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TSING-CHANG CHEN AND SHIH-YU WANG

Study on how monsoon circulation impacts tropical cyclones, analyzing 1979-2004 data with MJO modulation included.

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TSING-CHANG CHEN AND SHIH-YU WANG

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  1. Impact of the Intraseasonal Variability of the Western North Pacific Large-Scale Circulation on Tropical Cyclone Tracks TSING-CHANG CHEN AND SHIH-YU WANG Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa MING-CHENG YEN Department of Atmospheric Science, National Central University, Chung-Li, Taiwan ADAM J. CLARK Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa (Manuscript received 15 July 2008, in final form 25 October 2008)

  2. 1. Introduction • 1. The summer monsoon circulation in East and Southeast Asia is composed of • a. monsoon trough stretching from Indochian to the Philippine Sea • b. the western North Pacific anticyclone • c. the mei-yu-baiu rainband • 2. The life cycle of the East and Southeast Asian monsoon is formed by • a. mei-yu-baiu rainband in early summer • b. the western Pacific anticyclone in midsummer • c. the TC activity in late summer • 3. Because of the northward progression of these circulation elements, there is a phase lag of a monsoon life cycle between the southern and northern regions of East Asia. • 4. This phase lag is a distinct opposite-phase intraseasonal oscillation of monsoon circulation cells. • 5. This oscillation is caused by the northward migration of the 30-60-day mode monsoon trough-ridge system.

  3. 1. Introduction • 6. The characteristic of TC tracks in the western North Pacific are modulated by the northward progression of the monsoon circulation elements in Southeast Asia. • 7. Harr and Elsberry (1991) observed: • TC tracks alternate between clusters of straight and recurving paths with an intraseasonal time scale. • 8. Harr and Elsberry (1995a,b) found: • Transitions occurred between the recurrent patterns associated with active monsoon trough and the inactive monsoon condition. • 9. Lander (1996) observed: • When the axis of the monsoon trough is oriented along its climatological NW-SE direction, TCs tend to move northwestward.

  4. 1. Introduction • Analysis period • June-October from 1979 to 2004 • Data used in this study • Circulation analysis: • -- ECMWF Re-Analysis (1979~2002) • -- NCEP-DOE Global Reanalysis 2 (2003~2004) • Tropical cyclone • -- TC track from JTWC

  5. 2. MJO modulation on tropical cyclone tracks: 1996 Monsoon Index 30-60-day second-order Butterworth bandpass filter Monsoon westerlies↑ Trade easterlies↓ FIG. 1.Longitude–time (x–t) diagram of the 850-hPa zonal wind (U) at 15°N superimposed with the longitudinal positions of tropical cyclones (typhoon symbols) at 0000 UTC formed within the latitudinal zone of 0°–25°N. The corresponding typhoon symbols are colored in red (blue) when the propagation direction of the tropical cyclones is smaller than 45° away from the west in the second quadrant (45° away from the east in the first quadrant). The 30–60-day bandpass-filtered 850-hPa zonal wind at (15° N, 115°E) (U) (red line) is used as a monsoon index. The red-dashed oblong around 10 September indicates that the 850-hPa environmental flow embedded by tropical cyclones appearing within the time period indicated by this oblong will be illustrated in Fig. 3.

  6. 2. MJO modulation on tropical cyclone tracks: 1996 Straight-moving TCs occur with the inactive monsoon trough. Recurving TCs occur with the active monsoon trough. FIG. 1.Longitude–time (x–t) diagram of the 850-hPa zonal wind (U) at 15°N superimposed with the longitudinal positions of tropical cyclones (typhoon symbols) at 0000 UTC formed within the latitudinal zone of 0°–25°N. The corresponding typhoon symbols are colored in red (blue) when the propagation direction of the tropical cyclones is smaller than 45° away from the west in the second quadrant (45° away from the east in the first quadrant). The 30–60-day bandpass-filtered 850-hPa zonal wind at (15° N, 115°E) (U) (red line) is used as a monsoon index. The red-dashed oblong around 10 September indicates that the 850-hPa environmental flow embedded by tropical cyclones appearing within the time period indicated by this oblong will be illustrated in Fig. 3.

  7. 2. MJO modulation on tropical cyclone tracks: 1996 Trade easterlies along the southern periphery of the subtropical anticyclone The northern side of the monsoon trough became stronger FIG. 2. The 850-hPa streamlines and zonal wind (shadings) at 1200 UTC on (a) 4, (b) 6, (c) 8, (d) 12, (e) 14, and (f) 16 Sep 1996 superimposed with four tropical cyclones: Sally (1), TD 24 (2), Tom (3), and Violet (4). Tropical cyclones with sustained wind speeds>(≦)64 kt are marked by solid (open) typhoon symbols, while straight (recurving) tropical cyclone tracks are indicated by blue (red) solid lines.

  8. 2. MJO modulation on tropical cyclone tracks: 1996 E-W NW-SE SW-NE

  9. 2. MJO modulation on tropical cyclone tracks: 1996 The Monsoon westerlies became stronger The easterlies north of the monsoon trough appeared weaker The western subtropical anticyclone retreated eastward The typhoon deflected northward and recurved within the reverse-oriented monsoon trough

  10. 2. MJO modulation on tropical cyclone tracks: 1996 FIG. 3. Following Fig. 1, time series of 30–60-day bandpass-filtered 850-hPa zonal winds ( ) at (15°N, 115°E) superimposed with typhoons during their midlife within the latitudinal zone of 0°–25°N, for 26 typhoon seasons (1979–2004). Straight-moving (recurving) tropical cyclones are marked by blue (red) typhoon symbols. The same phases of the (850 hPa) index of every typhoon season are aligned. The year of each typhoon season is indicated at the top of every other time series of the (850 hPa) index.

  11. 3. Composite analysis a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks FIG. 4. Composite streamline charts superimposed with zonal wind speeds at (a) 300 and (b) 850 hPa, averaged when the midlife positions of recurving typhoons are located within the region (10°–30°N, 115°–155°E). (c),(d) As in (a),(b) except for straight-moving tropical cyclones. Differences are between composite streamlines of recurving and straight-moving tropical cyclones at (e) 300 and (f) 850 hPa. The mean trajectories of recurving and straight-moving typhoons indicated as thick red and blue dotted arrow lines are added in (a),(b) and (c),(d), respectively. The oceanic trough line is marked by a red-dashed line. The cyclonic (anticyclonic) centers are denoted by red Ls (blue Hs). midlife location within 10°–30°N, 115°–155°E (95% of 426 tropical cyclones during 1979~2004) 95% of 426 TCs during 1979~2004

  12. 3. Composite analysis The upper-level trough separating the Tibetan anticyclone and the western subtropical Pacific anticyclone deflects TCs northward a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks

  13. 3. Composite analysis The merger of Tibetan anticyclone and subtropical Pacific anticyclone steers TCs approximately straight west. a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks

  14. 3. Composite analysis Monsoon westerlies are stronger for the recurving track than for the straight stack Trade easterlies are stronger for the straight track than for the recurving track a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks

  15. 3. Composite analysis The monsoon trough and subtropical anticyclone are farther north for the recurving track than for the straight track a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks

  16. 3. Composite analysis a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks A short wave train emerges along the northwestern rim of the northwest Pacific, and forms a barotropic for vertically uniform structure.

  17. 3. Composite analysis a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks For the recurving tracks, the anomalous westerlies between Taiwan and Philippines hinder the northwestward propagation of TCs, while the anomalous cyclone circulation around Taiwan deflects TCs northward

  18. 3. Composite analysis a. Composite flow patterns associated with recurving and straight-moving tropical cyclone tracks The spatial structure of short wave train resembled that of North Pacific-Japan Oscillation (PJO) in response to the summer SST anomalies FIG. 5. As in Figs. 4e and f, but for a domain covering the entire North Pacific. Anomalous cyclonic (anticyclonic) circulations associated with the cross-Pacific short wave train are indicated by light pink (blue) shaded areas.

  19. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks SUMMER of 1996 Both monsoon westerlies and trade easterlies have a very coherent 30-60-day oscillation in intensity FIG. 6. Latitude–time (y–t) diagrams of (a) 850-hPa total zonal winds (u), and (b) the 30–60-day bandpass-filtered 850-hPa zonal winds ( ) at 135°E. When their directions changed to the sector between northwest and north, these typhoons are marked by red typhoon symbols, otherwise by dark blue typhoon symbols.

  20. 3. Composite analysis mean westerlies mean easterlies mean streamlines Maximum RMS of easterlies and westerlies mean zonal wind uE mean zonal wind uW Two indices introduced, ûWand ûE at the midlife of each TC located in this box b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks 26 TCs seasons (July-October) FIG. 7. (a) The climatological 850-hPa westerly (yellow–orange shadings) and easterly (blue shadings) zonal winds superimposed on streamlines for the tropical cyclone season (July–October) and a 5°×5 ° square encircling the maximum rms value of the 30–60-day bandpass-filtered westerlies (red hatched) at (10 ° N, 115 ° E) and easterlies (dark blue hatched) at (15 ° N, 165 ° E).

  21. 3. Composite analysis averaged value for all pairs of uW and uE indices for recurving TCs averaged value for all pairs of uW and uE indices for straight-moving TCs oblong covers the 95% confidence level separation line of bivariate normal random variables for two density functions b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks

  22. 3. Composite analysis Most recurving TCs cluster in [uW>0,uE<0] Most straight-moving TCs cluster in [uW<0,uE<0] ûWand ûE are more appropriate to depict the life cycle of the Southeast-East Asian monsoon than uW and uE Most recurving TCs cluster in [ûW>0, ûE>0] Most straight-moving TCs cluster in [ûW<0, ûE<0] b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks

  23. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks • The procedure of modified Knutson and Weickmann (1987) composite analysis: • 1) When the amplitude of the ûW(850hPa) index at any time during the typhoon season is larger than its average value by at least one standard deviation , the monsoon life cycle (with one trough and one ridge) is selected. If only one-half of the cycle reaches the one standard deviation criterion, this half cycle is used. • 2) The entire cycle of the ûW(850hPa) index is evenly divided into eight phases. Each phase covers a 5-day period centered on the third day. • 3) Tropical Cyclones with their midlife occurring within the 5-day period in any one of the eight phases are included in the composite chart of this phase.

  24. 3. Composite analysis The largest number of recurving TCs occurs when the ûW(850hPa) index reaches its maximum value The largest number of straight moving TCs occurs when the ûW(850hPa) index reaches its minimum value b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks FIG. 8. (a) Schematic phase diagram of the Southeast Asian monsoon depicted in terms of the û(850 hPa) index at (10°N, 115°E), and (b) the occurrence frequency of tropical cyclones with two different tracks for every monsoon phase.

  25. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks 5-day averaged composite of the 30-60-day filtered streamline FIG. 9. Composite 30–60-day bandpass-filtered 850-hPa streamlines superimposed with the filtered westerly (easterly) zonal wind anomalies in orange (blue) for eight phases of the û(850 hPa) index defined in Fig. 10a, and midlife positions of tropical cyclones (marked by dots). Trajectories of recurving and straight-moving tropical cyclones are depicted by red and blue lines, respectively. Numbers of these two types of tropical cyclones are given in the upper-left corner of each panel, with recurving tropical cyclones indicated by the red number at the top and straight-moving cyclones by the blue number at the bottom.

  26. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks The northward migrations of the 30-60-day anomalous cyclonic-anticyclonic circulations behave in a manner similar to that inferred from Fig. 6.

  27. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks The population contrast between recurving and straight-moving TCs reflects the impact of meridional locations of monsoon trough-ridge pattern, as well as the intensity and meridional locations of monsoon westerlies and trade easterlies Evidently, the characteristics of tropical cyclone tracks are affected by the global 30-60-day oscillation through its modulation on the Southeast-East Asian monsoon circulation

  28. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks FIG. 10. As in Fig. 9, but for 300 hPa.

  29. 3. Composite analysis FIG. 10. (300hPa) Fig. 9. (850hPa) b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks The barotropic nature of the composite 30-60-day monsoon mode by the vertically uniform anomalous circulations east of Taiwan TC tracks affected by not only upper-level flows but also low-level flows!

  30. 3. Composite analysis b. Composite 30-60-day flow patterns and associated with tropical cyclone tracks More recurving TCs develop east of 140°E during strong monsoons More straight moving TCs develop west of 140°E during weak monsoons FIG. 11. As in Fig. 9, but the midlife positions of tropical cyclones are replaced by their genesis locations. For simplicity, trajectories of tropical cyclones are not shown. Genesis locations of recurving and straight-moving tropical cyclones are marked by red and blue dots, respectively.

  31. 4. Dynamic diagnosis for changes in tropical cyclone track a. Vorticity budget of the monsoon flow FIG. 12. Various dynamic processes included in the 850-hPa vorticity budget of the monsoon flow during the two extreme phases of the û(850 hPa) index: (a) ζt , (b) -V ‧▽(f +ζ), (c) -(f +ζ)▽‧V, (d)-V‧▽ζ, and (e)-vβ for the maximum û(850 hPa) phase (left column); (f)–(j) as in (a)–(e) but for the minimum û(850 hPa) phase (right column).

  32. 4. Dynamic diagnosis for changes in tropical cyclone track A deepened monsoon trough with strong monsoon southwesterlies a. Vorticity budget of the monsoon flow

  33. 4. Dynamic diagnosis for changes in tropical cyclone track Two trough lines emanate from the monsoon trough a. Vorticity budget of the monsoon flow

  34. 4. Dynamic diagnosis for changes in tropical cyclone track Monsoon circulation reaches its breakpoint as established by the subtropical anticyclone a. Vorticity budget of the monsoon flow

  35. 4. Dynamic diagnosis for changes in tropical cyclone track The E-W-oriented positive and negative ζt zone are juxtaposed with the monsoon trough line A northward propagation of the monsoon trough a. Vorticity budget of the monsoon flow

  36. 4. Dynamic diagnosis for changes in tropical cyclone track Inactive circulation A newly formed monsoon trough is juxtaposed with weak positive and negative E-W-oriented ζt zone A northward propagation of the monsoon trough a. Vorticity budget of the monsoon flow

  37. 4. Dynamic diagnosis for changes in tropical cyclone track Relative vorticity advection should be positive ahead of a trough line and negative behind the trough line + Planetary vorticity advection is negative at the eastern end of the 2 monsoon troughs a. Vorticity budget of the monsoon flow

  38. 4. Dynamic diagnosis for changes in tropical cyclone track a. Vorticity budget of the monsoon flow The VA process is generally negative along the monsoon flow around the trough with centers behind the trough lines

  39. 4. Dynamic diagnosis for changes in tropical cyclone track a. Vorticity budget of the monsoon flow The VA process is generally negative along the monsoon flow around the trough with centers behind the trough lines The VS process is generally positive along the monsoon flow around the trough with centers behind the trough lines

  40. 4. Dynamic diagnosis for changes in tropical cyclone track The E-W-oriented positive and negative ζt zone are juxtaposed with the monsoon trough line The VA process is generally negative along the monsoon flow around the trough with centers behind the trough lines The VS process is generally positive along the monsoon flow around the trough with centers behind the trough lines a. Vorticity budget of the monsoon flow

  41. 4. Dynamic diagnosis for changes in tropical cyclone track The VA and VS processes are active only along the southern and northern rims of the subtropical anticyclone a. Vorticity budget of the monsoon flow

  42. 4. Dynamic diagnosis for changes in tropical cyclone track VA and VS along the monsoon trough are still applicable to form a meridional juxtaposition of positive and negative ζtbands The VA and VS processes are active only along the southern and northern rims of the subtropical anticyclone a. Vorticity budget of the monsoon flow

  43. 4. Dynamic diagnosis for changes in tropical cyclone track b. Interaction of a tropical cyclone with the monsoon flow average midlife position of TCs at (17.5°N,135°E) 850hPa FIG. 13. Horizontal wind structure of an idealized tropical cyclone adapted from Carr and Elsberry (1995): (a) radial profile of tangential wind and (b) spatial distribution of horizontal wind vectors.

  44. 4. Dynamic diagnosis for changes in tropical cyclone track + b. Interaction of a tropical cyclone with the monsoon flow FIG. 14. Various dynamic processes included in the 850-hPa vorticity budget of the monsoon flows embedded by an idealized cyclone (depicted in Fig. 12): (a)ζt, (b) -V‧▽(f +ζ), (c) -(f +ζ)▽‧V, (d) -V‧▽ζ, and (e)-vβ for the maximum û(850 hPa) phase (left column ); (f)–(j) as in (a)–(e) but for the minimum û (850 hPa) phase (right column). The scale of the vorticity tendency generated by different dynamic processes is indicated in the upper-right corner of (a). The possible moving direction of a tropical cyclone, as indicated by a red arrow in each panel, is determined by the direction from the center of the idealized tropical cyclone toward the maximum ζt(850 hPa).

  45. 4. Dynamic diagnosis for changes in tropical cyclone track Positive center appears north of the trough Negative center appears south of the trough Propagation the TC north-northwestward b. Interaction of a tropical cyclone with the monsoon flow

  46. 4. Dynamic diagnosis for changes in tropical cyclone track Planetary vorticity Generate the E-W differentiation of the vorticity tendency Propagate the TC westward b. Interaction of a tropical cyclone with the monsoon flow

  47. 4. Dynamic diagnosis for changes in tropical cyclone track Relative vorticity advection the basic characteristics is not changed by the presence of TC, except in magnitude Fig. 14(d) Fig. 12(d) b. Interaction of a tropical cyclone with the monsoon flow

  48. 4. Dynamic diagnosis for changes in tropical cyclone track The VA process exhibits a spatial structure similar to that during the same phase of monsoon circulation without a TC Fig. 14(b) Fig. 12(b) b. Interaction of a tropical cyclone with the monsoon flow

  49. 4. Dynamic diagnosis for changes in tropical cyclone track The positive VS may reduce the negative ζt produced by VA b. Interaction of a tropical cyclone with the monsoon flow

  50. 4. Dynamic diagnosis for changes in tropical cyclone track Positive center appears west of the TC Negative center appears east of the TC Facilitates the westward propagation of the TC b. Interaction of a tropical cyclone with the monsoon flow

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