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

www.climate.gov

El Nino

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 NOAA/CPC, IRI, Barnston et al. 2017

NMME Skill

La Nina

El Nino

www.climate.gov ENSO El Nino-Southern Oscillation Jet stream shifts are a source of predictability for forecasts of seasonal averages of U.S. precipitation and temperature 2015-16 2017-18

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

Gunturi and Tippett, Willis Re report

Opposite signal In Florida

• 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

Shifts in Mar-May SCS activity and ENSO

Probability of below-normal, normal and above normal SCS activity

Moderate ENSO

Weak ENSO

La Nina

El Nino

Lepore, Tippett & Allen, 2017

Brier skill score for probability of above and below normal 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

Skill of probabilistic forecasts of U.S. Mar-May SCS 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