Influences of Natural Variability and Anthropogenic Forcing on the Extreme 2015 Accumulated Cyclone Energy in the Western North Pacific
Abstract: The extreme value of the 2015 western North Pacific (WNP) accumulated cyclone energy (ACE) was mainly caused by the sea surface warming in the eastern and central Pacific, with the anthropogenic forcing largely increasing the odds of the occurrence of this event.
Discussions and Conclusions:
We have observed an extremely active TC season in the WNP in 2015, with an extremely high ACE and frequency of C45 TCs. The 2015 season may be caused mainly by warm ocean surface temperatures in the tropical Pacific, characterized by a strong El Niño event, with other climate modes (e.g., PMM) potentially playing a role. We have found that anthropogenic forcing has substantially increased the risk of having WNP ACE higher than or equal to such an extreme event. Although the changes in WNP ACE under anthropogenic forcing are still unclear (e.g., Emanuel 2013; Lin and Chan 2015), both GFDL FLOR and HiFLOR do suggest that the annual WNP ACE tends to become more extreme because of anthropogenic forcing. The two models also suggest that the variability of WNP ACE attributable to climate modes will increase at a much higher rate than as a result of anthropogenic forcing. The frequency of strong El Niño events is projected to increase due to greenhouse warming (Cai et al. 2014), which in turn could potentially lead to a higher frequency of WNP seasons with high values of ACE.