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Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December

Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December. Phil Klotzbach 33 rd Annual Climate Diagnostics and Prediction Workshop October 21, 2008. Gray et al. (1992). Problems with Earlier December Forecast Schemes.

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Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December

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  1. Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December Phil Klotzbach 33rd Annual Climate Diagnostics and Prediction Workshop October 21, 2008

  2. Gray et al. (1992)

  3. Problems with Earlier December Forecast Schemes • Predictor pool too large – attempting to hindcast too many tropical cyclone metrics • Too many predictors utilized in forecasting each parameter – likely overfit the data • - Tenuous links between individual predictors and tropical cyclone activity during the following year’s hurricane season (e.g., QBO) • - Prediction scheme developed over the full data period – no “independent” testing done

  4. Linear correlation between previous year’s QBO and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

  5. Objectives • - To develop a statistical scheme for forecasting NTC from 1 December that shows hindcast skill over the dependent dataset of 1950-1989 and “forecast skill” over an “independent” dataset from 1990-2007 • - To design this forecast utilizing NCEP/NCAR reanalysis • - To determine physical relationships between predictors utilized in the forecast and Atlantic Basin tropical cyclones

  6. Methodology - Correlate October-November NCEP/NCAR reanalysis fields of sea level pressure, sea surface temperature and 500 mb geopotential height with seasonal NTC over the period from 1950-1989 - Add predictors to the forecast using a stepwise regression technique - Only keep predictors that explain an additional 3% in NTC variance over the periods of 1950-1989, 1990-2007 and 1950-2007

  7. October-November Correlation between SST and the following year’s Atlantic NTC

  8. Predictor 1 Physics (+) October-November SST (55º-65ºN, 60º-10ºW) - Warm North Atlantic related to positive phase of the AMO - Strong auto-correlation between late fall North Atlantic SSTs and following summer/fall SSTs, both in the tropical and North Atlantic - Positive phase of the AMO is associated with a warm tropical Atlantic, low sea level pressures and reduced levels of vertical wind shear during the following August-October period

  9. Linear correlation between October-November SST in the North Atlantic (Predictor 1) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

  10. Predictor 2 Physics (+) November 500 mb Geopotential Height (67.5º-85ºN, 50ºW-10ºE) - Predictor correlates at -0.73 with November Arctic Oscillation (AO) index - Negative AO values imply weaker westerlies, more blocking, a weaker Azores high, weaker trades and warmer tropical Atlantic SSTs - Warm tropical Atlantic SSTs, weaker vertical wind shear and lower pressures are clearly evident during the August-October period

  11. Linear correlation between November 500 mb heights in the far North Atlantic (Predictor 2) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

  12. Predictor 3 Physics (+) November SLP (7.5º-22.5ºN, 175º-125ºW) - High pressure in the subtropical Northeast Pacific drives stronger trade winds encouraging mixing and upwelling – helping to initiate or prolong La Niña conditions - Strong trades also inhibit discharge of the warm pool in the western Pacific - La Niñais associated with anomalous upper-level easterlies during August – October, thereby reducing Atlantic basin vertical wind shear

  13. Linear correlation between November SLP in the subtropical Northeast Pacific (Predictor 3) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

  14. Individual Predictor Correlations with Seasonal NTC

  15. Stepwise Equation Regression Development with Seasonal NTC

  16. Ranking Methodology - Rank all statistical hindcast values from 1950-2007 - Assign observed NTC values based on the rankings assigned - Assign final hindcast NTC based on rank value - Hindcast NTC values restricted to range from 40 – 200

  17. Top 15 NTC Hindcasts 7 Major Hurricane Landfalls 1950-2007 Florida Peninsula and East Coast Bottom 15 NTC Hindcasts 2 Major Hurricane Landfalls 1950-2007 Florida Peninsula and East Coast

  18. Summary and Conclusions - Three predictors were selected that explained 54% of the variance in Net Tropical Cyclone activity over the 1950-2007 period - The scheme was developed over 1950-1989 and then tested over 1990-2007 - Utilizing these NTC hindcasts, one can issue United States landfall probability forecasts with skill well-exceeding climatology Full discussion of forecast model available in: Klotzbach, P. J., 2008: Refinements to Atlantic basin seasonal hurricane prediction from 1 December. J. Geophys. Res., 113, D17109, doi: 10.1029/2008JD010047

  19. Questions???

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