El Nino - Southern Oscillation (ENSO) and Florida Dry Season Tornadoes

El Nino - Southern Oscillation (ENSO) and Florida Dry Season Tornadoes
Bart Hagemeyer
NWS Melbourne, Florida

Note: The sections on ENSO and Florida weather and ENSO and Storminess should be reviewed prior to reading this section.

The image below compares the reported tornadoes from January through April and mean position of the jet stream for the two extreme El Nino’s of 1997-98 and 1982-83. The similarity in the jet stream pattern and resulting tornadoes is striking. These two years represent the two most active tornado seasons in Florida history.

A number of research projects have verified the importance of the jet stream, and in particular the passage of jet streaks of higher wind embedded within the mean jet stream, in providing an environment favorable for strong and violent tornadoes in the Florida dry season. While the passage of an extra-tropical cyclone and associated jet streak generally sets the stage for the potential for severe weather, it is the strength of the wind field that was found to have the highest correlation with tornado intensity or F-scale (Hagemeyer, B. C., and D. A. Matney, 1993: Relationship of twenty upper air indices to central Florida tornado outbreaks. Preprints 13th Conference on Weather Analysis and Forecasting. Vienna, VA, Amer. Meteor. Soc., 574-577).

Very strong La Nina’s are characterized by much lower than normal tornado activity. The image below compares the tornado reports and mean jet stream positions for the strong El Nino of 1982-83 with the strong La Nina of 1988-89. The 1982-83 dry season had a record number of strong and violent tornadoes, the 1988-89 dry season tied several other dry seasons for the least, with no strong and violent tornadoes reported.

Case Study - Deadliest Tornado Outbreak in Florida History

The following satellite and radar animations for the February 22-23rd 1998 central Florida tornado outbreak (the deadliest in state history) aptly illustrate howthe approach of a strong southerly jet streak aided in the development of tornadic supercells.

Water Vapor Satellite of approaching squall line with jet stream analysis
Animated Loop (high bandwidth recommended)

IR Satellite Animated Loop (high bandwidth recommended )

NEXRAD Doppler Radar (red triangles are tornado vortex signatures (TVS) from algorithm output Animated Loop (high bandwidth recommended)

Lightning as a Proxy for Severe Weather

As lightning climatologies improve and we gain longer time series we may find that lightning makes a useful proxy for severe weather activity on a seasonal basis. Goodman et al. found that for the 1997-98 ENSO event the most significant year-to-year changes in lightning frequency worldwide occurred along the Gulf Coast and within the Gulf of Mexico basin during the Northern Hemisphere winter. Note the striking difference in lightning activity between the strong El Nino during the 1997-98 winter and the following strong La Nina during the winter of 1998-99. (Goodman et al. 2000 "The 1997-98 El Nino Event and Related Wintertime Lightning Variations in the Southeastern United States" Geophysical Research Letters, Vol. 27, No. 4 541-544.)

Lightning Days in December, January, and February

Tornado Predictability from the ENSO Signal

With such striking differences in tornado activity between strong El Nino and strong La Nina years the obvious question is - can we predict tornadoes from the ENSO signal? Forecasting an individual tornado or even tornado outbreak based solely on the ENSO signal is problematic due largely to the time-space scales of the various phenomena involved. Indeed, one of the scientific concerns with the strong El Nino event of 1997-98 was that individual significant tornado events were often ascribed to El Nino in the popular press.

The so-called "EL Nino spawned tornado" is a misnomer and that is why the idea of an experimental forecast of tornadoes based on the ENSO signal is highly controversial. However, there is significant dry season to dry season variability in tornado activity over Florida. The number of dry season F2 tornado days, the number of $5 million dollar tornado events, and the amount of dry season tornado damage ($millions) plotted from 1980 through the 2000 dry season illustrate this variability. If one then considers "seasonal" measures there is considerable scientific basis for aviable physical relationship to work with. The following forecasts done in the same manner as the storminess and rainfall forecasts show some skill, especially during extreme phases of ENSO, but are not statistically significant:

There are at least two significant reasons why trying to predict tornadoes directly from the ENSO signal is problematic: 1) becausethe tornado database comes only from reported tornadoes, it is thought that many more tornadoes occur than are reported, especially from sparsely populated areas. In addition, not all tornado and high wind reports are thoroughly investigated, and not all tornadoes and high wind damage can be surveyed to determine the true nature of the damage. Tornado damage and strength assessment are subjective, and indeed the amount of damage depends on what areas the tornadoes hit, not necessarily the character of tornadoes themselves. These problems do not affect the storminess database which is objectively obtained. 2) The physical process of tornadogenesis in the dry season generally involves the spinup of a tornado from a thunderstorm mesocyclone. The fact that a tornado may or may not spinup is determined by processes on a very small space-time scale and can theoretically have nothing to do with sea surface temperatures in the tropical Pacific Ocean. Indeed, why some mesocyclones produce tornadoes and some do not is not well-understood. Anecdotal evidence from NEXRAD doppler radar observations at Melbourne over the past 10 years indicates that most mesocyclones do not, in fact, produce tornadoes. On the other hand, the synoptic environment that is favorable for tornadogenesis and severe weather is well known and understood. This is why storminess is perhaps a better measure of seasonal tornado potential, and a more useful proxy for assessing societal impact of ENSO.

Characteristic Time and Space Scales

Phenomena	Time Scale	Space Scale
Tropical Pacific Sea Temperature	Months to Seasons	Thousands of miles
Mean Storm Track and Jet Stream	Months	Thousands of miles
ET Cyclone Warm Sector	Days to weeks	Hundreds to thousands of miles
Jet Streak	Days	Hundreds to thousands of miles
Mesoscale Convective Complex in warm sector	6 to 12 hours to a day or two	Hundreds of miles
Supercell Thunderstorm	Several hours	Tens of miles
Mesocyclone	Minutes to hours	Five to 10 miles
Tornado	Seconds to minutes	Hundreds of feet

Online Severe Weather References from NWS Melbourne

NWS Melbourne On-Line Research Page

Hagemeyer, B. C., 1991: A lower-tropospheric thermodynamic climatology for March through September: some implications for thunderstorm forecasting. Wea. Forecasting, 6, 254-270.

Hagemeyer, B. C., and G. K. Schmocker, 1991b: Characteristics of east central Florida tornado environments. Weather and Forecasting, 6, 499-514.

_____, 1997: Peninsular Florida tornado outbreaks . Wea. Forecasting, 12, 399-427.

_____, 1998: Significant extratropical tornado occurrences in Florida during strong El Nino and strong La Nina events. Preprints, 19th Conference on Severe Storms. Minneapolis, MN, Amer. Meteor. Soc., 412-415. Copyright 1998 by AMS.

_____, 1999: El Nino and significant tropical and hybrid cyclone tornado events in Florida. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology. Dallas, TX, Amer. Meteor. Soc., 415-418.

_____, 2000: Development of an index of storminess as a proxy for seasonal severe storms forecasting in Florida. Preprints, 20th Conference on Severe Storms. Orlando, FL, Amer. Meteor. Soc., .

Hagemeyer, B.C, L. A. Jordan, A. L. Moses, S. M. Spratt, and D. F. Van Dyke, 2010: Climatological, meteorological, and societal implications for the large number of fatalities from central Florida Dry Season tornadoes during El Niño. Preprints, 22nd Conference on Climate Variability and Change, First Symposium on Planetary Atmospheres, First Environment and Health Symposium, 20th Conference on Probability and Statistics in Atmospheric Science, 24th Conference on Hydrology, 18th Conference on Applied Climatology, First Conference on Weather, Climate, and the New Energy Economy. Amer. Meteor. Soc., Atlanta, GA, CD-ROM J4.5.

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Florida region seasonal forecast development: Bart Hagemeyer.

Updated 8/10/10