Storm Prediction Center
Norman, OK 73069
and Frank B. Tatom
Engineering Analysis Inc.
Huntsville, AL 35801
The presence of a warm/cold surface layer of water in the equatorial Pacific Ocean
Storm Prediction Center
Norman, OK 73069
and Frank B. Tatom
Engineering Analysis Inc.
Huntsville, AL 35801
The presence of a warm/cold surface layer of water in the equatorial Pacific Ocean has been shown to be related to various climatological patterns around the world. As a whole, this phenomenon is called ENSO (El Nino/Southern Oscillation). The warm phase of the ENSO is known as El Nino phase, while the cold phase is called La Nina. This study explores the possible existence of a relationship between tornadic activity over portions of the United States and the ENSO phase.
The Storm Prediction Center's archive of United States tornadoes from 1950 through 1996 was used. Among the parameters in the data set includes the date and time of all reported tornadoes, the state, latitude, and longitude of each touchdown and lift-up, and the Fujita scale intensity rating (Schaefer et al. 1986).
The Climate Prediction Center's use of the mean sea surface temperature (SST) anomaly over the area extending from 5oN to 5oS and 180oW to 150oW is used for determination of an El Nino/La Nina. The criterion for an El Nino (warm episode) is an anomaly of at least 1oC, while for a La Nina (cold episode) the threshold was relaxed to 0.8oC (Livezey, 1998).
In this paper, an El Nino/La Nina year is one in which the temperature criterion over the key area was met for three or more consecutive months that included at least one Northern Hemisphere cold season month (December, January, February). These data were then compared to the tornado record for the year, or "tornado season" following the warm/cold event (Table 1).
For example, 1950 and 1951 tornadoes are both considered as La Nina related because of the cold SST anomaly that was present from January 1950 through February 1961. Similarly, the warm SST anomaly from January 1969 through April 1969 made all 1969 tornadoes El Nino affected.
3. UNITED STATES TORNADOES
As a preliminary study, a plot of all reported tornadoes over the contiguous United States as a function of the ENSO phase, was constructed (Fig. 1). Changes in tornado reporting efficiency since 1950 have caused an almost linear increase of nearly 15 reports a year. This dominates any ENSO effect.
Before any comparisons can be made, this bias in the observations must be removed. While the number of tornadoes reported was greatest in 1992, the difference between the expected value (i.e., the least squares fit value) and the observed value was higher in 1973. In fact, using the delta as the standard of comparison drops the rank of 1992 to fifth. The average rank in terms of the delta in tornado count for the 10 El Nino years was 24.6. This compares to an average rank of 24.0 for the 8 La Nina years, and 23.7 for the 29 neutral years.
The Kruskal-Wallis H test (Walpole and Meyers, 1985) provides a means to determine if the difference in the average ranks of three or more independent samples is significant. Essentially this test tells if one can safely reject the hypothesis that all the samples came from the same parent population. Applying this test to the data shows that at the 5% significance level there is no reason to reject the hypothesis that the number of tornadoes reported across the contiguous United States in a given year is independent of the ENSO phase.
4. TORNADO ALLEY
From meteorological considerations, it is more realistic to expect that ENSO effects on tornado occurrence would be localized to quasi-homogeneous regions. To test this, tornado data from eight states lying between the Mississippi River and the high plains (Texas, Oklahoma, Kansas, Nebraska, Iowa, Missouri, Arkansas, Louisiana) were analyzed separately. This region essentially contains the proverbial "tornado alley" where tornadoes are most prevalent.
When the "all tornado" data (Fig. 2a) is considered, both the most tornadoes, and the largest deviation from the linear trend occurred in 1982 (a neutral year). Two El Nino years (1987, 1988) exhibited the largest negative deviations from the expected value, but fourth highest positive deviation also occurred during the warm phase (1973). The average ranking for El Nino years is 24.5, for La Nina years it is 29.25, and for neutral years it is 22.3. The Kruskal-Wallis H test shows that the differences between the ranking for the three ENSO phases have no significance at the 5% level.
When only F2 or greater tornadoes are considered (Fig. 2b), the first thing that is noted is that while there is still a quasi-linear trend with time in the data the slope of least squares fit line is negative. On the average, about one less strong or violent tornado is reported each year. This bias probably arises from better data collection methods. Before about 1978 virtually all F-scale determinations were made by college students reading newspapers from state archives, in the 1990's trained NWS Warning Coordination Meteorologists do most of the rating.
An El Nino year, 1973, had the most activity and the largest delta. This compares to its fourth place ranking when all tornadoes were considered. While two La Nina years had large positive deltas, three had large negative ones. The average rank for the ENSO phases were all within two positions and again they were statistically indistinguishable from each other.
