Pang-chi Hsu, Chih-Hua Tsou Department of Earth Science, NTNU, Taiwan

1. Multi-scale Interaction between ENSO and Large-scale Circulation along the Tropical Storm Track Pang-chi Hsu, Chih-Hua Tsou Department of Earth Science, NTNU, Taiwan

2. Outline • Motivation • Data and Model Description • Numerical Experiments • Simulated Results • Summary

3. Motivation • Which one plays the active role ? ENSO-related warmSST anomaly, the enhancement and eastward-extension of low-level westerly, monsoon trough beneficial for TS formation and development all occur simultaneously.  Eddy energetics  Numerical experiment

4. Walker Circulation Eddy energy budget equations Zonal mean  Eddy Less study Time mean  Transient Eddy General circulation Oort (1964) No calculation Synoptic disturbance Lau and Lau (1992) MJO-TD type eddy Maloney and Dickson (2003) By 3-D EKE budget eq. & 2-D EAPE budget eq. General circulation Extratropical cyclones Lorenz (1955) Kung (1966) ﹕ Hu et al. (2004) 3-D EKE/EAPE diagnosis

5. Data • NCEP / DOE (NCEP R2) : 144 × 73 × 17 • NCEP monthly SST : 180 × 90 • JTWC 6-hourly best track • Based on Wang and Chan (2002), the TS activity over the WNP is closely related to Nino3.4 SSTA in summer. • The classifications of ENSO are stratified by JAS SSTA in Nino3.4 region into Warm years (SSTA ≥ 0.8 S.D.): 1982, 1987, 1991, 1997 Cold years (SSTA ≤ -0.8 S.D.): 1981, 1985, 1988, 1998

6. Model Description • ECHAM4.6 : MPI, Germany (Roecknet et al. 1996) • High resolution: T106 (320×160) L19 • 6-hourly output for TS identification and trajectory • Forced with observed monthly SST and sea-ice (AMIP run) for 1979-1999 Vitart et al. (1997) and Knutson et al. (2007)

7. ECHAM 4.6 T106 control run

8. Numerical Experiments Two numerical experiments are designed to clarify whether the ENSO-related SST anomaly plays an active role to the interanuual variations of large-scale circulations and TS activity.

9. Outline • Motivation • Data and Model Description • Numerical Experiments • Simulated Results • Summary

10. <Warm minus Cold> Obs CTL Overestimate (underestimate) the response of heating (cooling ) effect TS frequency has eastward bias Camargo et al. (2005)

11. ENSO-related SST anomalies • Strong westerly • Cyclonic shear (monsoon trough) favorable environment for TS activity • No warming and cooling • no westerly jet • A-C wavetrain still exists • but shifts northeastward insignificant interannual TS variation

12.   WHY? • The TS number and life span of TS during warm years are similar to JTWC, but the intensity of simulated TS is too weak(Bengtsson et al. 2007). • ENSO-related SST anomalies play an active role in modulating the TS activity over the WNP.

13. Eddy energetics along the TS track (obs) MKE to EKE EAPE to EKE • Enhanced eddy barotropic and baroclinic energy conversion might be beneficial for the southeastward extension of the TS formation region and the intensification of the TS during warm events. • The eddy baroclinic energy conversion plays a significant role in maintaining the subsequent development of eddies.

14. Less EKE converted from EAPE Insufficient EAPE generation and eddy baroclinic energy conversion

15. Summary • The results of numerical experiments support that the El Nino-related anomalous SST forcing plays an active role in setting the favorable large-scale environments for the formation and early-development of transient eddies including TS over the WNP. • The inadequate simulation of eddy diabatic heating and eddy baroclinic processes which might be resulted from insufficient resolution would lead to the underestimate of TS intensity in ECHAM4 T106.

16. Thanks for your attention