| Summary: | <p>El Niño–Southern Oscillation (ENSO) modulates severe thunderstorm activity in the US, with
increased activity expected during La Niña conditions. There is
also evidence that severe thunderstorm activity is influenced by the
Arctic Oscillation (AO), with the positive phase being associated
with enhanced activity. The combined ENSO–AO impact is relevant for
situations such as in early 2021, when persistent, strong positive
and negative AO events occurred during La Niña conditions. Here we
examine the relation of a spatially resolved tornado environment
index (TEI) with ENSO and the AO in climate model forecasts of
February, March, and April conditions over North America. Bivariate
composites on Niño 3.4 and AO indices show that TEI predictability
is high (strong signals and probability shifts) when the ENSO and AO
signals reinforce each other and low when they cancel each
other. The largest increase in the expected value and variance of
TEI occurs when Niño 3.4 is negative, and the AO is
positive. Signal-to-noise ratios are higher during El
Niño–negative AO than during La Niña–positive AO, but
probability shifts are comparable.</p>
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