| El Nino - Southern Oscillation (ENSO) and Florida Dry Season Tornadoes Bart Hagemeyer Note: The sections on ENSO and Florida weather and ENSO and Storminess should be reviewed prior to reading this section. Strong and violent tornadoes are rare in Florida because their formation requires many atmospheric ingredients to come together at just the right time and place to form. Hagemeyer (2000) developed an index of "Storminess" and found it to be a reasonable proxy for tornadoes and severe weather in Florida during the dry season. Severe weather, particularly the occurrence of strong and violent tornadoes (F2 and greater), in the Florida dry season have shown a striking relationship to extreme phases of ENSO in recent decades. A graph of F2 and greater dry season tornadoes since 1950 below shows three distinct peaks associated with strong El Ninos. The tornado database is not as accurate prior to about 1980 and the number of F2 and greater tornadoes in the 50s, 60s and 70s in Storm Data is highly suspect. However, since 1980 the two extreme EL Nino events of 1982-83 and 1997-98 have produced the most significant tornadoes. |
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.
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.
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).
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 how the 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 a viable 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:
Experimental
Forecast of Florida Region Dry Season $5 Million Tornado Events
Experimental Forecast
of Florida Region Dry Season F2 Tornado Days
Experimental
Forecast of Florida Region Dry Season Tornado Damage
There are at least two significant reasons why trying to predict tornadoes directly from the ENSO signal is problematic: 1) because the 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
Current Dry Season Storminess Monitoring Graphs Past Seasonal Storminess Forecasts 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. Copyright 1991 by AMS. _____, 1997: Peninsular Florida tornado outbreaks . Wea. Forecasting, 12, 399-427. Copyright 1997 by AMS _____, 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. Copyright 1999 by AMS. _____, 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., .. Copyright 2000 by AMS. Other On-line Research on ENSO and Severe Weather Schaefer, J. T. and F. B. Tatum, 1999: THE RELATIONSHIP BETWEEN EL NINO, LA NINA, AND UNITED STATES TORNADO ACTIVITY . Preprints, 19th Conference on Severe Storms. Minneapolis, MN, Amer. Meteor. Soc. Florida region seasonal
forecast development: Bart Hagemeyer.
Updated 8/1/01 bch |