North American April tornado occurrences linked to global sea surface temperature anomalies
Ocean temperature anomalies in April influence U.S. tornado frequencies through atmospheric teleconnections and moisture transport. Annual tornado occurrences over North America display large interannual variability and a statistical linkage to sea surface temperature (SST) anomalies. However, the underlying physical mechanisms for this connection and its modulation in a rapidly varying seasonal environment still remain elusive. Using tornado data over the United States from 1954 to 2016 in combination with SST forced atmospheric general circulation models, we show a robust dynamical linkage between global SST conditions in April, the emergence of the Pacific North American teleconnection pattern (PNA) and the year to year tornado activity in the Southern Great Plains (SGP) region of the United States. Contrasting previous studies, we find that only in April SST driven atmospheric circulation anomalies can effectively control the northward moisture laden flow from the Gulf of Mexico, boosting low level moisture flux convergence over the SGP. These strong large scale connections are absent in other months because of the strong seasonality of the PNA and background moisture conditions.
Science Advances