Future Changes in Extratropical Transition of Tropical Cyclones under Climate Warming: A Case Study on Hurricane Irene (2011)

According to a study by Hart and Evans (2001), 46% of North Atlantic tropical cyclones (TCs) underwent extratropical transition (ET) during the past 50 years. While many TCs weaken at this stage due to TC-unfavorable environmental conditions, a subset of these systems can re-intensify and produce severe weather in areas that rarely experience direct TC impacts. In recognition of the importance of ET events, numerous studies have been devoted to ET. In addition, many studies have investigated the influence of climate change on TCs. However, only a few studies have considered the question of how climate change will affect TCs undergoing ET. Here, we utilized a set of numerical model simulations to examine how warming affects Hurricane Irene (2011), which impacted the East Coast of the U.S. in late August 2011. To assess the effect of climate change, we used Weather Research Forecasting (WRF) model and adopt the pseudo global warming (PGW) method in which thermodynamic changes between the end of 21^st century and 20^th century, derived from an ensemble of 20 CMIP5 GCMs under RCP8.5 scenario, are applied to initial and boundary conditions for the future simulations. In order to remove differences due to changes in the effects of topography and land interactions between present-day and future simulations of Irene, pseudo-idealized experiments with no orography and entirely covered by ocean are utilized.

The results of our pseudo-idealized (i.e., entirely oceanic) simulations demonstrate that large-scale thermodynamic change increases the intensity of the transitioning storm, and also prolongs the duration of ET for Hurricane Irene, despite a highly similar initial synoptic weather pattern in current and future simulations. Storm-centered area-and-time average 3-hr accumulated precipitation during the ET is found to increase by ~68%. Bulk water budgets are assessed to determine the crucial factors for the increase. Increased moisture flux convergence with additional contributions from greater surface evaporation are responsible, consistent with earlier modeling studies (Braun 2004; Trenberth et al. 2007). Since the enhanced latent heat release develops due to the increased precipitation during the ET, the intensity of future transitioning Irene is stronger than that of the present-day Irene. Storm-centered PV analysis confirms that latent heat release in the warmer climate is responsible for the strengthening in minimum sea level pressure and wind speed. The results also indicate that the ET process itself may be prolonged in warmer climate due to a decrease in amplitude of mid-tropospheric trough, reduction of the meridional SST gradient, and weakening in vertical wind shear in the future. This suggests that storms such as Irene in a warmer environment could bring TC-like conditions farther poleward than at present.