NWC REU 2000
May 22 - July 28



Tornado Frequency in the Southern Plains as Related to SSTs in the Pacific Ocean

Jesse Sparks and Mike Richman



Forty years of tornado data are analyzed for their relation to Pacific sea surface temperature anomalies (SSTA) to detect linkages between the Pacific and Southern Plains tornado days. The Southern Plains is divided into eight regions that group counties with similar tornado day behavior. These regions serve as a springboard for further analyses. Visual inspection of the tropical and north Pacific Ocean suggest that above normal tornado days are associated with unique regions. These regions are identified as rectangular boxes in the western Pacific, central Pacific and eastern Pacific in mid-latitudes. Additionally, two areas in the ENSO region of the tropical Pacific are tested since they are hypothesized to be related to tornado behavior. Two tornado day regions, one in west Texas, the Oklahoma panhandle and western Kansas and the second in west central Texas extending into central Oklahoma are subject to an intensive analysis. This included identification of March through May years with above normal tornado days that are subsequently formed into a time series for compositing.


Results of compositing above normal years minus below normal years for each spring month on SSTA, 500 hPa geopotential heights and .210 sigma total vorticity are consistent in identification of ridges overlaying warm SSTAs in the mid-latitudes and troughs overlaying cold SSTAs. The height patterns frequently form Rossby wave trains over the US. The positive anomalous vorticity is consistently oriented along the base and eastern side of the aforementioned troughs and always terminate over or immediately adjacent to the tornado regions. This suggests that storm tracks are displaced from their normal positions by the anomalous SSTAs through a feedback mechanism with the overlaying atmosphere.

Paper available upon request.