Western Pacific Tropical Cyclogenesis Precursors

1. Western Pacific Tropical Cyclogenesis Precursors Ph.D. Prospectus Carl J. Schreck, III

2. Traditional Precursors Monsoon trough Monsoon gyres Easterly waves Tropical cyclone dispersion Equatorial Waves The Madden–Julian oscillation (MJO) Equatorial Rossby (ER) waves Mixed Rossby–gravity (MRG) waves Kelvin waves Tropical Cyclogenesis Precursors

3. Traditional Precursors Monsoon trough Monsoon gyres Easterly waves Tropical cyclone dispersion Equatorial Waves The Madden–Julian oscillation (MJO) Equatorial Rossby (ER) waves Mixed Rossby–gravity (MRG) waves Kelvin waves Tropical Cyclogenesis Precursors

4. Monsoon Trough • Roughly 70% of western Pacific TCs form in this climatologically favorable region (e.g., Gray 1968) • Shear line provides cyclonic vorticity • Confluence zone can induce wave growth through accumulation (Sobel & Bretherton 1999) Briegel & Frank (1997)

5. Monsoon Gyre • Monsoon trough is sometimes replaced with a large gyre (Lander 1994) • Multiple TCs can form in the southeast quadrant of the gyre • Gyre itself can become a large TC Lander (1994)

6. Easterly waves • Sometimes called “Tropical Depression (TD)-type disturbances” • Westward or northwestward propagating synoptic scale disturbances • Do not correspond to any linear equatorial wave solution • Attributions range from 8% (Lee et al. 2008) to 71% (Fu et al. 2007) of TC formations Ritchie & Holland (1999)

7. Rossby Dispersion from a Preexisting Tropical Cyclone • TC-like vortex in a shallow-water model can radiate Rossby waves to the southeast • Krouse et al. (2008) found that the TC must be moving westward relative to the background flow • Stronger TCs may be more likely to generate wave trains (Fu et al. 2007) Krouse et al. (2008)

8. Traditional Precursors Monsoon trough Monsoon gyres Easterly waves Tropical cyclone dispersion Equatorial Waves The Madden–Julian oscillation (MJO) Equatorial Rossby (ER) waves Mixed Rossby–gravity (MRG) waves Kelvin waves Tropical Cyclogenesis Precursors

9. TRMM Precipitation Spectrum • Includes both symmetric and antisymmetric signals (Roundy & Frank 2004) • Peaks in the spectrum tend to correspond with shallow-water equatorial wave solutions (Wheeler & Kiladis 1999) • Boxes indicate filter bands (Frank & Roundy 2006) • MRG waves and TD-type disturbance have similar periods, but wavelength is longer for MRG waves (Takayabu & Nitta 1993) Wavenumber Eastward Westward Total Spectrum No red background removed

10. The MJO • TCs develop in region of enhanced convection, convergence, and cyclonic vorticity (e.g., Liebmann et al. 1994; Frank & Roundy 2006) • May also provide favorable easterly vertical wind shear (Frank & Roundy 2006) and mid-level relative humidity (Camargo et al. 2009) • Higher frequency modes also grow within the active MJO (Maloney & Hartmann 2001) Frank & Roundy (2006)

11. ER waves Kiladis et al. (2009) Molinari et al. (2007) • TCs develop in region of cyclonic vorticity and enhanced convection (e.g., Frank & Roundy 2006; Molinari et al. 2007) • May favorably influence vertical wind shear (Frank & Roundy 2006) • Molinari et al. (2007) showed that a monsoon gyre was the first low in an ER wave packet

12. MRG waves • MRG waves may turn to the northwest and develop TD-type structures (e.g., Liebmann & Hendon 1990; Takayabu & Nitta 1993) • TCs develop in region of cyclonic vorticity and enhanced convection (e.g., Dickinson & Molinari 2002; Frank & Roundy 2006) Takayabu & Nitta (1993) Kiladis et al. (2009) Dickinson & Molinari (2002)

13. Kelvin waves Kiladis et al. (2009) Kiladis et al. (2009) • Less important than other waves for cyclogenesis (Frank & Roundy 2006) • TCs may develop in association with cyclonic vorticity in observed Kelvin waves (Bessafi & Wheeler 2006)

14. Primary Research Questions • What percentage of western Pacific TC formations can be attributed to the MJO, equatorial waves, and TD-type disturbances? • How do traditional and equatorial wave precursors interact to promote cyclogenesis?

15. Equatorial wave attributions • Spatially average filtered anomalies around genesis location • If this average exceeds a threshold, then attribute the genesis to that wave • In this example, the MRG/TD band averaged over 5°× 5° produces a 30 mm day-1 anomaly • How large of an area should anomalies be averaged over? • What threshold should be used for attributions?

16. Sensitivity of attributions • Percentage of storms attributed to each wave decreases as the threshold or averaging box increases • MRG/TD and ER waves are consistently associated with the most TC formations

17. Composite TC • All TCs 1998–2006, Eq – 25N • Assumed to represent TC, but could indicate waves signal • Anomalies averaged 5°× 5° around composite genesis: • MJO: 0.63 • Kelvin: 0.44 • ER: 1.59 • MRG/TD: 2.54

18. Composite TC • All TCs 1998–2006, Eq – 25N • Assumed to represent TC, but could indicate waves signal • Anomalies averaged 5°× 5° around composite genesis: • MJO: 1.89 • Kelvin: 1.32 • ER: 4.77 • MRG/TD: 7.62

19. Selection of Threshold and Averaging Area • Threshold: 7.62 mm day-1 • Maximum TC contribution to any filter band • Greater than 95% of filtered anomalies at all grid points • Box size: 5°× 5° • Smaller box would increase the TC contribution • Larger box may overly smooth wave anomalies

20. MRG/TD 57% Kelvin 4% MJO 2% ER 34% • Hurricane symbols indicate genesis locations of TCs attributed to a given band • Variance of filtered anomalies is contoured None 27% May–November 1998–2006

21. Typhoon Lingling • Example of genesis with an MRG wave • Also attributable to ER wave, but this signal is less clear • Other waves are inactive at genesis time

22. Typhoon Lingling • Example of genesis with an MRG wave • Also attributable to ER wave, but this signal is less clear • Other waves are inactive at genesis time

38. Proposed improvements to equatorial wave attributions • Separate MRG and TD-type disturbances using the idealized MRG dispersion relation • Compare the TRMM results with OLR and other datasets • Reduce noise by filtering with Roundy and Schreck’s (2009) extended empirical orthogonal function (EEOF) method • Incorporate other influences on genesis, such as low-level vorticity, mid-level relative humidity, and vertical wind shear • Examine how attributions vary in different parts of the basin or with ENSO

39. Primary Research Questions • What percentage of western Pacific TC formations can be attributed to the MJO, equatorial waves, and TD-type disturbances? • How do traditional and equatorial wave precursors interact to promote cyclogenesis?

40. Objective equatorial wave attributions MJO ER waves Kelvin waves MRG waves TD-type disturbances Subjective monsoon attributions Shear line Confluence region Monsoon gyres Fu et al. (2007) Synoptic wave trains Easterly waves Tropical cyclone energy dispersion Tropical Cyclogenesis Precursors

41. Issues to be addressed by comparing attributions • How do MRG waves, TD-type disturbances, TC dispersion, and the monsoon trough interact to form northwestward propagating wave trains? • How often are monsoon gyres and ER waves interrelated? • How frequently does genesis result from equatorial waves growing within the monsoon confluence region via wave accumulation? • What are the roles of the MJO and ER waves in modulating the monsoon trough? • What fraction of TCs develop in situ with no wave precursor? Why do they form?