|Florida ENSO Education Info|
El Nino - Southern Oscillation (ENSO) and Florida Dry Season Tornadoes
NWS Melbourne, Florida
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.
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.
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.
|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|
Seconds to minutes
|Hundreds of feet|
Hagemeyer, B. C., and G. K. Schmocker, 1991b: Characteristics of east central Florida tornado environments. Weather and Forecasting, 6, 499-514.
_____, 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.