The Roles of Climate Change and Climate Variability in the 2017 Atlantic Hurricane Season

The 2017 hurricane season was hyperactive with total accumulated cyclone energy exceeding 220 10⁴kn² as of October 31, the fourth highest since 1950. An analysis of the sea surface temperatures (SSTs) reveals that a long–term upward trend and the positive phase of the Atlantic Meridional Mode (AMM) contribute to the second warmest ocean surface on record over the Main Development Region (MDR). In particular, the SSTs for August/September, when TCs were hyperactive, were ~0.45°C above the 1982–2017 average, about one half of that corresponding to an upward trend and most of the remainder attributed to the AMM. The role of the other leading climate modes such as ENSO and the North Atlantic Oscillation was negligible. Mid–tropospheric humidity and vertical wind shear over the MDR, and tropical ocean heat content are shown to be closely linked to the AMM. The results show that even modest impacts of the leading modes of climate variability on the MDR SST can, when of the same sign as the global warming contribution (and in concert with favorable dynamical conditions such as that produced by the AMM), substantially increase the risk of enhanced TC activity.