10:15 AM CDT Monday, September 17, 2012
With the coming El Nino, I have been wondering if Severe Weather is more likely to occur with an El Nino or La Nina. This prompted some digging over the last couple weeks, and here’s the conclusion I have come to: it’s relative. Here’s why:
One of the many misunderstandings/myths that circulate is that severe weather is more likely with an El Nino because it is the warm phase of the Pacific. When one thinks of Tornadoes, Hail, etc. you think of warm or at least warmish weather. With that in mind, you would naturally assume an El Nino – the warm phase of ENSO – would bring more severe weather; however, one of the most important ingredients for severe weather is a strong jet stream.
Typical El Nino & La Nina Winter Jet Stream Patterns
Differential heating is what creates our wind and weather patterns. The greater the difference in temperatures, the stronger the winds. Thus, the greater the amount of cold air to the north, the more intense the jet stream will be. This would then suggest that a La Nina would result in more severe weather because La Nina events usually result in a colder air mass across Canada. Thus, there is a more organized (single) Polar jet, rather than two weaker jet streams as with an El Nino. When this Polar jet stream dips south from time to time, it has a much greater impact than a more persistent (but usually weaker) southern jet. Take a look at this excerpt from the National Weather Service in Midland:
This is one of the best articles I have read regarding this topic. Another good article that is related to this discussion is on Dr. Ryan Maue’s site (http://policlimate.com/2011/09/03/weather-amnesia/). Be sure to scroll to the bottom of the page to see John Monteverdi’s commentary on the article. Because of the factors mentioned above, only one thing can be said about these episodes. That is that Significant Severe Weather and Strong Tornadoes (EF-2+) have a greater tendency to occur during a La Nina than an El Nino. Once again, this is due to a stronger jet stream which adds more “spin” to the atmosphere. Other than that, each ENSO episode (La Nina or El Nino) is influenced by factors such as the teleconnection patterns (AO, NAO, etc.) and the Madden-Julian Oscillation (MJO). These three phenomena heavily make up our weather pattern that may or may not be conducive for Severe Weather across the Southern Plains.
For more information on this topic, check out the following websites:
Meteorologist Ryan Vaughn’s Weather Blog: http://ryansmorningblog.blogspot.com/2010/02/severe-weather-and-el-nino.html
SPC/NWS Study By Meteorologist Joseph T. Schaefer & Frank B. Tatom: http://www.spc.noaa.gov/publications/schaefer/el_nino.htm
Be sure to share your thoughts on this topic below!