Contributions of Interdecadal Pacific Oscillation, Atlantic Multidecadal Oscillation, and global oceanic warming to the secular change in United States tornado occurrence

Tornado has occurred frequently in the United States (US), with its seasonality and interannual variability tremendously discussed in previous studies. However, its long-term variabilities, including the interdecadal/multidecadal variabilities and the linear trend, have less been comprehensively discussed and quantified. In this study, we find that the tornado occurrence in the US shows salient interdecadal/decadal variabilities, in additional to the interannual variability. A modest positive linear trend is also present, qualitatively consistent with the thermodynamic influence of global oceanic warming (GOW) on convective available potential energy and low-level southerly winds. Two internal modes, namely the Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Oscillation (AMO), influence the interdecadal variabilities of tornado occurrence. The IPO stimulates a stationary wave train, propagating from the Pacific to North America, resembles the Pacific-North America pattern yet on the decadal time scale, and affects local circulations and hence tornado occurrence. The AMO induces a local dipole of circulation anomalies, resembling the North Atlantic Oscillation pattern, which triggers a northwest-southeast dipole change of tornado occurrence with the northwest component stronger. Furthermore, we quantitatively estimate the relative importance of IPO, AMO and GOW on the tornado occurrence from 1950 to 2021 based on a multi-regression model. The result shows that the IPO and AMO jointly contribute about 32.6 % (19.9 %) of the decadal (total) variance while the GOW contribution is secondary.