There have been a number of excellent posts on the family outbreak of tornadoes in the South last week. These include, for example,
I want to add a perspective to the discussion. First, as presented in my post
“Little difference was found in total tornado numbers between El Niño and La Niña events. However, significant differences were found in the number of violent tornadoes, and in large number tornado outbreaks. La Niña event years were found to have longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes.”
What occurs in La Niña springs in the United States is
- above average directional and wind speed vertical differences (helicity)
- above average convective available potential energy (CAPE).
Both helicity and CAPE are higher in La Niña springs because of the location of the polar jet stream further south of its normal position this time of the year. As defined by the National Weather Service the term helicity
“[a] property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2. “
With a more southerly polar jet stream, helicity values are larger in La Niña springs such as we are having this year.
The National Weather Service defines CAPE as
Convective Available Potential Energy [CAPE]. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)”
With a more southerly polar jet stream (and resulting colder air aloft), CAPE values are larger in La Niña springs such as we are having this year.
As we wrote in
Knowles, J.B., and R.A. Pielke Sr., 2005: The Southern Oscillation and its effect on tornadic activity in the United States. Atmospheric Science Paper No. 755, Colorado State University, Fort Collins, CO 80523, 15 pp. (Originally prepared in 1993, published as a Atmospheric Science Paper in March 2005).
“Colder than normal temperatures in the western US/Canada along with warmer than normal temperatures in the southern United States during La Niña events would act to strengthen the interactions between warm and cold air in the midwest (Barnston et al., 1991). There would be an increase in the number of days favorable for tornadic development. This would act to increase the number of violent tornadoes that occur during the late spring-early summer. Large multiple tornado outbreaks are more likely for the same reason.”
We have modeled the formation of tornadoes, as reported in several of our papers which documents the important role of helicity and CAPE in their formation and intensity; e.g.
Finley, C.A., W.R. Cotton, and R.A. Pielke, 2001: Numerical simulation of tornadogenesis in a high-precipitation supercell: Part I: Storm evolution and transition into a bow echo. J. Atmos. Sci., 58, 1597-1629.
Pielke, R.A., J. Eastman, L.D. Grasso, J. Knowles, M. Nicholls, R.L. Walko, and X. Zeng, 1995: Atmospheric vortices. In: Fluid Vortices, S. Green, Editor, Kluwer Academic Publishers, The Netherlands, 617-650.