The Present-Day Simulation and Twenty-First-Century Projection of the Climatology of Extratropical Transition in the North Atlantic

The Present-Day Simulation and Twenty-First-Century Projection of the Climatology of Extratropical Transition in the North Atlantic

15 APRIL 2017 L I U E T A L . 2739 The Present-Day Simulation and Twenty-First-Century Projection of the Climatology of Extratropical Transition in the North Atlantic MAOFENG LIU Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey GABRIEL A. VECCHI Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, and Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey JAMES A. SMITH Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey HIROYUKI MURAKAMI Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, and Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey (Manuscript received 2 May 2016, in final form 10 December 2016) ABSTRACT This study explores the simulations and twenty-first-century projections of extratropical transition (ET) of tropical cyclones (TCs) in the North Atlantic, with a newly developed global climate model: the Forecast- Oriented Low Ocean Resolution (FLOR) version of the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model version 2.5 (CM2.5). FLOR exhibits good skill in simulating present-day ET properties (e.g., cyclone phase space parameters). A version of FLOR in which sea surface temperature (SST) biases are artificially corrected through flux-adjustment (FLOR-FA) shows much improved simulation of ET activity (e.g., annual ET number). This result is largely attributable to better simulation of basinwide TC activity, which is strongly dependent on larger-scale climate simulation. FLOR-FA is also used to explore changes of ET activity in the twenty-first century under the representative concentration pathway (RCP) 4.5 scenario. A contrasting pattern is found in which regional TC density increases in the eastern North Atlantic and decreases in the western North Atlantic, probably due to changes in the TC genesis location. The increasing TC frequency in the eastern Atlantic is dominated by increased ET cases. The increased density of TCs undergoing ET in the eastern subtropics of the Atlantic shows two propagation paths: one moves northwest toward the northeast coast of the United States and the other moves northeast toward western Europe, implying increased TC-related risks in these regions. A more TC-favorable future climate, evident in the projected changes of SST and vertical wind shear, is hypothesized to favor the increased ET occurrence in the eastern North Atlantic. 1. Introduction (ET). The potential hazards from extratropically tran- sitioning TCs include heavy rainfall and expanded gale- Tropical cyclones (TCs) moving poleward can lose force wind (Jones et al. 2003). Tropical Storm Agnes in their tropical characteristics and evolve into extra- 1972 (DiMego and Bosart 1982a,b), Hurricane Floyd in tropical storm systems through interaction with baro- 1999 (Atallah and Bosart 2003; Colle 2003), and Hurri- clinic environments in midlatitudes (Jones et al. 2003), a cane Irene in 2011 (Liu and Smith 2016), for example, process generally referred to as extratropical transition produced extreme rainfall and severe flooding in the northeastern United States. All three produced heavy Corresponding author e-mail: Maofeng Liu, maofeng@princeton. rainfall with a left-of-track concentration of rain at far edu range from the storm circulation (Atallah and Bosart DOI: 10.1175/JCLI-D-16-0352.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 2740 JOURNAL OF CLIMATE VOLUME 30 2003; Colle 2003; Atallah et al. 2007). More recently, Crucial questions left unanswered by the above studies Hurricane Sandy in 2012 produced catastrophic storm concern the potential role of projected multidecadal cli- surge over the coast of the northeastern United States, mate change on the number and intensity of ET events. especially over New Jersey and New York City. The in- High-resolution climate models provide a framework for tensification of Sandy was closely associated with ET simulating, predicting and projecting TC activity (e.g., (Galarneau et al. 2013). Case studies of historical ET Zhao et al. 2009; Murakami and Wang 2010; Murakami storms that have reached Europe highlight their damages et al. 2012; Chen and Lin 2013; Vecchi et al. 2014; through both direct impacts (Thorncroft and Jones 2000; Murakami et al. 2015; Walsh et al. 2015). Studies using Agustí-Panareda et al. 2005; Feser et al. 2015)andremote climate models found that ET was a key element of a impacts (Grams and Blumer 2015). Hurricane Gonzalo projected increase in storms impacting western Europe (2014) moved from the northern part of the United under global warming (Haarsma et al. 2013; Baatsen et al. Kingdom to the Balkans for several days as an extra- 2015). Studies on future ET activity in the NA, however, tropically transitioned cyclone and produced strong wind remain sparse. This paper aims to provide a comprehen- and heavy rainfall, which led to fatalities and economic sive study on the simulation and projection of ET activity losses (Feser et al. 2015). Grams and Blumer (2015) in the NA using a global high-resolution coupled climate found that ET of Hurricane Katia (2011) was responsible model. The significance of this basin is highlighted by the for the development of a downstream trough that trig- dense population and the economic importance in the gered severe thunderstorms in central Europe. northeast coast of the United States and western Europe. The potential hazards associated with ET highlight High-quality best-track data available in this basin allow a the importance of understanding ET events. Previous detailed validation of the climate model simulation. studies have focused on the dynamic and thermody- As a first step, we examine the capability of our model namic evolution of ET events (e.g., Harr and Elsberry in simulating the present-day climatology of ET. Then 2000; Klein et al. 2000; Jones et al. 2003; Arnott et al. we use the model to simulate the response of ET under 2004; Davis et al. 2008; Kitabatake 2008; Galarneau warming scenarios. The paper is organized as follows. In et al. 2013). The interaction of TCs with midlatitude section 2, we introduce data and methods used in this troughs plays a crucial role in determining the post- study. Results are presented in section 3, and a summary transition intensity and structure of the storm systems and conclusions are given in section 4. (e.g., Harr and Elsberry 2000; Hanley et al. 2001; Klein et al. 2002; McTaggart-Cowan et al. 2003; Ritchie and 2. Data and methods Elsberry 2003; Hart et al. 2006; Galarneau et al. 2013; a. Data Leroux et al. 2013; Griffin and Bosart 2014). In addition to these studies, there has been research In this study, we use the Atlantic Hurricane Database investigating the climatological properties of ET over (HURDAT) best-track data (6-h interval) from National different oceans. Hart and Evans (2001) examined the ET Oceanic and Atmospheric Administration (NOAA)’s climatology in the North Atlantic (NA), and found that National Hurricane Center (NHC). To match the track- 46% of TCs experienced ET during the period 1950–99. ing scheme used in the climate model (Murakami et al. There is a pronounced seasonal cycle associated with ET 2015; Harris et al. 2016), only TCs with 3-day duration and in the NA. The latitude with the highest frequency of ET tropical storm intensity, or greater, are used. HURDAT ranges from 358 to 458 (Hart and Evans 2001). A slightly best-track data provide the timing when a TC changes lower ET ratio (40%) was found in the western North into an extratropical cyclone (EX) (McAdie et al. 2009). Pacific (WNP) during the period 1979–2004 (Kitabatake An EX in HURDAT is defined as a system whose major 2011). An even smaller percentage (only 9%) of TCs in energy source is baroclinic (McAdie et al. 2009). Despite the eastern North Pacific (ENP) undergo ET (Wood this rather subjective definition, the ET information from and Ritchie 2014). The westward extension of the deep HURDAT is compared with climate model results. subtropical high from North America to the ENP is Two reanalysis datasets are used in this study: the 6-h suggested to be the main reason of the decreased ET National Centers for Environmental Prediction (NCEP) ratiofromtheNAandtheWNPtotheENP(Wood and Climate Forecast System Reanalysis (CFSR; Saha et al. Ritchie 2014). Increasing trends of ET frequency and 2010) at 0.58 horizontal resolution and the 6-h, 1.258 ratio were found in both the Northern and Southern Japanese 55-Year Reanalysis (JRA-55; Kobayashi et al. Hemispheres over the period of 1970–2012, although 2015). The CFSR and JRA-25 (an earlier version of there were differences in the magnitude of trends JRA-55) reanalysis fields have been shown to have the and statistical significances across basins (Mokhov lowest TC position differences from best-track data et al. 2014). compared to European Centre for Medium-Range 15 APRIL 2017 L I U E T A L . 2741 Weather Forecasts (ECMWF) 40-Year Re-Analysis 2006–2100 for simulations of the future climate. To eval- (ERA-40), ECMWF interim reanalysis (ERA-Interim, uate the changes of ET activity, we explore five ensembles hereafter ERA-I), and Modern-Era Retrospective over two simulation periods: the present-day (PD) run Analysis for Research and Applications (MERRA) over 1961–2005, and the global warming (GW) run over (Schenkel and Hart 2012). Murakami (2014) found that the last 45 years (2056–2100) of the twenty-first century as JRA-55 and CFSR showed the highest detection rate in we expect to cover the ending year of the simulation. terms of hitting rate and false alarm rate compared to The tracking scheme applied here is the same as the other reanalysis datasets.

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