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Luiz Felippe Gozzo, Rosmeri P. da Rocha, Michelle S. Reboita, and Shigetoshi Sugahara

1. Introduction A subtropical cyclone (SC) is a low pressure system that presents both extratropical and tropical structure during its development. These hybrid cyclones are nonfrontal, with a low tropospheric warm core and an upper-level cold core ( Hart 2003 ). They can be formed with such characteristics and maintain them throughout the cyclone lifetime, or they may originate as an intermediary stage in a tropical or extratropical transition ( Ritchie and Elsberry 2001 ; Hulme and Martin

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Christopher A. Davis

1. Introduction What are subtropical cyclones? Cyclones forming in the subtropics, roughly 23–35° from the equator, have long presented challenges for basic understanding, classification, and forecasting. They have been regarded as having some characteristics of extratropical and tropical cyclones simultaneously. As noted by Evans and Guishard (2009) and Guishard et al. (2009) , they are inconsistently included in the database for tropical cyclones. Their distinction from conventional

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Juan J. González-Alemán, Francisco Valero, Francisco Martín-León, and Jenni L. Evans

1. Introduction Subtropical cyclones (STCs) are low pressure systems showing characteristics of both tropical and extratropical cyclones, thus being between the theoretical extremes of cyclones. They have a hybrid thermal structure with cold upper-tropospheric and warm lower-tropospheric thermal anomalies as main feature. In recent years, there has been a growing interest in STCs because of their recognition as damaging weather systems, such as pre–Hurricane Karen’s landfall over Bermuda in

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Leone Cavicchia, Andrew Dowdy, and Kevin Walsh

1. Introduction The subtropical east coast of Australia is often affected by low pressure systems that can cause high-impact weather. Such systems are collectively known as east coast lows (ECLs). All cyclones crossing the area are generally considered ECLs irrespective of whether they are midlatitude cyclones entering the region from the southern boundary, transitioning tropical cyclones, or are formed locally, and irrespective of the mechanisms responsible for their formation. Building upon

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Stuart A. Browning and Ian D. Goodwin

1. Introduction Subtropical cyclones typically combine characteristics of tropical and extratropical cyclones ( Evans and Guishard 2009 ) to produce one of the most complex and destructive maritime storm types observed in either hemisphere. Although large anticyclones typically dominate the subtropics, intense cyclonic systems can develop via several mechanisms: transition from the tropics, transition from the extratropics, or in situ cyclogenesis. Throughout their life cycle subtropical storms

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Alicia M. Bentley, Lance F. Bosart, and Daniel Keyser

either the favorable interaction of a preexisting lower-tropospheric cyclonic vorticity center with an upper-tropospheric trough (e.g., Molinari et al. 1995 ) or the tropical transition (TT; Davis and Bosart 2003 , 2004 ) of a subtropical cyclone (STC) (e.g., Roth 2002 ; Guishard et al. 2007 ; Evans and Guishard 2009 ; Guishard et al. 2009 ; González-Alemán et al. 2015 ; Bentley et al. 2016 ). A recent climatology of NATL STCs that undergo TT constructed by Bentley et al. (2016) from the

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Mark P. Guishard, Jenni L. Evans, and Robert E. Hart

a poorly forecast tropical storm. Oceanic storms with ambiguous origins and/or structures have often been operationally classified as subtropical. The operational definitions of cyclone types relevant to this study are a follows (as published by the U.S. National Weather Service; OFCM 2007 ): Subtropical cyclone . A nonfrontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to

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Wataru Yanase, Hiroshi Niino, Kevin Hodges, and Naoko Kitabatake

intermediate types between TCs and ECs, which are hereafter referred to as hybrid cyclones (HCs): for example, subtropical cyclones ( Otkin and Martin 2004 ; Evans and Guishard 2009 ; Guishard et al. 2009 ; Evans and Braun 2012 ), extratropical transitioning TCs ( Hart and Evans 2001 ; Jones et al. 2003 ; Hart et al. 2006 ; Kitabatake 2008 , 2011 ), and tropical transitioning disturbances originating from the midlatitudes ( Davis and Bosart 2004 ; Mctaggart-Cowan et al. 2013 ). These different

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Leone Cavicchia, Acacia Pepler, Andrew Dowdy, and Kevin Walsh

1. Introduction The portion of the east coast of Australia extending between the subtropics and the midlatitudes (the area corresponding approximately to the latitudes between 25° and 40°S and the longitudes between 150° and 160°E) experiences a frequent occurrence of intense low pressure systems. This type of cyclone is generally known as east coast lows (ECLs). Such systems can occur year-round; the most intense events, however, are normally observed during the transition to the cold season

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Ron McTaggart-Cowan, Thomas J. Galarneau Jr., Lance F. Bosart, Richard W. Moore, and Olivia Martius

1. Introduction The global tropical cyclone (TC; all acronyms defined in Table 1 ) climatology of Gray (1968) serves as a primary reference for TC development mechanisms and environmental properties associated with TC formation. While the majority of events included in the study were classified as “tropical” (type A, 83%), the remainder were assigned to “subtropical” and “hybrid” categories. The dominance of type-A developments has led to a focusing of TC research on formations that follow

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