Unprecedented Northern Hemisphere Tropical Cyclone Genesis in 2018 Shaped by Subtropical Warming in the North Pacific and the North Atlantic

Northern Hemisphere tropical cyclone (TC) genesis (TCG) frequency in June-September of 2018 reached a record high since 1965. This unprecedented TCG was mainly driven by the North Pacific (NP) and the subtropical North Atlantic (NA). TCs in the western North Pacific (WNP) predominately took a northward-recurving track, resulting in the highest number of named storm days in the subtropical WNP on record. TCs in the eastern North Pacific propagated anomalously westward with three Category 4-5 hurricanes reaching the central North Pacific. The unprecedented TCG in the NP was caused by a persistent weakening of the NP subtropical high that was tightly coupled with the subtropical NP sea surface temperature (SST) warming. In the NA, enhanced TCG was mainly found in the subtropics owing to a favorable large-scale environment forced by subtropical SST warming. The result here has important implications for understanding variability in TC activity on the hemisphere-scale.