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Or in other words, does it increase the amount of heat+moisture advection from the Gulf of Mexico into the Central United States? And is this increase associated with increased CAPE/tornado/thunderstorm activity?

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    $\begingroup$ When you say "does it increase the amount of heat+moisture advection from the Gulf of Mexico" do you mean that the waters in the Gulf of Mexico have warmed up because of the El Nino event, therefore the evaporation is higher? $\endgroup$ – Isopycnal Oscillation Nov 18 '14 at 6:57
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According to the NOAA document A Comparison of Rawinsonde Data from the Southeastern United States during El Niño, La Niña, and Neutral Winters (Sankovich et al.) report that

Schaefer and Tatom (1998) considered the number of tornadoes per year in the United States and sea surface temperatures (SST) in various portions of the Pacific Ocean to try to discern an impact of ENSO on the occurrence of tornadoes. Their statistical inquiries showed no major influence, but their data does show a signal that more tornadoes tend to occur in the mideastern states during the La Niña phase.

However, another study cited by Agee and Zurn-Birkhimer (1998) state

that tornado occurrences do not favor one ENSO phase but rather exhibit a shift in geographic location. For example, their results suggest that more tornadoes occur in the lower Midwest, Ohio Valley, Tennessee Valley, and mid-Atlantic region during the La Niña phase than in any other phase.

This is further concluded by Bove (1998) concluded that

increased tornado activity over the Ohio and Tennessee River Valleys during the cold La Niña phase.

Sankovich et al. conclude that

Like all climatological signals, they [ENSO] represent long-term trends. They do not provide a tool for making seasonal or other extended forecasts; rather, they show that other things being equal, the atmospheric stratification over the Southeastern United States is typically less compatible for the occurrence of severe thunderstorms (3/4” or larger hail, thunderstorm gusts of 58 mph or faster, tornados) during a La Niña winter than during Neutral or El Niño winters.

In the paper A seasonal-scale climatological analysis correlating spring tornadic activity with antecedent fall–winter drought in the southeastern United States (Shepherd et al. 2009), state that to

remove the uncertainty associated with the ENSO debate, we consider a new approach by focusing on antecedent drought conditions for a region.

Whereby, they conclude that the relationship between drought and incidences of tornadic and severe thunderstorm behaviour to possess a

statistically significant tendency for fall–winter drought conditions to be correlated with below-normal tornado days the following spring in north Georgia (i.e. 93% of the years) and other regions of the Southeast. Non-drought years had nearly twice as many tornado days in the study area as drought years and were also five to six times more likely to have multiple tornado days

References:

  • Schaefer, J. T., and F. B. Tatom, 1998: The relationship between El Niño, La Niña, and United States tornadoes, Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., 416-419.

  • Agee, E., and S. Zurn-Birkhimer, 1998: Variations in USA tornado occurrences during El Niño and La Niña, Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., 287-290.

  • Bove, M.C., 1998: Impacts of ENSO on United States tornadic activity, Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., 313-316.

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