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Extratropical Transition of Southwest Pacific Tropical Cyclones. Part I Applied Aviation Sciences - Prescott College of Aviation 3-2002 Extratropical Transition of Southwest Pacific rT opical Cyclones. Part I: Climatology and Mean Structure Changes Mark R. Sinclair Embry-Riddle Aeronautical University, [email protected] Follow this and additional works at: https://commons.erau.edu/pr-meteorology Part of the Atmospheric Sciences Commons, and the Meteorology Commons Scholarly Commons Citation Sinclair, M. R. (2002). Extratropical Transition of Southwest Pacific rT opical Cyclones. Part I: Climatology and Mean Structure Changes. Monthly Weather Review, 130(3). https://doi.org/10.1175/ 1520-0493(2002)130<0590:ETOSPT>2.0.CO;2 © Copyright 2002 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (http://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected]. This Article is brought to you for free and open access by the College of Aviation at Scholarly Commons. It has been accepted for inclusion in Applied Aviation Sciences - Prescott by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. 590 MONTHLY WEATHER REVIEW VOLUME 130 Extratropical Transition of Southwest Paci®c Tropical Cyclones. Part I: Climatology and Mean Structure Changes MARK R. SINCLAIR Embry-Riddle Aeronautical University, Prescott, Arizona (Manuscript received 12 October 2000, in ®nal form 4 September 2001) ABSTRACT A database of tropical cyclone best track and intensity information for the southwest Paci®c Ocean basin is used to construct a 28-year climatology for tropical cyclones that move into middle latitudes. Of the nine or so tropical cyclones that form each year, an average of about three can be expected to migrate south of 358S, with the greatest fraction in March. Storms entering the Tasman Sea west of New Zealand (NZ) move almost due south on average and retain greater intensity than those to the east of NZ, where storms decay quickly while moving rapidly away to the southeast. Storms east of NZ are embedded in a stronger, more zonal ¯ow than those to the west, which move poleward ahead of a larger-amplitude trough. During El NinÄo years, tropical cyclones that move into middle latitudes exhibit stronger zonal motion and occur over a wider range of longitudes than during La NinÄa years. Storm intensity is only weakly correlated with concurrent SST anomalies, suggesting that atmospheric circulation is the dominant in¯uence on storm properties. Average structure changes during extratropical transition (ET) are identi®ed using the NCEP±NCAR reanalysis dataset, for a subset of 33 transitioning storms during 1980±97. Composites are used to construct a three- dimensional conceptual model of the transformation from a mature hurricane to an asymmetric baroclinic midlatitude cyclone. Southwest Paci®c tropical cyclones encounter the baroclinic westerlies early in their lives, accounting for their average eastward (and poleward) motion. At maximum average intensity near 208S, baroclinic effects are already important, with warm frontogenesis appearing in the southeast quadrant and out¯ow aloft into a downstream subtropical wind maximum that moves poleward with the storm. By 258S, the average TC has lost the characteristic symmetric anticyclonic out¯ow aloft and acquired the characteristics of a baroclinic midlatitude storm, including regions of warm and cold frontogenesis, a vertical motion dipole and a westward tilt with height. From about 308S poleward, a second upper-tropospheric wind maximum appears west of the storm, with strengthening cyclonic vorticity advection aloft. Below about 400 hPa, the storm retains the vertical, warm cyclonic core as it migrates poleward. 1. Introduction changes associated with the poleward movement of a TC are termed extratropical transformation. Extratropical transition (ET) occurs when a tropical As TCs move into middle latitudes, they often main- storm moves into middle latitudes and progressively ac- tain suf®cient vigor to produce damaging winds, high quires characteristics of a baroclinic midlatitude system. seas, and heavy rain in middle latitudes. Occasionally, As the tropical cyclone (TC) moves poleward, it loses TC remnants reintensify in the extratropics to become its tropical characteristics when it moves over cooler potent midlatitude storms capable of in¯icting loss of water and encounters the increasing vertical wind shears life and severe property damage. Several memorable ET associated with the midlatitude westerlies. During this events have occurred in the southwest Paci®c Ocean time, it typically loses its distinctive symmetric upper- basin. In April 1968, TC Gisele reintensi®ed as it moved level circulation and cloud shield, which commonly south over New Zealand (NZ), producing winds gusting shears off poleward and eastward. Eventually, increas- to 75 m s21 in the capital city of Wellington, and sinking ing thermal advection brings cooler and drier air into the interisland ferry Wahine with the loss of 51 lives the western sector of the storm, which progressively (Hill 1970). Cyclone Bola dumped over 900 mm of rain acquires the asymmetric cloud and thermal character- and produced hurricane-force winds in regions of north- istics of a midlatitude frontal cyclone. These structure ern NZ in March 1988 (Sinclair 1993b). A colorful de- scription of the ``Cyclone of '36'' (an ET event), prob- ably the most damaging storm of the twentieth century Corresponding author address: Dr. Mark R. Sinclair, Embry-Rid- to hit NZ, is found in Brenstrum (1997). Notable ex- dle Aeronautical University, 3800 Willow Creek Road, Prescott, AZ 86301. amples from the Northern Hemisphere (NH) include the E-mail: [email protected] regeneration of Hurricane Hazel as a powerful extra- q 2002 American Meteorological Society MARCH 2002 SINCLAIR 591 tropical storm over Canada (PalmeÂn 1958) and Hurri- piled basic statistics for North Atlantic ET events. For cane Agnes which produced one of the worst natural the Southern Hemisphere (SH), Foley and Hanstrum ¯ooding disasters in United States history over the (1994) assembled statistics and composite MSL pressure northeast of the country (Carr and Bosart 1978; DiMego analyses for TCs affecting the west coast of Australia. and Bosart 1982a,b; Bosart and Dean 1991). While max- For the southwest Paci®c Ocean basin, a number of imum winds in these transformed storms may be weaker basic summaries of TC activity have been undertaken than in the parent hurricane, the areas of damaging wind, (Kerr 1976; Revell 1981; Holland 1983; Thompson et high seas, and heavy rain are typically much larger (Hart al. 1992; Radford et al. 1996); its relation to the phase and Evans 2001), increasing the potential for loss of of the El NinÄo±Southern Oscillation (ENSO) cycle has life and property damage. been examined (Nichols 1984; Revell and Goulter In comparison with the effort devoted to the respec- 1986a,b; Hastings 1990; Basher and Zheng 1995); and tive study of tropical and extratropical cyclones, the descriptions of individual TCs have been published (Re- transformation from TC to extratropical cyclone is only vell 1985, 1986, 1987). However, these studies largely now starting to receive attention. Research agendas have omit any explicit discussion of ET. The decadal sum- been typically restricted to either tropical or midlatitude maries of Kerr (1976), Revell (1981) and Thompson et storms, with hybrid transition cases often falling be- al. (1992) analyze the tracks and intensities of all TCs tween the cracks. For example, there has never been a between 1949 and 1989, including the extratropical major ®eld campaign devoted to ET. This lack of at- phase. Their careful analysis provided much of the raw tention is surprising considering the loss of life and track and intensity data on which part of the present billions of dollars of damage caused by past ET events, study is based (see section 2). More detailed synoptic and the fact that they continue to be rather poorly fore- and diagnostic analysis of individual ET events that cast (Browning et al. 1998; Klein et al. 2000). Once the have impacted NZ includes the studies of Hill (1970), tropical storm ``goes extratropical,'' national hurricane Tomlinson (1975), Trenberth (1977), Littlejohn (1984), centers typically cease monitoring efforts like aircraft and Sinclair (1993a,b). While these case studies have reconnaissance, even though the potential for reinten- revealed speci®c insights about particular storms, there si®cation remains. Consequently, data on storms that remains a need for more systematic study of a larger retain or regain damaging winds and/or precipitation in number of past ET events, as advocated by the RPI middle latitudes are often incomplete, especially where Workshop participants. storms move away from land. This dif®culty is com- A preliminary climatology of TCs entering middle pounded by the practice of keeping tropical and extra- latitudes in the southwest Paci®c was prepared in an tropical storm databases separate, making it dif®cult to unpublished 1995 report to the Meteorological Service obtain coherent information on past ET events through- of New Zealand (C. Revell 1996, personal communi- out the life of the storm. Assessment of hazard risk is cation), using data from 196 cases of ET during 1969± thus hampered by the dearth of statistical information 89.
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