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

Gray et al. (1992)

- 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 Problems with Earlier December Forecast Schemes

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)

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

- 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 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 , and Methodology

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

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

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)

Predictor 2 Physics (+) November 500 mb Geopotential Height (67.5º-85ºN, 50ºW-10ºE) - Predictor correlates at 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

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)

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 Niña is associated with anomalous upper-level easterlies during August – October, thereby reducing Atlantic basin vertical wind shear

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)

Predictor (r) (40 Years) (r) (18 Years) (r) (58 Years) Individual Predictor Correlations with Seasonal NTC

Predictor (r 2 ) (40 Years) (r 2 ) (18 Years) (r 2 ) (58 Years) , , 2, Stepwise Equation Regression Development with Seasonal NTC

Ranking Methodology - Rank all statistical hindcast values from 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

Top 15 NTC Hindcasts Bottom 15 NTC Hindcasts Major Hurricane Landfalls 2 Major Hurricane Landfalls Florida Peninsula and East Coast

- Three predictors were selected that explained 54% of the variance in Net Tropical Cyclone activity over the period - The scheme was developed over and then tested over Utilizing these NTC hindcasts, one can issue United States landfall probability forecasts with skill well-exceeding climatology Summary and Conclusions 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: /2008JD010047

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