A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis

Abstract

Tropical cyclogenesis is generally considered to occur in regions devoid of baroclinic structures; however, an appreciable number of tropical cyclones (TCs) form in baroclinic environments each year. A global climatology of these baroclinically influenced TC developments is presented in this study. An objective classification strategy is developed that focuses on the characteristics of the environmental state rather than on properties of the vortex, thus allowing for a pointwise ''development pathway'' classification of reanalysis data. The resulting climatology shows that variability within basins arises primarily as a result of local surface thermal contrasts and the positions of time-mean features on the subtropical tropopause. The pathway analyses are sampled to generate a global climatology of 1948-2010 TC developments classified by baroclinic influence: nonbaroclinic (70%), low-level baroclinic (9%), trough induced (5%), weak tropical transition (11%), and strong tropical transition (5%). All basins other than the North Atlantic are dominated by nonbaroclinic events; however, there is extensive interbasin variability in secondary development pathways. Within each basin, subregions and time periods are identified in which the relative importance of the development pathways also differs. The efficiency of tropical cyclogenesis is found to be highly dependent on development pathway. The peak efficiency defined in the classification subspace straddles the nonbaroclinic/ trough-induced boundary, suggesting that the optimal environment for TC development includes a baroclinic contribution from an upper-level disturbance. By assessing the global distribution of baroclinically influenced TC formations, this study identifies regions and pathways whose further study could yield improvements in our understanding of this important subset of TC developments.

Objective determination of the extratropical transition of tropical cyclones in the Northern Hemisphere

“…Finally, if a storm does not move at least 10 degrees in latitude over its whole life cycle, no transition is recorded even if one 'appears' in the CPS CA because this is not reconcilable with understandings of ET as the process of storms of tropical genesis moving into the midlatitudes. By implementing these constraints results were considerably improved, for example removing the higher latitude baroclinically induced TCs (McTaggart-Cowan et al, 2013) and monsoonal storms in the northern Indian Ocean that superficially resemble ET in the CPS.…”

Characteristics and Impacts of Extratropical Rossby Wave Breaking during the Atlantic Hurricane Season

“…Besides the possible impacts of the NAO, extratropical frontal systems may affect Atlantic TC activity by contributing to TC formation, which is known as the tropical transition Bosart 2003, 2004;McTaggart-Cowan et al 2013). Notably, the tropical transition is often accompanied by potential vorticity (PV) streamers associated with anticyclonic Rossby wave breaking (RWB) (Davis and Bosart 2004;Galarneau et al 2015).…”

Development of North Atlantic Tropical Disturbances near Upper-Level Potential Vorticity Streamers

“…The environmental factors that are favorable for TC development are well documented and include warm sea surface temperatures (SSTs), moist conditions in midlevels, strong divergence in upper levels, weak vertical wind shear, and preexisting low-level cyclonic vorticity (e.g., Gray 1968;DeMaria et al 2001). It is known, however, that TCs also develop in environments with moderate to strong deep-layer vertical wind shear (e.g., Bracken and Bosart 2000;Davis and Bosart 2001, 2002, 2003 and over relatively cool SSTs (e.g., McTaggart-Cowan et al 2006). In fact, previous research has demonstrated that half of all TC developments over the North Atlantic occur in environments with moderate to strong baroclinicity and attendant vertical wind shear (e.g., McTaggart-Cowan et al 2008, 2013Hess et al 1995;Elsner et al 1996).…”

“…It is known, however, that TCs also develop in environments with moderate to strong deep-layer vertical wind shear (e.g., Bracken and Bosart 2000;Davis and Bosart 2001, 2002, 2003 and over relatively cool SSTs (e.g., McTaggart-Cowan et al 2006). In fact, previous research has demonstrated that half of all TC developments over the North Atlantic occur in environments with moderate to strong baroclinicity and attendant vertical wind shear (e.g., McTaggart-Cowan et al 2008, 2013Hess et al 1995;Elsner et al 1996). These baroclinically influenced TC developments encompass subtropical cyclones that undergo tropical transition (TT; e.g., Bosart 2003, 2004;Evans and Guishard 2009;Guishard et al 2009;Hulme and Martin 2009a,b) and precursor low-level cyclonic vorticity centers that interact with upper-tropospheric troughs (e.g., Sadler 1976Sadler , 1978Lander 1994;Molinari et al 1995;Chen et al 2008).…”