Climate and Weather: Information and Education

What is ENSO?

El Niño is an abnormal warming of sea surface of eastern tropical Pacific Ocean (Figure 1). The opposite of El Niño is La Niña which happens when sea surface temperature of Eastern tropical Pacific is less than average. The abnormal warming or cooling happens in a see-saw pattern which includes reversing surface air pressure between eastern and western tropical Pacific Ocean and causes low surface pressure in the eastern tropical Pacific and high pressure in the western Pacific or vice-versa. The sea and atmospheric oscillation which develops due to these changes in sea surface temperature and pressure is known as El Niño Southern Oscillation or ENSO. Other than these warm and cool phases there is a third phase when none of these two phases exist, is known as Neutral phase. Neutral Phase also includes the time in between the transition of the warming or cooling to normal (average sea surface temperature and pressure) situations.


Figure 1. Standardized sea surface temperature (color) anomalies (departures from the long term average) for El Nino and La Nina.

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The current phase of ENSO

El Niño/ La Niña and Tornadoes

The ENSO phases also affect the number of tornadoes in USA but the relationships in between the ENSO phases and number and fatality of tornadoes are complicated. The results of a study relating the sea surface temperature anomaly and outbreaks of tornado of last one century during March to May was tabulated showing the timing and phases of ENSO. The results suggest more elaborated future studies to get more reliable result in this case.

Date Tornadoes Fatalities Location ENSO Phase
April, 1908 >41 324 Central-Southeastern US La Niña
May-June, 1917 >78 383 Central-Southeastern US La Niña
March 18, 1925 9 >747 Mississippi Valley-Ohio Valley Neutral
May, 1930 >90 110 Great Plains, Mississippi Valley El Niño
March 21-22, 1932 >20 >330 Southeastern US El Niño
April 5-6, 1936 17 >436 Southeastern US Neutral
March-May 1942 - >270 Great Plains El Niño
April 9-10, 1947 8 181 Southern Great Plains Neutral
May, 1949 >82 66 Central-Southeastern US Neutral
April-May 1953 33 >144 Southern Great Plains, Upper MS Valley El Niño
April 11-12, 1965 51 265 Central US Neutral
April 3-4, 1974 148 330 Eastern US La Niña
April 2-3, 1982 61 29 Southern Plains-Mississippi Valley Neutral
May 31, 1985 43 88 US-Canada, Eastern Great Lakes La Niña
May 1995 391 11 Central and Southern US El Niño
April-May 1999 140 50 Southern Great Plains La Niña
May 2003 543 >48 Southern Great Plains, Midwest, Southeast US Neutral
May 2004 384 7 Great Plains-Midwest Neutral
May 2008 >100 >40 Great Plains, Mississippi Valley, Southeast La Niña
April 2011 875 361 Southeast US La Niña

Table 1: Notable Tornado Outbreaks - 1900-2011