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Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development

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Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development REVISITING THE 26.5°C SEA SURFACE TEMPERATURE THRESHOLD FOR TROPICAL CYCLONE DEVELOPMENT BY RON MCTAGGArt-COWAN, EMILY L. DAVIES, JONATHAN G. FAIrmAN JR., THOMAS J. GALARNEAU JR., AND DAVID M. SCHULTZ A measure of tropospheric depth and bulk convective stability replaces the traditional sea surface temperature–based threshold as a necessary ingredient for tropical cyclogenesis from baroclinic precursors. he development of a tropical cyclone is the net The value of 26.5°C is conveniently the closest result of numerous processes that promote the half-degree Celsius to 80°F, a fact thought to have T growth of a weak perturbation into an intense contributed to its selection [Sadler (1964, p. 352), self-sustaining circulation. A list of five necessary based on Palmén (1956)]. This threshold was found ingredients is typically used to assess the favor- to be globally applicable by Gray (1968) despite being ability of an environment for tropical cyclogenesis: developed based on experience in the Gulf of Mexico, high sea surface temperature (SST), a steep vertical and the western North Atlantic and western North temperature gradient (reduced stability), high lower- Pacific basins; however, alternative values based on tropospheric relative humidity, low wind shear, and early observational studies include 26.1° (Fisher 1958) a nonzero Coriolis force (Palmén 1948; Riehl 1954; and 26.8°C (Wendland 1977). More recently, Dare Miller 1958; Gray 1968; Lee et al. 1989; DeMaria et al. and McBride (2011) present a global climatology of 2001). We focus here on the SST element of this list. SSTs associated with tropical cyclogenesis, finding Since the statement by Palmén (1948, p. 31) that that almost 7% of formations occur over waters whose “hurricanes can be formed only in the oceanic regions temperature at the time of formation lies below the outside the vicinity of the Equator where the surface 26.5°C threshold. They propose an adjustment of this water has a temperature above 26–27°C,” a 26.5°C value to 25.5°C, such that only 1.4% of developments threshold for tropical cyclogenesis has become so occur over subthreshold SSTs. However, Dare and well established that it appears in many current McBride (2011) also find that 26.5°C is a reasonable textbooks (Wallace and Hobbs 2006; Williams 2009; threshold when the SST is averaged over the 2-day Ahrens 2009; Laing and Evans 2011; Ackerman period leading up to storm formation. and Knox 2015) and review articles [Galvin (2008), Tropical cyclone development over the relatively rounding up to 27°C]. However, the precise value of cold waters of the northeastern North Atlantic Ocean the SST threshold has been a matter of debate since is found by Mauk and Hobgood (2012) to be associ- its inception. ated with the presence of baroclinicity in the storm AMERICAN METEOROLOGICAL SOCIETY NOVEMBER 2015 | 1929 environment, consistent with the increasing rec- such an element to the list of ingredients required for ognition of the potential importance of baroclinic tropical cyclogenesis will help to refine our under- processes in tropical cyclogenesis (Bosart and Bartlo standing of, and improve our ability to, predict this 1991; Bosart and Lackmann 1995; Davis and Bosart important class of development events. 2004; McTaggart-Cowan et al. 2008, 2013; Evans and Guishard 2009; Guishard et al. 2009). The majority DATA AND METHODS. This study employs of low-SST formations documented by Mauk and four global datasets that cover a common 25-yr Hobgood (2012) are classified as strong tropical period from 1989 to 2013: tropical cyclone best transitions, a development pathway characterized by tracks, high-resolution SST, atmospheric analyses, the presence of a well-defined extratropical precur- and cyclone development pathway classifications. A sor that evolves into a warm-core system through total of 1,757 tropical cyclones are included across all the vertical redistribution of mass and momentum basins, thus allowing for the development of robust by sustained convection (Davis and Bosart 2003, statistics even for relatively rare events. As a result, 2004). Considering both the weak and strong forms the term significant will be used hereafter in the strict of tropical transition (TT), McTaggart-Cowan et al. statistical sense to indicate the rejection of the null (2013) find that 16% of all tropical cyclones develop hypothesis at the 99% confidence level. from baroclinic precursors. All tropical cyclone tracking information used In this study, we investigate the significant dif- in this study is derived from the International Best ferences that exist between environments associated Track Archive for Climate Stewardship (IBTrACS), with tropical cyclogenesis over waters on either side version 4, revision 5 (Knapp et al. 2010). The subset of the 26.5°C threshold. The presence of upper-level of best track data from the World Meteorological baroclinic disturbances during low-SST formation Organization’s Regional Specialized Meteorological events motivates a development pathway–specific Centers is used to determine storm location and analysis of the relevance of an SST-based threshold estimated intensity. The tropical storm wind speed for cyclogenesis. For pathways involving the TT of a threshold for the North Atlantic basin [35 kt (18 m s–1)] precursor baroclinic disturbance, a 22.5°C maximum is used to determine the development time of the threshold of the coupling index (computed as the cyclone.1 This definition focuses on the point at difference between upper- and lower-level equiva- which the precursor vortex becomes a self-sustaining lent potential temperatures) is preferable to the SST circulation (Laing and Evans 2011) and is consistent threshold in terms of both effectiveness and physical with the definition adopted by Dare and McBride relevance to the problem at hand. The addition of (2011). The study thereby concentrates on the early intensification stage of developing storms rather than on precursor tropical depressions that may or may not AFFILIATIONS: MCTAGGArt-COWAN—Numerical Weather intensify. Of the 2,125 tropical cyclones in the 1989– Prediction Research Section, Environment Canada, Dorval, Quebec, 2013 dataset, 2,026 reach the 35-kt threshold after a Canada; DAVIES—School of Earth, Atmospheric and Environmental median of 30 h of precursor tracking. An analogous Sciences, University of Manchester, Manchester, United Kingdom; investigation performed using a development- FAIrmAN AND SCHULTZ—Centre for Atmospheric Science, School time definition of the first IBTrACS entry shows of Earth, Atmospheric and Environmental Sciences, University of limited sensitivity as described in the “Sensitivity Manchester, Manchester, United Kingdom; GALARNEAU—Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona to development-time definition” section of the CORRESPONDING AUTHOR: David Schultz, Centre for supplement. Additionally, any storm with an initial Atmospheric Science, School of Earth, Atmospheric and intensity estimate greater than or equal to 35 kt in Environmental Sciences, University of Manchester, Simon Building, the best track record is rejected from further analysis Oxford Road, Manchester M13 9PL, United Kingdom because the early intensification stage is deemed to E-mail: [email protected] have been missed. This criterion eliminates a further The abstract for this article can be found in this issue, following the 269 storms, leaving a total of 1,757 storms for this table of contents. study (83% of the original dataset). In the “Sensitivity DOI:10.1175/BAMS-D-13-00254.1 A supplement to this article is available online (DOI:10.1175/BAMS-D-13 1 In the western North Pacific basin, the Koba et al. (1991) -00254.2) pressure–wind relationship is used to estimate the initial In final form 4 November 2014 intensity because wind speeds below 35 kt are not reported ©2015 American Meteorological Society in the Japan Meteorological Agency best track (Knapp et al. 2013). 1930 | NOVEMBER 2015 to initial intensity restriction” section of the supple- storms in this study. McTaggart-Cowan et al. (2013) ment, this condition is shown to be effective at elimi- use a linear discriminant analysis (Friedman 1989) to nating invalid IBTrACS entries without noticeably assign each tropical cyclone to one of five categories affecting results. Dare and McBride (2011) consider depending on a pair of metrics: lower-level thickness only formations that occur equatorward of 35°, a asymmetry (Th) and upper-level quasigeostrophic condition that is not applied here in recognition of forcing for ascent (Q). For the purposes of the current the fact that TT can occur at a relatively high latitude. study, the TT categories (weak TT and strong TT) are However, consistent with Dare and McBride (2011), considered independently, whereas the remaining storms classified as either subtropical or extratropical pathways (nonbaroclinic, low-level baroclinic, and in the best track archive are not considered in this trough induced) are combined into a non-TT group investigation in order to eliminate nontropical systems (Table 1). 2 The study thereby remains focused on the from the dataset. TT pathways in which the ingredients required for The Reynolds et al. (2007) SST dataset, available low-SST tropical cyclogenesis are found to reside. daily with a 0.25° grid spacing, is employed throughout The metric-based divisions between the develop- this study. The analyzed state is interpolated linearly ment pathways are shown in Fig. 2, from which it is for each storm to the formation time derived from the evident that the bulk of events fall into the non-TT best track record. A pair of definitions of development category (Table 1). An important distinction between SST is evaluated by Dare and McBride (2011): the point SST at formation time at the storm center and the maximum SST over the previous 48 h along the precursor track. In this study, we instead adopt a storm-centered area-averaging approach over a 2° radius in order to obtain a representative SST on the storm scale without introducing a potential bias from storms with short preformation tracks.
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