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El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms

El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms. Master’s Thesis Presentation by LT Paula Hildebrand March 15, 2001 Advisor: Prof. Tom Murphree Second Reader: Prof. Pat Harr. Motivations for This Study.

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El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms

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  1. El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms Master’s Thesis Presentation by LT Paula Hildebrand March 15, 2001 Advisor: Prof. Tom Murphree Second Reader: Prof. Pat Harr

  2. Motivations for This Study • Clarify impacts of EN and LN events on North Atlantic TCs • Explore mechanisms for EN/LN impacts in the North Atlantic • Follow on to LT Bruce Ford’s study of EN and LN impacts on WPAC TCs

  3. Global Scale Background - Ford (2000)

  4. Hypotheses EN/LN events occur in the tropical Pacific Alterations of global tropical circulation Changes in Tropical Easterly Jet (TEJ) Development of anomalous extratropical wave trains Changes in North Atlantic shear and steering flow Changes in North Atlantic TC activity: - numbers and intensities - formation sites - tracks

  5. Past Studies Focused on overall number of TCs Mainly statistical analyses Used localized indices to select EN/LN years Used individual years to identify EN/LN impacts Analyzed conditions in and near North Atlantic This Study Considers numbers, intensities, formation sites, and tracks Analyzes large-scale dynamics Uses a Pacific-wide index during TC season to select EN/LN years Uses 10 year composites to represent EN and LN events Explores teleconnections from Pacific via TEJ and extratropical wave trains Background

  6. MEI : a Pacific-wide index Years Selected Selection based on MEI average during North Atlantic TC season (Jun-Nov) Long Term Mean (LTM): 1970-1999 Selection of EN and LN Years

  7. Data and Methods NHC best track and NCEP reanalysis fields • EN/LN differences in TC activity: • - formation numbers and sites • - TC tracks and landfall EN/LN differences in basin scale factors: - vertical shear, SST, OLR, UL & LL vorticity EN/LN differences in global scale factors: - TEJ, wave trains, UL westerlies, trades

  8. LTM EN LN Ave TC intensity (kts) per month LTM EN LN Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TC Numbers and Intensities Ave # of TCs >25kts per month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

  9. 1 - Gulf of Mexico (GOMEX) 2 - Subtropical Atlantic (SATL) 3 - Western Caribbean (WCAR) 4 - Central Tropical Atlantic (TATL) 5 - West Africa (WAF) 1 2 3 4 5 Regions With Differences in TC Formations

  10. TC Formations & Vertical Shear in JAS El Niño La Niña El Niño formations Vertical shear, U200-U850 La Niña formations

  11. El Niño formations Sea Surface Temperature Anomalies La Niña formations TC Formations & SST Anomaly in JAS El Niño La Niña

  12. El Niño formations Outgoing Longwave Radiation Anomalies La Niña formations TC Formations & OLR Anomaly in JAS El Niño La Niña

  13. El Niño formations 200 hPa geopotential height anomaly Vertical shear, U200-U850 La Niña formations Formations, Shear, & UL Height Anomaly in JAS El Niño La Niña

  14. Global Scale Factors in JAS Anomalous 200 hPa Heights and Winds L EN H H H L L LN

  15. TC Formations & Vertical Shear in SON El Niño La Niña El Niño formations Vertical shear, U200-U850 La Niña formations

  16. El Niño formations Sea Surface Temperature Anomalies La Niña formations TC Formations & SST Anomaly in SON El Niño La Niña

  17. El Niño formations Outgoing Longwave Radiation Anomalies La Niña formations TC Formations & OLR Anomaly in SON El Niño La Niña

  18. El Niño formations 200 hPa geopotential height anomaly Vertical shear, U200-U850 La Niña formations Formations, Shear, & UL Height Anomaly in SON El Niño La Niña

  19. Global Scale Factors in SON Anomalous 200 hPa Heights and Winds H L H L L L L H H H EN L H L H H H H L L L LN

  20. Comparison of TC Tracks During EN & LN • Actual tracks • Westernmost longitude • Formation in relation to ridge axis

  21. TC Tracks & 200 hPa Heights in JAS El Niño La Niña El Niño formations 200 hPa geopotential height anomaly TC tracks La Niña formations

  22. TC Tracks & 200 hPa Heights in SON El Niño La Niña El Niño formations 200 hPa geopotential height anomaly TC tracks La Niña formations

  23. SON JAS EN: 8 LN: 19 EN: 12 LN: 26 El Niño positions La Niña positions Hurricane Westernmost Longitudes Would you want to be in the coastal regions during a LN?

  24. TC Formations & 500 hPa Heights in JAS El Niño La Niña 500 hPa heights TC formations

  25. TC Formations & 500 hPa Heights in SON El Niño La Niña 500 hPa heights TC formations

  26. Anomalous Steering Flow in SON El Niño La Niña 500 hPa height anomalies 500 hPa wind anomalies

  27. Track and Steering Flow Differences • height anomalies due to extratropical wave trains • anomalous wind • anomalous steering EN 500 hPa height anomalies 500 hPa wind anomalies

  28. LN EN Basin Factors that Explain Formation Differences Formations: Differences and Explanations • Differences: • More tropical formations during LN • More subtropical formations during EN • Explanations: • Vertical shear due to anomalous TEJ and extratropical wave trains • SST anomaly • OLR anomaly

  29. LN LN LN Tracks: Differences and Explanations • Differences: • More westward tracks during LN • More landfalls during LN • Explanation: • TC formation sites • Wind anomalies due to anomalous extratropical wave trains

  30. Global Climate Synoptic TC Questions or Comments… and don’t forget toTHINK OUT OF YOUR BOX!

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