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This study assesses the long-term changes in tropical cyclones (TCs) formations and intensities in the Western North Pacific (WNP) from 1970-2006. The analysis includes the examination of intraseasonal to multidecadal changes and the relationships between TC changes and known large-scale environmental factors (LSEFs), including global warming (GW) hypothesis. The study also highlights the limitations of prior studies and presents regression models to assess the sensitivity of TC formations and accumulated cyclone energy to LSEF variations.
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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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
