Dominant Role of Subtropical Pacific Warming on the Extreme 2015 Central Pacific Hurricane Season

The 2015 hurricane season in the Eastern and Central Pacific Oceans (EPO and CPO), particularly around Hawaii, has been extremely active – including the first instance of three simultaneous Category 4 hurricanes in the EPO and CPO. Why has the 2015 hurricane season been so active in the EPO and CPO? A number of factors have been linked to enhancements of EPO and CPO hurricane activity, such as greenhouse-induced global warming, as well as natural variations, particularly eastern equatorial Pacific Ocean (EEqP) warming during El Niño events. Here we show, using a suite of targeted high-resolution model experiments, that the extreme 2015 EPO and CPO hurricane season was not primarily induced by the 2015 El Niño's EEqP warming, but by warming in the subtropical Pacific Ocean (a “warm blob”). This warming is not typical of El Niño, but is associated with a different mode of climate variability, the “Pacific Meridional Mode”, superimposed on long-term anthropogenic warming. We have undertaken a systematic assessment of the mechanisms controlling this extreme hurricane season. We leverage seasonal forecasts with a state-of-the-art high-resolution dynamical model that correctly predicted both the 2015 El Niño and an unprecedented number of hurricanes in the EPO and CPO (and approaching Hawaii) during July–November 2015, when initialized before this hurricane season. We further show that long-term warming of the subtropical Pacific Ocean is key to projections for increased risk of such active tropical cyclone years in the CPO and Hawaiian Islands over the next few decades.