Tuesday, 19 July 2011
Salon B (Asheville Renaissance)
El Niño and La Niña conditions occur when 5-month running means of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N5°S, 120°170°W) exceed or drop 0.4°C for 6 months or more. Data for years 1980 through 2009 were filed into an El Niño, La Niña or Normal category by examining the SSTs from NOAA's Climate Prediction Center (CPC). For each category, corresponding tornado data (CPC) for the United States were compared to each ENSO phase to identify disparities in frequency and geographic location. ENSO phase changes from Normal to El Niño/La Niña resulted in corresponding shifts in tornadic activity both spatially and temporally in the Central United States during the April-July time frame. For insight as to why this occurred, reanalysis fields of 500mb height anomalies from NOAA NCEP-DOE (Reanalysis 2) were examined to determine variances from the normal upstream synoptic patterns over the Eastern Pacific and Western United States. These analyses revealed a distinct 500mb height fall over of the southern and central Rockies and a separate height enhancement over the Pacific Northwest during El Niño years. The supportive upper-level dynamics of this pattern along with favorable low-level shear are conducive for tornadic activity east of the Rockies and correlates well with actual CPC observations of tornadoes in this region during El Niño years.
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