Dynamics and Characteristics of the Inter-Tropical Convergence Zone (ITCZ) during the Active Season

1. The Inter-Tropical Convergence Zone (ITCZ) during the active season (May—Oct) on daily data Chia-chi Wang Dept. Atmospheric Sciences Chinese Culture University Acknowledgment: Prof. Gudrun Magnusdottir (Earth System Science, University of California, Irvine) Dr. Chia Chou and Dr. Wei-liang Lee (Research Center for Environmental Change, Academia Sinica, Taiwan)

2. Outline • Introduction • Approaches • Modeling studies • Data analysis • Some current understanding for the ITCZ • Summary • Future work

3. ITCZ Waliser and Gautier 1993. Averaged from 17 years of monthly Highly reflective cloud (HRC) data. The number of days per month the given grid point was covered by a large-scale deep convective system, subjectively determined.

4. ITCZ breakdown HURSAT-Basin, VS, 19--27 Sept, 2000 Timescale ~ 1—3 weeks

5. The ITCZ The produced disturbances Relative vorticity ζ (QuikSCAT) Aug. 22, 2000 (before breaking) Aug. 27, 2000 (ITCZ breaking)

6. 2000/08/07 2000/08/11 2000/08/09 2000/08/13 QuikSCAT surface wind anomaly and relative vorticity. (mean: all available JJA wind) 1E-5 1/s Wang et al. 2010

7. The number of ITCZ breakdown occurrences (subjectively determined) Wang and Magnusdottir (2006) Frequency of occurrence is about 2 weeks on average

8. Motivations • To understand the basic dynamics of the ITCZ on synoptic timescale. • An efficient way to pool vorticity in regions in the tropics which represents the early stages of tropical cyclonegenesis. • Might contribute meridional moisture transport (need more data and study).

9. Mechanisms for breakdown • Westward Propagating Disturbances (WPDs), including easterly waves (e.g., Zehnder and Powell 1999) • Vortex roll-up mechanism through barotropic instability (Schubert and collaborators, Wang and Magnusdottir 2005)

10. Mechanisms for reformation • The meridional changes of Coriolis force due to Earth rotation (Kirtman and Schneider 2000, W. Chao 2000, Chao and Chen 2004) • Wind-evaporation feedback (Wang et al. 2010)

11. Vortex roll-up simulation

12. Potential Vorticity ~850 hPa PVU Barotropic instability (strong horizontal wind shear)

13. Characteristics of the ITCZ on synoptic timescale: • The breakdown can be self-induced • The new ITCZ re-forms within a few days  A strong mode of synoptic timescale lifecycle

14. 2-D FFT Power spectrum data Frequency (cpd) 1/5 time 1/30 3 2 1 1 2 Longitude Westward moving Eastward moving wavenumber Space-time spectral analysis

15. Dataset • ERA-40 reanalysis, May—Oct, 1979—2001 • 1.125 x 1.125 degree, • 6 hourly temporal resolution • Relative vorticity at 850 hPa

16. Northern Hemisphere (EQ-25N) Southern Hemisphere (25S-EQ) wavenumber wavenumber Magnusdottir and Wang 2008

17. 5N—20N, 180—80W Variance of the filtered vorticity (0.02 X 10-10 s-2) Magnusdottir and Wang 2008

18. North Eastern Pacific Phase speed = 8 m/s Longwave approximation: Non-dispersive waves u = -2.6 m/s easterly u = 0 m/s Dispersion curve for equatorial Rossby waves (n = 1) Magnusdottir and Wang 2008

19. ITCZ composite (lag-regression between the raw vorticity data and the ITCZ index) Magnusdottir and Wang 2008

20. North Eastern Pacific he= 30m, 70 m, 150m Equivalent depth: the depth after you convert the depth of a wave in a multi-layer system to that in a one-layer system Magnusdottir and Wang 2008

21. Equivalent depth • Wheeler and Kiladis (1999), the same method, 2.5X2.5 twice daily OLR  he = 15 ~ 50 m for equatorial convectively coupled waves (forced by wet convection). • Dry convection  he = 200 ~ 400 m (forcing) • What we found : 30~150 m

22. Dry case simulation using UCLA QTCM1 Prescribed heating: 6 K/day, 5 days. Red dash line. 850 hPa Wang et al. 2010

23. All-physics-on Wang et al. 2010

24. 2000/08/07 2000/08/11 2000/08/09 2000/08/13 QuikSCAT surface wind anomaly and relative vorticity. (mean: all available JJA wind) 1E-5 1/s Wang et al. 2010

25. Evap (color), sfc wind speed (contour) and wind vector K K Contour interval 5 m/s Wang et al. 2010

26. A disturbance can induce surface convergent flow on its southwest. • Surface wind induces surface evaporation (energy source). • wind-evaporation feedback The tail can be seen as a new ITCZ.  Suggests a possible mechanism for ITCZ reformation (timescale is within a couple of days).

27. Summary • The EP ITCZ goes through stages of formation, undulation, breakdown, and dissipation on synoptic timescale. • The EP ITCZ is a constructive feature. It is not a group of thermal-driven convective cells lined up in a row. • A non-dispersive wave packet propagates westward with a constant phase speed. • The ITCZ may reform via wind-evaporation feedback within a couple of days.

28. Future work(1) • the vertical structure of the tropical convection. • The interaction with other phenomena (on other timescales) such as MJO and ENSO.

29. MJO index defined by Maloney and Kiehl, 2002 Wang and Magnusdottir (2006)

30. Magnusdottir and Wang 2008 25-day running variance of the filtered vorticity El Nino El Nino El Nino La Nina El Nino El Nino La Nina El Nino La Nina La Nina

31. Future work(2) • The role of the ocean? • TMI SST (30-day high pass filtering): warmer SST before breakdown, cooler SST during/after breakdown. • Coupled with an ocean model with simple dynamics (ex. parameterized Ekman pumping) • Different meridional SST gradients • Coupled with daily SST (i.e., larger SST variation)