteristics simulated by the ECHAM bear many similarities to those in the NCEP/NCAR reanalysis. The most notable feature is the meridional asymmetry of vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears to north of the convection center. The maximum low-level specific humidity also shifts to its north. Two internal atmospheric dynamics mechanisms are proposed to explain the northward propagation. The first is the vertical shear mechanism. The key process involves the generation of barotropic vorticity due to baroclinic and barotropic mode coupling in the presence of the vertical shear of mean flows. The so-induced barotropic vorticity further causes PBL convergence, leading to the northward shift of convective heating.

46. Summary 3. The second mechanism is the moisture-convection feedback. Two processes contribute to the northward shift of the low-level moisture. One is the moisture advection by the mean southerly in the PBL. Another is the moisture advection by the perturbation wind due to the mean meridional specific humidity gradient. The asymmetry in specific humidity further contributes to the northward shift of convective heating. 4. An eigenvalue analysis indicates that the northward propagation of the BSISO is an unstable mode of the summer monsoon mean flow. It is primarily caused by the internal atmospheric dynamics, although the air-sea interaction also plays a role.