1. Seasonal Influences upon and Long- Term Trends in the Length of the Atlantic Hurricane Season Juliana M. Karloski and Clark Evans Atmospheric Science Program, University of Wisconsin- Milwaukee karloski@uwm.edu // evans36@uwm.edu

2. Background Kossin (2008, Geophys. Res. Lett.), their Figure 4 season end season start becomin g later in season becoming earlier in season 90% confidence interval

3. Motivating Questions If the period of record extends into the present, is the Atlantic hurricane season getting longer? When considering the entire basin, is the Atlantic hurricane season getting longer? How does the large-scale environment vary between early- and late- starting and ending Atlantic hurricane seasons?

4. Methods: Length of Season Tropical cyclone formation data obtained for 1979-2013 from the NHC HURDAT2 database. –Unlikely that tropical cyclones (N = 433) were systematically missed in this period (Landsea 2007). –No attempt made to account for possible variability in operational classification practices in this period. Quantile regression is used to quantify trends in TC formation date (5 th -95 th percentiles, Δ = 5%). –Relatively insensitive to individual outliers; considers topology of entire data set and not just its tails.

5. Results: Length of Season There exists no statistically-significant trend in extended MDR season length when the record extends into the present.

6. Results: Length of Season Kossin (2008, Geophys. Res. Lett.), their Figure 1 5% 95% 7 Aug.16 Oct. 2001-2007 5 th Pct. Dates 2001-2007 95 th Pct. Dates

7. Results: Length of Season There exists no statistically-significant trend in full basin Atlantic hurricane season length, particularly for 1979- 2013.

8. Methods: Large-Scale Conditions Linear regression between 10 th (90 th ) percentile date and June (November) monthly-mean fields. –Atmospheric: ERA-Interim 850 hPa relative vorticity, 850-300 hPa vertical wind shear magnitude, 600 hPa relative humidity, 500 hPa geopotential height –Oceanic: NOAA Extended Reconstructed SST v3b A Student’s t test is used to determine whether linear regressions are statistically-significant to 90, 95, and 99% confidence.

9. Example Regression Analysis November Monthly-Mean SST vs. 90 th Percentile Formation Date Contoured: 1979-2013 monthly mean field Shaded: linear regression slope, multiplied by the standard deviation of the monthly-mean field Hatched: slope is stat. significant to ≥ 90%, 95%, 99% confidence Decrease field by one standard deviation, lengthen season by shaded # of days. Increase field by one standard deviation, lengthen season by shaded # of days.

10. Pre-10 th Percentile Genesis Events Pathway classification follows McTaggart-Cowan et al. (2013, Mon. Wea. Rev.)

11. June – 10 th Percentile Date SST 600 hPa RH 500 hPa Φ 850 hPa ζ

12. Post-90 th Percentile Genesis Events Pathway classification follows McTaggart-Cowan et al. (2013, Mon. Wea. Rev.)

13. November – 90 th Percentile Date SST 600 hPa RH 500 hPa Φ 850 hPa ζ

14. Summary and Conclusions The Atlantic hurricane season is not becoming longer, independent of the region considered. Early- and late-forming TCs occur when atmospheric and oceanic conditions supportive of their respective genesis pathways exist. Only a portion of the variance in season length is captured, and there exist multiple pathways to particularly short and long hurricane seasons.