ENSO and U.S. severe convective storm activity Michael K. Tippett Columbia University Willis Research Network Autumn meeting Nov 1, 2017
Summary What is ENSO? Unusual warming or cooling of tropical Pacific sea surface temperature, usually peaking in winter, with global climate impacts especially in the tropics What is the impact of ENSO on U.S. severe convective storm activity? Combining storm reports with environmental indices shows enhanced spring SCS activity during La Nina conditions, (opposite in Florida) Can ENSO be used to predict U.S. spring SCS activity? Yes, the Dec-Feb ENSO state is a good indicator for Mar-May SCS activity Dec-Feb ENSO state can be predicted as early as October. Monthly forecasts of SCS environmental indices Physics-based forecasts Future work ENSO Modulation of intensity, outbreaks
What is ENSO? Unusual warming or cooling of tropical Pacific sea surface temperature, usually peaking in winter, which impacts global climate especially in the tropics
ENSO El Nino- Southern Oscillation Unusual warming or cooling of tropical Pacific sea surface temperature, usually peaking in winter La Nina El Nino www.climate.gov
ENSO state summarized by NINO 3.4 index Events last 9-12 months and occur about every 3-5 years 5 N-5 S, 170-120 W
Current ENSO state and forecasts Forecast skill varies with lead time and target period NMME Skill NOAA/CPC, IRI, Barnston et al. 2017
ENSO www.climate.gov El Nino-Southern Oscillation Jet stream shifts are a source of predictability for forecasts of seasonal averages of U.S. precipitation and temperature La Nina 2015-16 2017-18 El Nino
What is the impact of ENSO on U.S. severe convective storm activity? Combining storm reports with environmental indices gives a clearer picture: Enhanced spring SCS activity during La Nina conditions, except in Florida
Tornado and hail reports from NOAA/SPC Hail Tornadoes
Tornado and hail occurrence is associated with favorable environments 1. Instability, updraft potential, CAPE 2. Vertical wind shear, 0-6km shear, Storm Relative Helicity Tornado Environment Index (TEI) Hail Environment Index (HEI) These indices can be Observed (weather balloons) Estimated (reanalysis = observations + physics) Forecast (physics-based models) Next week Next month Next century
More SCS activity during Mar-May in most regions during La Nina Caveat: Need to know Mar- May ENSO state Allen, Tippett & Sobel, 2015
Can ENSO be used to predict U.S. spring SCS activity? Yes, the Dec-Feb ENSO state is a good indicator and can be predicted as early as October.
March-May tornado/hail reports/environment blend conditional on Dec-Feb NINO 3.4 value Opposite signal In Florida Gunturi and Tippett, Willis Re report
Shifts in Mar-May SCS activity and ENSO Shifts in MAM SCS activity conditional on DJF ENSO state Calendar opportunity: DJF NINO 3.4 is well predicted as early as October Substantial variability in addition to ENSO ENSO shifts odds of above or below normal activity Tippett & Lepore https://www.climate.gov/news-features/blogs/enso/enso-and-tornadoes
Probability of below-normal, normal and above normal SCS activity El Nino Weak ENSO Moderate ENSO La Nina Lepore, Tippett & Allen, 2017
Skill of probabilistic forecasts of U.S. Mar-May SCS activity Forecast based on observed Dec-Feb NINO 3.4 Available early March Forecast based on forecast Dec-Feb NINO 3.4 Available early October Brier skill score for probability of above and below normal activity Lepore, Tippett & Allen, 2017
Comparison with ENSO and Atlantic hurricane Atlantic hurricane More active during La Nina But ASO ENSO difficult to predict more than a few months in advance Forecast info available in summer Activity is not landfall U.S. SCS More active during La Nina DJF ENSO predictable many months in advance Forecast info in early winter Does the shared ENSO connection mean U.S. SCS correlates with Atlantic hurricane?
Monthly forecasts of SCS environmental indices Based on NOAA s Climate Forecast System
WRN forecasts of TEI and HEI produced at the start of May 2017 Preliminary tornado and hail reports
Future work Impact of ENSO on outbreaks and intensity What can environments tell us about intensity? Stronger tornadoes Large hail Changes in outbreak statistics Seasonality Regional features Detailed skill assessment of environmental index forecasts