Influences of Natural Variability and Anthropogenic Forcing on the Extreme 2015 Accumulated Cyclone Energy in the Western North Pacific
Introduction. The 2015 tropical cyclone (TC) activity measured by the ACE [computed as the sum of the square of the maximum surface wind speed (MSW) over the TC duration when MSW is greater than 34 knots; e.g. Bell et al. 2000] was extremely high in the western North Pacific Ocean (Figs. 26.1a,b and 26.2a) The 2015 WNP ACE is the second highest since 1970 (with the highest being 1997) based on the Joint Typhoon Warning Center best track data for 1970 2014 and Unisys data for 2015 (http:/weather.unisys.com /hurricane/ the highest since 1977 based on the Japan Meteorological Agency (JMA) and the highest since 1970 based on Shanghai Typhoon Institute (STI) data. Higher (lower) WNP ACE is generally observed during El Nino (La Nina) years, because TCs are formed more southeastward (northwestward) and stay longer (shorter) over the warm ocean surface (e.g. Camargo and Sobel 2005; Chan 2007) This shift in genesis and difference in tracks leads to a higher occurrence of the most intense typhoons, which is the main cause of a high ACE during El Nino years. An extremely strong El Nino event developed in 2015. While there has been major progress in the understanding of the El Nino Southern Oscillation (ENSO) WNP ACE association, the modulation of WNP ACE by anthropogenic forcing is still a challenging scientific question (e.g. Emanuel 2013; Lin and Chan 2015) Using observations and a suite of climate model experiments, this study attempts to assess whether and to what extent internal climate modes (e.g. ENSO) and anthropogenic forcing contributed to the extreme 2015 WNP ACE event.