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TC Activity in WNP, murphree@nps.edu, Oct08

Typhoon Saomai & Tropical Storm Bopha, Western North Pacific, 08Aug06. Long Term Changes in Tropical Cyclone Activity in the Western North Pacific Tom Murphree and David Meyer Naval Postgraduate School (NPS) murphree@nps.edu and dwmeyer@nps.edu.

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TC Activity in WNP, murphree@nps.edu, Oct08

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  1. Typhoon Saomai & Tropical Storm Bopha, Western North Pacific, 08Aug06 Long Term Changes in Tropical Cyclone Activity in the Western North Pacific Tom Murphree and David Meyer Naval Postgraduate School (NPS) murphree@nps.edu and dwmeyer@nps.edu 33rd Annual Climate Diagnostics and Prediction Workshop 21 October 2008 1 TC Activity in WNP, murphree@nps.edu, Oct08

  2. Long Term Changes in WNP Tropical Cyclone Activity • Overview • Assessment of changes in tropical cyclone (TC) formations and • intensities in western North Pacific during 1970-2006: • Intraseasonal to multidecadal changes • Long term net changes • Relationships between TC changes and known large scale environmental factors (LSEFs) • Relationships between TC changes and global warming (GW) Hypothesis: Assessments of GW impacts on TCs need to account for all LSEFs that affect TCs, and do so at time and space scales at which LSEFs affect TCs. 2 2 TC Activity in WNP, murphree@nps.edu, Oct08

  3. Long Term Changes in WNP Tropical Cyclone Activity • Prior Studies • LSEFs strongly affect TC formations (e.g., McBride 1995): • SST • Vertical shear • Vertical velocity • Absolute vorticity • Relative humidity • ENLN impacts on TC activity (e.g., Chan 2000, Ford 2000): • Impacts on formation sites, intensities, tracks • GW impacts on TCs so far (e.g., Chan and Liu 2004,Emanuel 2005, Webster and Holland 2005, Klotzbach 2006, Landsea 2007): • Little or no increase in TC numbers • Increase in TC intensities via SST increases • Impacts from interannual-multidecadal variations dominate • TC data inadequate to determine GW impacts • Limitations of prior studies: • Coarse resolution: basin and seasonal averages • Focus on one LSEF: SST • Uncertainties in TC data • Distinction and attribution: GW impacts vs. other impacts 3 3 TC Activity in WNP, murphree@nps.edu, Oct08

  4. Long Term Changes in WNP Tropical Cyclone Activity • Data • TC activity: JTWC best track data (no adjustments) • LSEFs: NCEP reanalysis 1 and 2 • Period: 1970-2006 • Main TC season: May-December • Region: western North Pacific (WNP): 0-40° N, 115–185°E • Main development region (MDR): 0-20° N, 115–165°E • Temporal resolution: one week • Spatial resolution: 5°x5° • Number of TCs: 989 4 4 TC Activity in WNP, murphree@nps.edu, Oct08

  5. TC Numbers, WNP 1950-2006 Global Surface T Anomaly 1850-2007 1970 1970 From CRU/UEA Long Term Changes in WNP Tropical Cyclone Activity Selection of analysis period 5 TC Activity in WNP, murphree@nps.edu, Oct08

  6. Long Term Changes in WNP Tropical Cyclone Activity • Methods • Identify major temporal and spatial patterns of changes in: • TCs: formations and accumulated cyclone energy (ACE) • LSEFs • Develop regression models that relate TC formation probability and ACE to local LSEFs at weekly and 5x5 resolutions • Validate models using independent TC and LSEF data • Use models to assess sensitivity of TC formations and ACE to LSEF variations • Relate GW to LSEF changes, and, thus, to TC changes • By-product: Models for sub-seasonal forecasting of TC formations and general tropical convection 6 6 TC Activity in WNP, murphree@nps.edu, Oct08

  7. No. of TCs 0° N Long Term Changes in WNP Tropical Cyclone Activity Number of TC Formations, 1970-2006 Main development region (MDR): 0-20°N, 115-165°E 7 7 TC Activity in WNP, murphree@nps.edu, Oct08

  8. Annual ACE Annual TC Numbers TC Number ACE (kts2) Year Year Long Term Changes in WNP Tropical Cyclone Activity Changes in TC Numbers and ACE, 1970-2006 • Large interannual-multidecadal variations (linked to ENLN) • Long term net increases in TC formations and ACE Can changes in LSEFs explain these long term changes in TC activity? 8 8 TC Activity in WNP, murphree@nps.edu, Oct08

  9. SST (C) Year Long Term Changes in WNP Tropical Cyclone Activity LSEF Changes – SST in MDR 9 9 TC Activity in WNP, murphree@nps.edu, Oct08

  10. SST Change, JASO, 1970-2006 (°C) TC Unfavorable TC Favorable Long Term Changes in WNP Tropical Cyclone Activity LSEF Changes – SST • Overall SST increase in MDR (+0.4 °C) is favorable for increases in TC formations and intensities, and consistent with GW. • Some SST decreases in TC active areas of WNP. SST increases consistent with long term increases in TC numbers and intensities. 10 TC Activity in WNP, murphree@nps.edu, Oct08

  11. Vertical Velocity (-Pa/s) Year Long Term Changes in WNP Tropical Cyclone Activity LSEF Changes – Vertical Velocity in MDR 11 TC Activity in WNP, murphree@nps.edu, Oct08

  12. TC unfavorable TC favorable Long Term Changes in WNP Tropical Cyclone Activity LSEF Changes – Vertical Velocity Vertical Velocity Change, JASO, 1980-2006 (-Pa/s) • Overall decrease in MDR is unfavorable for TC formation and ACE increases. • Other LSEFs (shear, vorticity, RH) show no pronounced long term changes. Have vertical velocity decreases countered impacts of SST increases on TC formations? 12 12 TC Activity in WNP, murphree@nps.edu, Oct08

  13. Long Term Changes in WNP Tropical Cyclone Activity Regression Modeling of TC – LSEF Relationships 1. TC formation probability = log(p/(1-p)) = a + b·SST + c·VertVel - d·Shear – e·Shear2 + f·AbsVort + g·Equat 2. ACE = A + B·SST + C·VertVel + D·Shear - E·Shear2 + F·AbsVort + G·RH Sensitivity to LSEF Changes:Increase in each LSEF by one standard deviation leads to following changes in formation probability and ACE: • TC formation relatively sensitive (insensitive) to vertical velocity (SST). • ACE moderately sensitive to SST and relatively insensitive to vertical velocity. 13 13 TC Activity in WNP, murphree@nps.edu, Oct08

  14. Modeled TC formation probabilities, 01-07 Oct 2006 10% 25% 40% 55% 70% Verifying observations of TC formations from best track data Long Term Changes in WNP Tropical Cyclone Activity Validation of TC Formation Probability Model • Modeled TC formation probabilities for individual week at 5x5 degree resolution (calculated using independent LSEF data) • Developing sub-seasonal TC formation forecasts based on model 15 15 TC Activity in WNP, murphree@nps.edu, Oct08

  15. Long Term Changes in WNP Tropical Cyclone Activity • Summary • Shortcomings of TC and LSEF data sets are problematic. • TC formations and intensities have undergone long term increases since 1970. • SST and vertical velocity are the only LSEFs that experienced net long term changes in WNP during 1970-2006. • SST (vertical velocity) changes were favorable (unfavorable) for increases in TC formation and intensity. • Impacts of SSTs on TC formations may have been minimized by vertical velocity decreases. • GW may have contributed to increases in TC formations and ACE, by way of SST increases but mitigated by vertical velocity decreases. • But attribution is problematic, given: • Shortcomings of data sets • Large interannual to multidecadal variations • Uncertainties about LSEF-TC relationships and GW signatures • LSEFs affect both TC formations and intensities, and need to be considered at realistic time and space scales in assessing GW impacts. 16 16 TC Activity in WNP, murphree@nps.edu, Oct08

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