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Tom Murphree Dept of Meteorology Naval Postgraduate School Monterey, CA 93943-5114 Bruce Ford

Mechanisms for El Nino and La Nina Induced Anomalies in Tropical Cyclone Formation and Motion in the Northwest Pacific. Tom Murphree Dept of Meteorology Naval Postgraduate School Monterey, CA 93943-5114 Bruce Ford Clear Science, Inc. Jacksonville, FL 32211

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Tom Murphree Dept of Meteorology Naval Postgraduate School Monterey, CA 93943-5114 Bruce Ford

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  1. Mechanisms for El Nino and La Nina Induced Anomalies in Tropical Cyclone Formation and Motion in the Northwest Pacific Tom Murphree Dept of Meteorology Naval Postgraduate School Monterey, CA 93943-5114 Bruce Ford Clear Science, Inc. Jacksonville, FL 32211 27th Hurricanes and Tropical Meteorology Conference 28 April 2006 Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  2. Background • A number of prior studies have investigated relationships between NW Pacific TC activity and El Nino (EN) and La Nina (LN), including: Chan 1985, Lander 1994, Chen 1998, Chan 2000, Ford 2000, Wang and Chan 2002, Chia and Ropelewski 2002, Wu 2004. • Most studies focused on statistical relationships. Relatively little attention to mechanisms. • Our objective: characterize large scale mechanisms affecting TC activity that can be attributed to EN and LN. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  3. Data • NCEP/NCAR monthly atmospheric reanalysis fields • SODA-POP v1.4.2 monthly ocean reanalysis fields • JTWC best track data • Multivariate ENSO Index (MEI) • Study period: 1970-2003 Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  4. Methods • Constructed composites of ten strongest EN and LN events during Aug-Nov of study period. • Identified characteristic anomalies in large scale factors affecting TC activity. • Related anomalies to corresponding TC formations, intensities, and tracks  favorable and unfavorable anomalies. • Focus: Aug-Nov (ASON) of TC season in which EN and LN events were intensifying or reaching their maximum intensities. • Emphasis for today: mechanisms affecting formations. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  5. EN Anomaly (J m-2 10-6) LN Anomaly (J m-2 10-6) Upper Ocean Thermal Energy, EN & LN, ASON, Upper 250 m • Positive (negative) UOTE anomalies suggest where TC formation might be • more (less) likely. But these indications are inconsistent with formation • differences shown in prior studies (e.g., Chan 2000, Ford 2000). • However, tropical atmo. circulation responds strongly to UOTE anomalies. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  6. 30N H Eq H 30S Central Pacific 30N L Eq L 30S Central Pacific upper level divergence upper level convergence tropospheric warming tropospheric cooling Upper Tropospheric Responses to Equatorial Warming and Cooling • Responses in form of quasi-stationaryRossby and Kelvin waves. • Low level responses opposite to upper level responses. cf. Matsuno (1966), Gill (1980). Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  7. EN LN ZAnomalies, EN and LN, ASON, 200 hPa • Arrows: anomalous wind direction. • Quasi-stationary Rossby-Kelvin wave responses seen throughout TC • season (Jun-Dec), with responses intensifying from summer to winter. • Similar to well-known responses in NH winter, but weaker and relatively • more pronounced over NW Pacific than in winter. • Low level response roughly opposite to upper level response. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  8. EN LN favorable unfavorable Relative VorticityAnomalies, EN and LN, ASON, 200 hPa • Upper level relative vorticity anomalies governed by Rossby-Kelvin wave • responses. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  9. EN LN favorable unfavorable Relative VorticityAnomalies, EN and LN, ASON, 850 hPa • Lower level relative vorticity anomalies governed by Rossby-Kelvin wave • responses. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  10. EN LN favorable unfavorable Wind DivergenceAnomalies, EN and LN, ASON, 200 hPa • Upper level divergence anomalies governed by Rossby-Kelvin wave • responses. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  11. EN LN favorable unfavorable Wind DivergenceAnomalies, EN and LN, ASON, 1000 hPa • Lower level divergence anomalies governed by Rossby-Kelvin wave • responses. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  12. EN LN favorable unfavorable Relative HumidityAnomalies, EN & LN, ASON, 850 hPa • Lower level humidity anomalies governed by Rossby-Kelvin wave • responses by way of upper and lower level circulation anomalies. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  13. EN LN favorable unfavorable OLRAnomalies, EN and LN, ASON • Convective anomalies governed by Rossby-Kelvin wave • responses by way of upper and lower level circulation anomalies. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  14. EN, U200 - U850 (m s-1) LN, U200 - U850 (m s-1) Vertical Shear, EN and LN, ASON Compared to long term mean: EN: tropical region of low shear extended to east subtropical region of low shear contracted to southwest LN: tropical region of low shear contracted to west subtropical region of low shear extended to northeast Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  15. EN LN Z200 Anomaly, Shear, and TC Formation, EN and LN, ASON • Shear: black contours. Z’200: color. Formation sites: black boxes. • EN-LN variations in shear driven by Rossby wave response. • In both tropical and subtropical regions: anomalous upper level easterly • (westerly) winds reduce (increase) shear. • Formation differences consistent with shear and prior circulation and • moisture differences governed by Rossby-Kelvin wave responses.. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  16. Tropics: 5N-15N LN LTM EN lower shear in LN m s-1 zero shear lower shear in EN Subtropics: 20N-30N In both regions, shear differences due mainly to upper level wind anomalies associated with Rossby-Kelvin wave response. m s-1 zero shear lower shear in LN 0E 60E 120E 180E 120W 60W 0W W PacificE Pacific Area Averaged Vertical Shear,EN & LN, ASON Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  17. EN LN Z200 Anomaly and Tracks, EN and LN, October • Track differences are affected by differences in formation sites and steering • flow, both of which are strongly governed by Rossby-Kelvin responses to EN • and LN. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  18. EN LN Circulation Anomalies, and Shear, Formations, and Tracks, EN and LN, Late Summer and Fall Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

  19. Conclusions • Major differences between EN and LN periods in NW Pacific TC formations, intensities, and tracks attributable to anomalies in large scale environmental factors. • Anomalies in large scale factors governed by quasi-stationary Rossby-Kelvin wave responses to EN and LN. • Wave responses driven by thermodynamic factors • (e.g., UOTE). • Similar conclusions for interdecadal variation in NW Pacific TC activity. • Beware of relying solely on thermodynamic reasoning to estimate impacts of thermal variations (e.g., EN, LN, global warming) on TC activity. Murphree & Ford, Tropical Met Conf, Apr06, murphree@nps.edu

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