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El Nino and La Nina Effects on Tropical Cyclones. LT Bruce W. Ford June 9, 2000 Master’s Thesis Presentation. The Mechanisms. Advisor: Tom Murphree Second Reader: Pat Harr. Motivation. Clarify role of EN and LN events on western Pacific TCs Explore mechanisms behind TC effects

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El Nino and La Nina Effects on Tropical Cyclones


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    1. El Nino and La Nina Effects on Tropical Cyclones LT Bruce W. Ford June 9, 2000 Master’s Thesis Presentation The Mechanisms Advisor: Tom Murphree Second Reader: Pat Harr 1

    2. Motivation • Clarify role of EN and LN events on western Pacific TCs • Explore mechanisms behind TC effects • Explore impacts on Atlantic TCs • Test to see if our results apply to a single event (1999) 2

    3. Hypotheses • Equatorial Pacific heating anomalies lead to alterations in western Pacific circulations, and thereby to altered TC numbers, formation sites, intensities, and tracks. • EN and LN events aid in creating anomalous wave trains which extend into the Atlantic, thereby altering Atlantic TC activity. 3

    4. Study Methodology Past Studies This Study • Considers numbers, formation sites, intensities, and tracks • Analyses large-scale dynamics • Examines both EN and LN events • Covers 1949-1999 • Explores teleconnections between basins • Focused on overall numbers of TCs • Mostly statistical • Focused on EN events • Conducted too early to include events of the 90’s • Did not consider links to other basins 4

    5. Data • NCEP/NCAR Reanalysis Data • Monthly fields, Jan 1948-present • Covers global heating and circulation fields, subject to reanalysis errors • 2.5o latitude x 2.5o longitude grid (144x73 points) • JTWC Best Track Data • 1536 storm files in dataset, 1945-1999 • Covers western North Pacific TCs, but limited east of dateline • NHC Best Track Data • 571 storm files in dataset, 1945-1999 • Covers North Atlantic TCs 5

    6. Methods • Event identification: Multivariate ENSO index (MEI) • Incorporates SLP, U and V surface winds, SST, Tair, OLR • Composites used: • 9 strongest EN and LN events • All events since 1949 EN and LN events • Anomalies calculated using 1968-1996 mean 6

    7. Number of Western Pacific TC Reports Number of Best Track Reports Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 7

    8. Average Number of Western Pacific TCs Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 8

    9. Average Number of Strong Western Pacific TCs Strong -  80 knots (41 ms-1) maximum intensity Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 9

    10. El Niño Events La Niña Events Average Number of Weak Western Pacific TCs Weak - < 80 knots (41 ms-1) maximum intensity Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Climo 10

    11. August TC Formation Sites El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear, U200-U850 La Niña formations 11

    12. 30N H Eq H 30S 180E 30N L Eq L 30S 180E Circulation Response to Equatorial Heating, Z200 Rossby-Kelvin Wave Response El Niño La Niña Tropospheric cooling Tropospheric warming 12

    13. Tracks Schematic Western Pacific Vertical Shear, August La Niña U200 Climatology El Niño 20oN 150oE U850 -4 0 4 ms-1 U200 15oN 165oE U850 -4 0 4 ms-1 13

    14. Tracks September TC Formation Sites El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 14

    15. October TC Formation Sites El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 15

    16. November TC Formation Sites El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 16

    17. September TC Tracks El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 17

    18. October TC Tracks El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 18

    19. November TC Tracks El Niño La Niña El Niño formations 200 hPa geopotential height anomaly Vertical shear La Niña formations 19

    20. Westernmost Longitudes September October 65% greater chance of LN landfall November 285% greater chance of LN landfall El Niño TCs La Niña TCs No EN landfalls 20

    21. SST Anomalies, August – November Climo El Niño La Niña 21

    22. OLR Anomalies, August – November Climo climo El Niño La Niña 22

    23. Z200 Anomalies, August – November Climo climo El Niño La Niña 23

    24. Shear Anomalies, August – November Climo climo El Niño La Niña Atlantic 24

    25. El Niño / La Niña Effects on Western N. Pacific TCs El Niño La Niña 25

    26. EN and LN Effects on WestPac TCs 26

    27. Mechanisms - Pacific  SST in equatorial Pacific  Convective heating in equatorial Pacific  Circulation in tropical and subtropical Pacific responses) (Tropical Rossby-Kelvin and extratropical Rossby wave  Vertical shear in tropical and subtropical western Pacific  TC activity: formation sites, numbers, intensities, tracks Wave Train 27

    28. El Niño Tropospheric Circulation Anomalies August-November 28

    29. HypothesizedMechanisms - Atlantic  SST in global tropics  Convective heating in global tropics  Circulation in global tropics and subtropics extratropical Rossby wave responses) and (Tropical Rossby-Kelvin  Vertical shear in tropical and subtropical North Atlantic  TC activity: formation sites, numbers, intensities, tracks 29

    30. El Niño Events La Niña Events 1999 Numbers of Atlantic TC Reports Number of Best Track Reports Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Climo 30

    31. Average Number of Atlantic TCs Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 31

    32. Average Number of Strong Atlantic TCs Strong -  80 knots (41 ms-1) maximum intensity Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 32

    33. Average Number of Weak Atlantic TCs Weak - < 80 knots (41 ms-1) maximum intensity Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak El Niño Events La Niña Events Climo 33

    34. Summary • Western North Pacific TC numbers, formation sites, intensities, and tracks are effected by equatorial Pacific circulation anomalies brought about by EN and LN events. • SST changes do not seem to be reason for more (less) intense storms in EN (LN). • Extratropical wave trains alter upper level flow which modifies Atlantic vertical shear and TC activity. • Atlantic alteration mechanisms are dynamically similar to those in the Pacific. 34

    35. Future Research • Examine year following event peaks • Conduct in-depth study of Atlantic(formations, tracks, etc.) • Develop indices to monitor and forecast EN and LN impacts. • Apply these results to improve medium and long-range TC forecasting (Systematic Approach) • Conduct modeling studies of EN and LN wave trains and impacts on TCs 35

    36. Questions/Discussion 36

    37. Number of Best Track Reports Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Back 37

    38. Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Back 38

    39. Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Back 39

    40. Number of TCs Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Back 40

    41. Intensity (knots) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak 41

    42. SST Climatology (1968-1996) SST Anomalies 42

    43. OLR Climatology (1968-1996) OLR Anomalies 43

    44. SLP Climatology (1968-1996) 44

    45. 200 hPa Zonal Wind Climatology (1968-1996) 45

    46. 850 hPa Zonal Wind Climatology (1968-1996) 46

    47. Vertical Wind Shear Climatology (1968-1996) Shear Anomalies 47

    48. 200 hPa Geopotential Height Climatology (1968-1996) Z200 Anomalies 48

    49. 850 hPa Geopotential Height Climatology (1968-1996) 49

    50. Number of Best Track Reports Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec peak peak Back 50