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Long-term trends suggest shifts toward earlier tornado season peaks, and yet fail to examine the role of year-to-year climate variability. Here, El Nino-Southern Oscillation phase is demonstrated to influence annual cycle characteristics of United States tornadoes. Observations and favorable environments show substantial modification of the peak spatial distribution and the temporal onset of tornado occurrence. La Nina produces an earlier annual peak probability by 1.5-2weeks, with a higher overall fraction of events in March and April. In contrast, El Nino leads to a week delay in the maximum probability and enhances a second peak in the fall months. Consequently, this suggests that climate change is not the sole driver of changes to seasonal onset and peak, and climate variability plays an important role in modulating the annual cycle.

Plain Language Summary What drives the onset of tornado season, and can we point to a changing climate as the cause of a trend toward an earlier season start date? In this paper, climate variability driven by equatorial Pacific sea surface temperature variations (El Nino-Southern Oscillation) is shown to modulate the characteristics of when the majority of tornadoes in any given year occur, and the timing of the season peak. La Nina is shown to shift the bulk of tornado season earlier by 1.5-2weeks relative to years when no Pacific sea surface temperature anomaly exists, while El Nino delays the maximum daily frequency by a week. This result contrasts earlier studies that show long-term trends of a similar or smaller magnitude, which had previously suggested that climate variability does not modulate the annual cycle. The results here provide the potential for guidance as to season onset in developing seasonal tornado outlooks.