170 WEATHER AND FORECASTING VOLUME 11 Speci®c Tropical Cyclone Track Types and Unusual Tropical Cyclone Motions Associated with a Reverse-Oriented Monsoon Trough in the Western North Paci®c MARK A. LANDER University of Guam, Mangilao, Guam (Manuscript received 27 January 1995, in ®nal form 7 November 1995) ABSTRACT In its simplest description, the large-scale low-level circulation of summer over the western North Paci®c Ocean can be described in terms of low-latitude southwesterlies, a monsoon trough, and a subtropical ridge. When the axis of the monsoon trough is in its normal orientation (NW±SE), tropical cyclones tend to move northwestward on tracks close to those expected from climatology. As an episodic event, the axis of the monsoon trough extends farther north and east than normal and acquires a reverse (SW±NE) orientation. When the monsoon trough becomes reverse oriented, tropical cyclones within it tend to exhibit north-oriented motion and other speci®c unusual motions such as eastward motion at low latitude and binary interactions with other tropical cyclones along the trough axis. Approximately 80% of the tropical cyclones that are associated with a reverse- oriented monsoon trough move on north-oriented tracks. A tropical cyclone track type, de®ned herein as the ``S''-shaped track, is primarily associated with reverse orientation of the monsoon trough: 23 of 35 cases (66%) of S motion during the period 1978±94 occurred in association with a well-de®ned reverse-oriented monsoon trough. 1. Introduction the low-level monsoon ¯ow of the tropical WNP: the reverse-oriented monsoon trough (RMT). The RMT is It is universally recognized that the large-scale pe- shown to be associated with a speci®c tropical cyclone riodic reversal of wind currents over the Indian sub- (TC) track type and with certain speci®c unusual TC continent and in other regions commonly acknowl- motions. edged to possess a monsoonal climate (e.g., northern Very few attempts have been made to discriminate Australia and sub-Saharan Africa) is due to the sea- TC track type based upon the large-scale pattern of the sonal changes in the differential heating of continents WNP monsoonal ¯ow. Harr and Elsberry (1991, 1993, and oceans. Lacking the sharp land±sea contrast, a sim- and 1995a, hereafter HE91, HE93, and HE95a) at- ilar lower-tropospheric low-pressure trough is often tempted to discriminate TC track type based upon the found over the tropical western North Paci®c (WNP) anomalies of the large-scale circulation of the WNP. in the summer. This trough has several dynamic and Their results showed that, under certain conditions, a kinematic features that distinguish it from the inter- discrimination of TC track type can be made based tropical convergence zones (ITCZs) of the Atlantic and upon the pattern of the large-scale circulation. Carr and eastern North Paci®c (see Atkinson 1971; Sadler 1975; Elsberry (1994, hereafter CE94) developed a forecast- and Sadler et al. 1987). In order to emphasize these er's guide in which subjective recognition of the large- differencesÐin particular, the presence of deeply scale ¯ow pattern is crucial to the proper application of moist southwesterly wind ¯ow to the south of the their forecasting techniques for TC motion. The RMT trough axis in the WNPÐthe lower-tropospheric low- has been adopted by CE94 as one of the environmental pressure trough there will herein be called a monsoon ¯ow patterns that corresponds with speci®c TC mo- trough. tions. The monsoon trough of the WNP undergoes sub- Data sources are listed in section 2. A climatology stantial migrations and major changes to its shape and of the WNP monsoon trough and a summary some of orientation. This paper de®nes a recurring and easy-to- the commonly observed patterns of the monsoon cir- recognize major departure from the normal pattern of culation of the WNP are found in section 3. Section 4 presents a synthesis of the TC track types found in HE91 and HE95a with TC track types described in other sources [e.g., Sandgathe 1987; JMA (Japan Me- Corresponding author address: Dr. Mark A. Lander, Water and Energy Research Institute, University of Guam, UOG Station, Man- teorological Agency) 1976] that was performed in or- gilao, Guam 96923. der to form a small set of TC track types that included E-mail: [email protected] the majority of TCs and that left as few TC tracks as q 1996 American Meteorological Society /ams 3q04 0202 Mp 170 Thursday May 23 04:06 PM AMS: Forecasting (June 96) 0202 Unauthenticated | Downloaded 10/06/21 09:32 PM UTC JUNE 1996 LANDER 171 1) the hand-plotted surface synoptic charts plotted operationally at the Joint Typhoon Warning Center (JTWC), Guam, which contain SLP and winds re- ported from ships, land stations, and drifting buoys; cloud drift winds; and gradient-level wind reports from available upper-air stations; 2) high-resolution visible and infrared satellite im- agery accessible at the JTWC; 3) the U.S. Navy's operational numerical analyses of SLP and other data ®elds (e.g., 1000-hPa stream- lines); and 4) the Annual Tropical Cyclone Reports (ATCRs) issued by the JTWC. The charts of SLP and sequences of satellite imagery were the primary products used to determine the struc- ture and the evolution of the selected cases of reverse orientation of the WNP monsoon trough. A survey of past ATCRs and an archive of full-disk satellite im- agery helped to identify some of the commonly ob- served patterns of the monsoon circulation of the WNP. The positions and intensities (at 6-h intervals) of TCs in the WNP are published by the JTWC in their ATCRs. Also contained in issues of the ATCR are brief narrative discussions of the highlights of each TC. 3. The monsoon trough of the western North Paci®c a. Climatology FIG. 1. The low-level circulation during the summer in the Tropics of the western North Paci®c: (a) the long-term average and (b) a In its simplest description, the large-scale low-level schematic example of the low-level circulation associated with a re- circulation of summer over the WNP can be described verse-oriented monsoon trough. Bold zig-zag lines indicate ridge in terms of low-latitude southwesterlies, a monsoon axes, and the bold dashed line indicates the axis of the monsoon trough, and a subtropical ridge (Fig. 1a). The de®ning trough. Arrows indicate wind direction. The locations of Guam (G) and Tokyo (T) are indicated. characteristic of the WNP monsoon is the presence of deep, moist, southwesterly wind ¯ow to the south of the trough axis. Though loosely anchored to the region of highest sea surface temperature, the over-water mon- possible with indeterminate classi®cation (e.g., the soon trough of the tropical WNP undergoes substantial ``odd'' track type of Sandgathe). Typical characteris- migrations and major changes to its shape and orien- tics of the RMT are provided in section 5. In section 6, tation (unlike the monsoon trough over south Asia, the TC track type and speci®c unusual motions of TCs, which is ®rmly anchored by topography, and unlike the which form in an RMT, are discussed. The possible ITCZs of the eastern Paci®c and the Atlantic, whose bene®ts to TC forecasting of categorizing the large- scale ¯ow pattern are discussed in section 7. Section 8 summarizes the ®ndings. The general association of re- verse orientation of the monsoon trough of the WNP with a speci®c TC track type and with certain speci®c unusual TC motions, and the domination of reverse ori- entation of the WNP monsoon trough throughout the summer and fall of 1994, prompted the writing of this paper. 2. Data sources The evolution of the low-level wind, sea level pres- sure (SLP), deep convective clouds, and the TC mo- FIG. 2. Normal migration of the axis of the monsoon trough indi- tion associated with an RMT were documented using cated by its mean monthly positions during June±November (after the following data sources: Atkinson 1971). /ams 3q04 0202 Mp 171 Thursday May 23 04:06 PM AMS: Forecasting (June 96) 0202 Unauthenticated | Downloaded 10/06/21 09:32 PM UTC 172 WEATHER AND FORECASTING VOLUME 11 FIG. 3. Illustration of the extreme year-to-year differences that can occur in the monthly mean low-level wind ¯ow of the tropical western North Paci®c. (a) The resultant ¯ow of an August that featured an episode of an RMT monsoon trough: streamline analysis of the surface wind during August 1989. (b) An August with persistent easterly wind anomalies in the low latitudes and with the monsoonal southwesterlies displaced far to the north: streamline analysis of the surface wind during August 1988. axes do not normally stray far from their mean monthly mean, there is considerable interannual and episodic positions). The axis of the summer monsoon trough of variation. the WNP usually emerges from east Asia at about 207± 257N and extends southeastward to a terminus south- b. Interannual and episodic variation east of Guam (137N, 1457E). Long-term averages of low-level wind ¯ow and sea level pressure (see Sadler Eliminated by the averaging process is information et al. 1987) show that, during the boreal summer, the concerning the character and geographical bounds of mean eastward penetration of low-level winds with a episodic excursions and systematic migrations of the westerly component is to 1457E. Eastward of this lo- monsoon trough of the WNP. Large year-to-year dif- cation, the low-pressure trough is associated with the ferences in the individual summer monthly averages of con¯uent easterly wind ¯ow of the ITCZ.