DECEMBER 2001 ANNUAL SUMMARY 3015

Western North Paci®c, North Indian Ocean, and Tropical Cyclones of 1997

MARK A. LANDER AND CHARLES P. G UARD University of , Mangilao, Guam

(Manuscript received 14 February 2000, in ®nal form 3 January 2001)

ABSTRACT This paper is an annual summary of the western North Paci®c, north Indian Ocean, and Southern Hemisphere tropical cyclones of 1997. (Note: for the Southern Hemisphere, the 1997 annual total is accrued from July 1996 to June 1997.) The statistics presented are derived from records at the Joint Typhoon Warning Center (JTWC), Guam. Although the text focuses on the tropical cyclones that occurred in the western North Paci®c during 1997, it also includes brief summaries of the tropical cyclones in the north Indian Ocean, south Indian Ocean, and the South Paci®c. The 38 tropical cyclones in the Southern Hemisphere during 1997 were a record high, and the 23 typhoons in the western North Paci®c were second only to the 24 typhoons there in 1971. In the north Indian Ocean, the annual number of tropical cyclones was below normal. The large-scale circulation anomalies, and many aspects of the tropical cyclone distribution, were those typical of a major El NinÄo. Highlights of the 1997 tropical cyclone distribution in the western North Paci®c also include an unprec- edented number of very intense tropical cyclonesÐ11 became supertyphoonsÐand a large eastward displacement of the genesis locations.

1. Introduction hereafter AH) TC wind±pressure relationship. In some cases, the wind and pressure are reliably measured (such This summary of 1997 western North Paci®c, north as during the passage of Supertyphoon Paka over Indian Ocean, and Southern Hemisphere tropical cy- Guam), and the peak wind and minimum sea level pres- clones (TCs) was compiled from the archives of the sure may deviate from the AH relationship. To evaluate Joint Typhoon Warning Center (JTWC), Guam (JTWC the accuracy of the operational application of Dvorak's 1997). The JTWC is a joint U.S. Navy±Air Force ac- techniques, Velden et al. (1998) compared Dvorak TC tivity with a forecast area of responsibility that extends intensity estimates with those made simultaneously by from 180Њ westward to the coast of Africa, north and south of the equator. Seventy percent of the world's TCs aircraft. With 346 total matches of Dvorak intensity es- develop in this area. The Naval Paci®c Meteorology and timates (converted to minimum sea level pressure) made Oceanography Command at Pearl Harbor, , pro- by operational tropical analysis centers compared with vided TC advisories for Southern Hemisphere TCs east aircraft measurements in 10 recent Atlantic hurricanes, of 180Њ that are included in this summary. In compiling the mean bias of the Dvorak estimates was 5.91 hPa annual statistics on TCs within each of the basins where too high, and the rms error was 10.61 hPa. On Dvorak's the JTWC has TC advisory responsibilities, the calendar T-number intensity scale (with a range of T1±T8), this year is used for annual statistics in the Northern Hemi- rms error is roughly 0.5 T number. sphere, and the yearlong period ending on 30 June of Most of the measured winds cited for speci®c TCs in the same calendar year is used for the Southern Hemi- section 2b are the peak gusts. Many TC wind measure- sphere. ments in the western North Paci®c (WNP) are recorded In the majority of cases, the JTWC determines TC at sites on high islands or in rough terrain. The peak intensity by applying the well-known Dvorak tech- gusts at such sites are deemed more meaningful than niques (Dvorak 1975, 1984) to visible and infrared sat- the sustained winds, especially in areas where the ratio ellite imagery. The minimum central sea level pressure of the peak gust to sustained wind becomes very high. is then derived from the Atkinson and Holliday (1977, Overland sustained winds are dif®cult to relate to their corresponding overwater value unless the land rough- ness and exposure is well understood. The peak gust Corresponding author address: Mark A. Lander, Water and En- gives a better benchmark for what the potential destruc- vironmental Research Institute of the Western Paci®c, University of Guam, Mangilao, GU 96923. tive force of the wind may have been. Only at well- E-mail: [email protected] exposed places such as Waglin Island, Hong Kong, does

᭧ 2001 American Meteorological Society

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FIG. 2. Number of tropical cyclones of tropical storm or greater FIG. 1. Western North Paci®c area locator chart showing the po- intensity in the western North Paci®c (1960±97). sitions of some of the islands (e.g., Guam), cities (e.g., Manila), and geopolitical groupings of the islands (e.g., Caroline Islands) refer- enced frequently in the text. 4) a substantial eastward displacement of the mean gen- esis location for all TCs; 5) the formation east of the 180 meridian of two TCs the ratio between the peak gust and the sustained wind Њ that moved into the WNP and became superty- tend to be consistent and small. phoons; Because JTWC's primary focus is on the TCs of the 6) the landfall in the Philippines of only one TC that WNP, the summary of the TCs in this basin is more was at tropical storm intensity or greater; detailed than are the summaries of TCs in the other ba- 7) the simultaneous existence in the Philippine Sea of sins. An extensive summary for the WNP is found in two supertyphoons, each possessing an intensity of section 2, which is subdivided into two sections: an over- 160 kt (82 m sϪ1); and view of the annual statistics coupled with a discussion 8) a controversial potentially record-breaking land wind of the large-scale circulation, and a recap of the TC ac- speed of 236 mi hϪ1 (105 m sϪ1) measured on Guam tivity by month. Brief summaries for the north Indian during Typhoon Paka. Ocean and Southern Hemisphere are found in sections 3 and 4. Concluding remarks appear in section 5. Some of these unusual characteristics of the TCs in the WNP during 1997 are certainly related to the large-scale atmospheric and oceanic circulation anomalies associ- 2. Western North Paci®c tropical cyclones: ated with a strong El NinÄo±Southern Oscillation January±December 1997 (ENSO) event. Of the items in the above list, 3±6 are a. Statistics and large-scale circulation typical features of El NinÄo years. By some measures (e.g., the magnitude of the warming of the SST in the The WNP basin is bound to the south by the equator, eastern equatorial Paci®c), the El NinÄo of 1997 was one to the west by the Asian coastline, and to the east by of the strongest in recorded history (Climate Prediction the 180Њ meridian. It includes the South China Sea and Center 1997). other partly landlocked gulfs and seas such as the Yel- The annual number of TCs assigned a number by the low Sea, the Sea of Japan, and the Gulf of Thailand. JTWC in the WNP during 1997 (Table 1) was slightly Many small islands are scattered throughout this basin, above normal: 33 versus the climatological average of and are often used as reference points in the discussions 31. The year of 1997 included 11 supertyphoons, 12 of the TCs, either to provide a reference for a TC's less intense typhoons, 8 tropical storms, and 2 tropical location, or to provide direct ground-based observations depressions. The calendar year total of 31 TCs of at of the TCs or their environment. The islands of Micro- nesia are mentioned frequently; either individually (e.g., Guam, Kwajalein, and Pohnpei), or as part of their re- spective political or geographical group (e.g., the Mar- iana Islands, the , and the Caroline Is- lands) (Fig. 1). The distribution, character, and behavior of the TCs of the WNP during 1997 included 1) a very high number of supertyphoons [i.e., those typhoons with maximum sustained 1-min average surface winds Ն 130 kt (Ն67msϪ1)]; 2) the highest annual number of typhoons since 1971; 3) an early start of TC activity with a higher-than-av- erage number during the ®rst half of the year; FIG. 3. Number of western North Paci®c supertyphoons (1960±97).

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TABLE 1. Western North Paci®c 1997 tropical cyclone statistics. Tropical Maximum Minimum cyclone 1-min wind SLP no. Name Classa Datesb (m sϪ1) (mb) 01W Hannah TS 19±24 Jan 26 987 02W Isa STY 11±23 Apr 75 892 03W Jimmy TS 22±25 Apr 28 984 04W Kelly TS 7±10 May 23 991 05W Levi TS 25±30 May 23 991 06W Marie TY 26 May±2 Jun 46 954 07W Nestor STY 6±15 Jun 72 898 08W Opal TY 15±21 Jun 46 954 09W Peter TY 23±29 Jun 33 976 10W Rosie STY 18±28 Jul 72 898 11W Scott TS 24±25 Jul/27 Jul±2 Augc 28 984 12W Tina TY 29 Jul±9 Aug 46 954 13W Victor TY 30 Jul±3 Aug 33 976 14W Winnie STY 8±19 Aug 72 898 15W Yule TY 16±23 Aug 33 976 16W Ð TD 18±19 Aug 15 1000 17W Zita TY 21±23 Aug 39 967 18W Amber TY 21±30 Aug 57 933 19W Bing STY 21 Aug±5 Sep 69 904 20W Cass TS 28±30 Aug 23 991 02C d Oliwa STY 2±17 Sep 72 898 21W David TY 11±20 Sep 49 949 22W Fritz TY 20±25 Sep 39 968 23W Ella TS 21±25 Sep 21 994 24W Ginger STY 22±30 Sep 75 892 25W Hank TS 3±4 Oct 21 994 26W Ð TD 4±7 Oct 15 1000 27W Ivan STY 13±24 Oct 82 872 28W Joan STY 13±24 Oct 82 872 29W Keith STY 27 Oct±8 Nov 80 879 30W Linda TY 31 Oct±9 Nov 33 976 31W Mort TY 10±16 Nov 33 976 05C d Paka STY 2±21 Dec 82 901e

a TDÐtropical depression, wind speed less than 17 m sϪ1; TSÐtropical storm, wind speed 17±32 m sϪ1; TYÐtyphoon, wind speed 33 msϪ1 or higher; STY; supertyphoon, subset of the typhoon category with wind speed greater that 66 m sϪ1. b Dates begin at 0000 UTC and include only the period of warning. c Scott regenerated and thus has two periods of warning. d These TCs formed in the central North Paci®c and were named by the Central Paci®c Hurricane Center, Honolulu. e The Atlantic TC wind±pressure relationship was used for Paka because the environmental SLP was closer to Atlantic norms, and the minimum SLP recorded in the on Guam was too high for its wind speed using the normal western North Paci®c wind±pressure relationship (Atkinson and Holliday 1977). least tropical storm intensity was three higher than the During the spring of 1997, El NinÄo (i.e., very warm climatological average (Fig. 2). The calendar year total SST in the central and eastern equatorial Paci®c) de- of 23 typhoons was ®ve above the long-term average, veloped rapidly, and was coupled with a large drop in and is the highest annual number of typhoons recorded the magnitude of the Southern Oscillation index (Fig. in the WNP basin since 1971, when there were 24. The 4). Unusually persistent low-level westerly wind ¯ow executive summary of JTWC's 1997 Annual Tropical became established at low latitudes in the WNP. This Cyclone Report (JTWC 1997) describes 1997 as, ``the westerly wind ¯ow was also displaced eastward from year of the super typhoon.'' A supertyphoon is a TC its normal domain (Fig. 5). The setup of low-level, low- with a maximum sustained 1-min wind Ն 130 kt (Ն67 latitude westerly wind ¯ow early in the year led to the msϪ1), which is close to the 135 kt (69 m sϪ1) threshold early establishment of a near-equatorial trough across of the Saf®r±Simpson category 5 hurricane. While the Micronesia from the western Caroline Islands eastward JTWC has long been the only agency to use the ``su- into the Marshall Islands. This trough supported the per-'' category, the India Meteorology Department re- development of several TCs early in the year. The only cently has adopted the ``supercyclone'' category. The 11 statistic of numbers of TCs in the WNP found by Lander supertyphoons is an unprecedented number, exceeding (1994) to be signi®cantly correlated with an ENSO in- by 4 the previous annual high of 7 supertyphoons re- dex is an increase in the number of TCs occurring in corded in the years 1971, 1987, 1989, and 1991 (Fig. 3). the ``early season'' (de®ned in his paper as the period

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of an extreme eastward displacement of the monsoon trough. During the period 1960±91, the ®ve years with the highest annual average of the Southern Oscillation index (SOI) (i.e., 1975, 1973, 1988, 1971, and 1974) had an average of 2.4 TCs form east of 160ЊE. The ®ve years with the lowest annual average SOI (i.e., 1982, 1987, 1991, 1977, and 1972) had an average of 7.4 TCs form east of 160ЊE (cf., Lander 1994). In Fig. 6b, the area east of 160ЊE and south of 20ЊN is designated as the ``El NinÄo box.'' During 1996, only 1 TC formed within the El NinÄo box, while 10 TCs formed there during 1997 (including Oliwa and Paka). During 1997, there was a tendency for low-level mon- soon westerly winds to persist at low latitudes and for the axis of the monsoon trough to remain near 10ЊN across Micronesia (Figs. 5 and 7). For much of the year, low-level westerly wind anomalies persisted throughout Micronesia with the largest westerly wind anomalies FIG. 4. Anomalies from the monthly mean for eastern equatorial located at low latitudes near and to the east of the 180Њ Paci®c Ocean sea surface temperature (SST) (hatched) in ЊC and the meridian (Climate Prediction Center 1997). Corre- Southern Oscillation index (SOI) (shaded) for the period 1996±97. sponding conditions in the upper troposphere consisted The rise in the eastern equatorial Paci®c SST during 1997 is of near- record magnitude: J ϭ Jan. [Adapted from Climate Prediction Center of easterly wind anomalies over most of the low lati- (1997).] tudes of the WNP. These large-scale atmospheric ¯ow pattern anomalies of 1997 were nearly everywhere the reverse of those persisting in the WNP for most of 1995 1 March±15 July). During 1997 there were eight TCs and 1996, and are typical of those observed during an in this early season window versus an average of four. El NinÄo year. The annual mean genesis location of TCs that form Despite the nearly continuous presence of the mon- in the WNP is related to the status of ENSO: it tends soon trough and abundant deep , the number to be east of normal during El NinÄo years and west of of TCs of at least tropical storm intensity was near nor- normal during those years characterized by large-scale mal. The TCs of 1997 tended to emerge one by one climatic anomalies opposite to those of El NinÄo, years from the eastern portion of the basin and then recurve known as La NinÄa or ENSO cold phase. Consistent with or move north, with each subsequent development at the TC distribution typically associated with El NinÄo low latitude tending to occur after the prior TC had (or an ENSO warm phase), the annual mean genesis exited the Tropics. Many of the TCs were large, very location for all TCs during 1997 was substantially east intense, and slow moving. There were relatively few of normal (Fig. 6a). This was a pronounced change from cases of multiple TCs (i.e., the simultaneous occurrence the TC distributions during 1995Ða La NinÄa year (Tren- of two or more) in the WNP during 1997. The most berth 1997)Ðand during 1996 when this position was noteworthy case of multiple TCs was the simultaneous west of normal. A breakdown of the genesis locations and spatially proximate formation and development of of all the individual WNP TCs of 1997 (Fig. 6b) shows Ivan (27W)1 and Joan (28W) in October. While in the that most formed east of 140ЊE, 16 formed east of 155ЊE, Philippine Sea, each attained an extreme intensity of and 11 formed east of 160ЊE. Two of the TCs that 160 kt (82 m sϪ1); the ®rst time in the JTWC archives formed east of 160ЊE, Oliwa and Paka, were named in that two TCs of such extreme intensity coexisted in the the central Paci®c by the Central Paci®c Hurricane Cen- WNP. The westernmost of these TCs, Ivan (27W), was ter in Honolulu before moving into the WNP. By virtue of their formation in the monsoon trough, these two 1 The JTWC appends the following suf®xes to the TC number: TCs may reasonably be regarded as typical WNP TCs W ϭ western North Paci®c, A ϭ Arabian Sea, B ϭ Bay of Bengal, of monsoonal origin, albeit under the unusual condition S ϭ South Indian Ocean, P ϭ South Paci®c.

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FIG. 5. Comparison between climatological (black) and analyzed (shaded) mean monthly winds with a westerly component for the WNP in 1997. For reference, the star indicates the location of Guam. The outline of Australia appears in the lower left of each panel except for Jun±Sep where the Korean peninsula and Japan appear in the upper left. Box domain from left to right is 120ЊE±180Њ; bottom to top is 20ЊS±20ЊN (except for Jun±Sep when bottom to top is 0Њ±40ЊN to include the subtropics of the WNP). The climatology is adapted from Sadler et al. (1987). The 1997 monthly mean winds were adapted from the Climate Prediction Center (1997).

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FIG. 7. Schematic illustration of the low-level circulation pattern which dominated the WNP for much of 1997. The summer ¯ow pattern is indicated, but the ¯ow of the other seasons was similar in that westerlies persisted at low latitudes in the eastern half of the basin. Arrows indicate wind direction, dashed line indicates the axis of the monsoon trough; Cs indicate cyclonic circulation centers, G ϭ Guam, and T ϭ Tokyo.

FIG. 6. (a) Mean annual genesis locations for the period 1970±97. 1997. Composite best tracks are provided for the periods For 1997, the location is indicated by the arrow. The star lies at the 1 January±2 August (Fig. 9a), 21 July±22 September intersection of the 27-yr average latitude and longitude of genesis. For statistical purposes, genesis is de®ned as the ®rst 25-kt (13 m sϪ1) (Fig. 9b), and 17 September±31 December (Fig. 9c). intensity on the JTWC best track. (b) Point of formation of signi®cant tropical cyclones in 1997 as indicated by the initial intensity of 25 kt (13 m sϪ1) on the best track. The symbols indicate solid dots ϭ b. Summary of monthly activity 1 Jan±15 Jul, open triangles ϭ 16 Jul±15 Oct; and, X ϭ 16 Oct±31 Dec. 1) JANUARY During November of 1996, episodes of strong low- the only TC of at least tropical storm (TS) intensity to level monsoon westerlies began to occur in the low make landfall in the Philippine archipelago during 1997. latitudes of the WNP. Most of the WNP TCs of No- A low number of landfalling TCs in the Philippines and vember and December 1996 were associated with these along the coast of Asia (excluding Japan) has been cited episodes of enhanced low-level westerly ¯ow. The si- as an effect of El NinÄo (e.g., Dong 1988). During the multaneous occurrence of TCs in the Southern Hemi- decade 1987±96, an annual average of approximately sphere, some of them twins to WNP TCs, was a notable seven TCs of at least TS intensity made landfall in the characteristic of the TC distribution as 1996 came to a Philippine archipelago. close. Twin TCs form simultaneously north and south Despite the low number of TCs to make landfall in of the equator, at low latitude, and at nearly the same eastern Asia, two that didÐWinnie (10W) and Linda longitude; they are often almost mirror images of one (31W)Ðcaused much loss of life and great destruction another (e.g., Lander 1990). Although the Southern at their respective landfall sites in China and Vietnam. Hemisphere became the dominant site of TC formation Mainland Japan, the Ryukyu Islands, the Volcano and by January 1997, there were some episodes of enhanced Bonin Islands, and the and other island westerly wind ¯ow at near-equatorial latitudes in the groups of Micronesia were each affected by several ty- WNP that enhanced the potential for the formation of phoons. The last TC of 1997 in the WNP, Supertyphoon an off-season TC. Paka (05C), affected the Marshall Islands and the islands During such a time of enhanced westerly ¯ow along of Guam and in the Mariana Islands. On Guam, a the equator, the tropical disturbance that became Hannah controversial potentially record-breaking land surface (01W) formed in the near-equatorial trough south of the wind gust of 236 mi hϪ1 (105 m sϪ1) occurred during Marshall Islands (®rst JTWC warning valid at 0600 passage of Paka over that island on the night of 16 UTC 19 January). Moving on a long westward track for December [the extant record surface wind gust of 231 over two weeks, it reached a peak intensity of 50 kt (26 mi hϪ1 (103 m sϪ1) occurred at the observatory atop msϪ1) and then dissipated in the Philippine Sea. Al- , NH, in April 1934]. Paka's contro- though Tropical Storm Hannah was in most aspects a versial wind gust is discussed in the next section. typical off-season TC, it was, in retrospect, an early An illustration of the TC activity in the entire JTWC manifestation of an unusual large-scale tropical circu- area of responsibility during 1997 is provided in Fig. 8. lation pattern that would see many of the TCs of 1997 Table 1 lists the TCs in the western North Paci®c during form well east of normal.

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2) FEBRUARY It then encountered a shear line (see the appendix) and dissipated over water during the ®nal week of April. In keeping with February's climatology as the month of lowest TC frequency in the WNP, there were no num- bered TCs in the WNP basin during February. 5) MAY As May began, monsoon westerlies persisted across 3) MARCH the low latitudes of the eastern half of Micronesia, and Tropical Storm Kelly (04W) formed in the southern The quiescent conditions of February continued into Marshall Islands (®rst JTWC warning valid at 0600 March, and there were no numbered TCs. UTC 7 May). It was a relatively weak TC that moved slowly to the northwest. It later turned westward, ac- celerated, and dissipated over water in an unfavorable 4) APRIL environment of westerly vertical wind shear. During most of April, a monsoon trough stretched During mid-May, the monsoon trough weakened, across Micronesia, and westerly low-level winds per- amounts of large-scale deep convection lessened, and sisted at low latitudes. During the second week of the the Tropics of the WNP became inactive. The next ep- month, sea level pressures fell across the eastern Car- isode of TC development commenced during the ®nal oline Islands, abundant deep convection increased, and week of May when two TCs formed at opposite ends a monsoon depression developed [see the appendix for of the basin: Tropical Storm Levi (05W) in the South JTWC de®nitions and descriptions of monsoon depres- China Sea (®rst JTWC warning valid at 1800 UTC 25 sions, tropical depressions, and other types of TCs; see May) and Typhoon Marie (06W) at low latitudes near Harr and Elsberry (1996) for a detailed description of 160ЊE (®rst JTWC warning valid at 1800 UTC 26 May). the transformation of a monsoon depression to a tropical As a tropical depression, Levi moved eastward across storm]. The monsoon depression moved westward, con- Luzon where it caused severe ¯ooding in Metro-Manila. solidated into a tropical storm, intensi®ed, and became After entering the Philippine Sea, it turned to the north, Supertyphoon Isa (02W) (®rst JTWC warning valid at intensi®ed, and reached its peak of 45 kt (23 m sϪ1). It 1800 UTC 11 April). Isa was the ®rst of the 11 super- eventually recurved on 28 May, merging with the Mei- typhoons of 1997. The abundant deep convection as- yu front (Chou et al. 1990; Chen et al. 1998) south of sociated with the monsoonal band from which Isa Japan. Marie initially moved westward, then turned to emerged dropped 20 in. (ϳ500 mm) of rain in 24 h on the north and maintained a northward track for several Pohnpei. These torrential rains contributed to a deadly days. While intensi®cation was initially slow, Marie landslide that killed 19 people on the island. On the eventually reached a peak intensity of 90 kt (47 m sϪ1). night of 16 April, Isa passed 140 n mi (260 km) south Shortly thereafter, Marie recurved and became extra- of Guam, where a peak wind gust of 61 kt (31 m sϪ1) tropical. was measured and where peripheral rainbands of the typhoon produced 24-h rainfall amounts of 6±10 in. 6) JUNE (150±250 mm) across the island. The system reached its peak intensity of 145 kt (75 m sϪ1) as it moved slowly Levi and Marie were still active in early June as they northward, northwest of Guam (Fig. 10a). The spring accelerated into midlatitudes, became extratropical, and onset of persistent westerly low-level winds in Micro- crossed the 180Њ meridian to become midlatitude lows nesia was cited in near±real time as evidence that a northwest of Hawaii. Meanwhile in the Tropics of the strong El NinÄo event was under way (Paci®c ENSO WNP,low-latitude monsoon westerlies continued to per- Applications Center 1997). The ``year of the superty- sist across Micronesia, and three TCsÐNestor (07W), phoon'' was off to an early start. Opal (08W), and Peter (09W)Ðformed in the monsoon As Isa was recurving, Tropical Storm Jimmy (03W) trough. Supertyphoon Nestor (07W) began as a mon- formed at a low latitude in the near-equatorial trough soon depression in the Marshall Islands and became the that extended across the southern Marshall Islands (®rst second supertyphoon of 1997 (Fig. 10b). Developing in JTWC warning valid at 0600 UTC 22 April). This small the southern Marshall Islands, Nestor was the farthest TC moved northwest and intensi®ed, reaching a peak east a TC has become a typhoon in June since JTWC of 55 kt (28 m sϪ1) as it made a turn to the northeast. records began in 1959. The monthly total of three ty-

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FIG. 9. (a) Composite best tracks for the western North Paci®c Ocean tropical cyclones for the period 1 Jan±2 Aug 1997. (b) Composite best tracks for the western North Paci®c Ocean tropical cyclones for the period 21 Jul±22 Sep 1997. (c) Composite best tracks for the western North Paci®c Ocean tropical cyclones for the period 17 Sep±31 Dec 1997. Each track begins at the ®rst identi®cation of a cyclonic vortex that became the TC, and the dot on each track indicates the location of the ®nal JTWC warning. Note that most tracks continue past the ®nal warning to include extratropical stages of the TC.

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FIG. 10. (Continued)

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FIG. 10. An image gallery of the 11 supertyphoons of 1997: (a) Isa at peak intensity when northwest of Guam [(2131 UTC 19 Apr visible Geostationary Meteorological Satellite (GMS) imagery]. (b) Nestor near its peak intensity when approximately 355 km east-northeast of (1547 UTC 9 Jun enhanced infrared imagery using the NOAA-NESDIS BD enhancement curve). (c) Rosie near peak intensity (2131 UTC 22 Jul visible GMS imagery). (d) Winnie nears its peak intensity of 140 kt (72 m s Ϫ1) as it approaches the (2133 UTC 11 Aug visible GMS imagery). (e) Bing's intensity was 130 kt (67 m s Ϫ1) in this image showing a smooth eyewall with a very well de®ned eye (0334 UTC 1 Sep visible GMS imagery). (f) the low-angle morning sun nicely highlights the features of the cloud tops of Oliwa's eyewall and peripheral rainbands as the typhoon reached its peak of 140 kt (72 m s Ϫ1) (2034 UTC 9 Sep visible GMS imagery). (g) possessing extensive banding features, Ginger nears its peak intensity of 145 kt (75 m s Ϫ1) (2132 UTC 26 Sep visible GMS imagery). (h) The low sun-angle of evening brings out relief in Keith's bands and central cloud shield at a time when its intensity had weakened slightly between two separate peaks of 155 kt (79 m sϪ1) (0632 UTC 2 Nov visible GMS imagery). (i) Ivan and Joan cross the supertyphoon threshold in this image on their way to their extreme intensities of 160 kt (82 m s Ϫ1) (2132 UTC 16 Oct enhanced infrared GMS imagery, enhancement curve is MB). (j) concentric eyewalls appear in IR imagery as Paka makes its way across the island of Guam (0732 UTC 16 Dec infrared GMS imagery). This IR image is nearly coincident with the NEXRAD image presented in Fig. 13.

Unauthenticated | Downloaded 10/03/21 08:34 PM UTC 3026 MONTHLY WEATHER REVIEW VOLUME 129 phoons in June 1997 was also an extreme event for a After becoming a tropical storm, the northward-moving June, equaled before only in 1963 and again in 1965. TC intensi®ed to a typhoon by 0000 UTC 21 July and The tropical disturbance that became Supertyphoon reached its peak intensity of 140 kt (72 m sϪ1) at 1200 Nestor was ®rst identi®ed on 1 June as a persistent area UTC on July 22. Twelve hours later, Rosie began to of convection located about 70 n mi (130 km) south- weaken and slowly accelerated toward the north-north- southwest of (®rst JTWC warning valid at 0000 east. The system made landfall near Okayama on the UTC 6 June). On the morning of 8 June, the TC veered Japanese island of Shikoku around 0800 UTC on 26 away from the Mariana Islands on a north-northwest- July as a minimal typhoon with 65 kt (33 m sϪ1) winds. ward track and continued intensifying for four days, Crossing over land, Rosie rapidly weakened as the main reaching peak intensity of 140 kt (72 m sϪ1) at 1200 convection sheared away from the low-level circulation UTC on 10 June, approximately 200 n mi (370 km) center. It continued to weaken in the Sea of Japan as northeast of Saipan. The system later turned to the north the exposed low-level circulation center and remnant and recurved. Iwo Jima recorded a wind gust to 102 kt rain were tracked southeastward back over Japan (51 m sϪ1) at 1500 UTC 12 June when the typhoon and into the Paci®c where it dissipated. Rosie left two passed about 20 n mi (37 km) to the east. A few hours dead in Japan, and its passage resulted in power failures, later, at 0200 UTC 13 June, the typhoon passed about landslides, and widespread damage to buildings in the 30 n mi (56 km) west of Chichijima, where a wind gust southern and central parts of the country. to 96 kt (50 m sϪ1) was recorded. While most of WNP TCs of 1997 developed in the As Nestor (07W) was recurving, another monsoon monsoon trough at low latitudes, Tropical Storm Scott depression, originating in the eastern Caroline Islands, (11W) formed north of 20ЊN in direct association with consolidated and ultimately became Typhoon Opal a cyclonic circulation in the tropical upper tropospheric (08W) (®rst JTWC warning valid at 0000 UTC 15 June). trough (Sadler 1975). As a tropical depression (®rst Opal moved northward and reached its peak intensity JTWC warning valid at 0600 UTC 24 July), the system of 90 kt (47 m sϪ1) at 1200 UTC 17 June when located was sheared from the out¯ow of Supertyphoon Rosie in the Philippine Sea about midway between Guam and (10W) and failed to intensify as it moved on an unusual Taiwan. It later became the ®rst TC of the season to hit southeast track. After a few days, it reversed direction Japan, when it made landfall in southern Honshu. Opal for 24 h. It then made a right turn and reached its peak then accelerated north of Tokyo, entered the Paci®c intensity of 55 kt (29 m sϪ1) while moving to the north- Ocean, and became extratropical. east. On 2 August, it approached the 180Њ meridian, After Opal (08W) recurved, yet another monsoon de- merged with a frontal system, and dissipated. pression, also originating in the eastern Caroline Islands, The tropical disturbance that became Typhoon Tina consolidated, intensi®ed, and ultimately became Ty- (12W) originated in the monsoon trough in the eastern phoon Peter (09W) (®rst JTWC warning valid at 0600 Caroline Islands. For over a week, organization was UTC 23 June). During the last week of June, Peter ap- very slow as the disturbance moved to the northwest. proached Luzon, but abruptly turned north and became On 29 July, the system became Tropical Depression a minimal typhoon of 65 kt (34 m sϪ1) as it neared the (TD) 12W (®rst JTWC warning valid at 1800 UTC 29 Ryukyu Islands. After Peter reached 30ЊN, it turned to July), and on 5 August it reached its 90-kt (47 m sϪ1) the northeast, made landfall in Kyushu, and traversed peak intensity. Tina then turned to the north, passed nearly the entire length of Honshu. Exiting Honshu on between Taiwan and Okinawa, made landfall in southern 28 June, the weakened tropical storm reentered the Pa- Korea, crossed the Sea of Japan, and dissipated as it ci®c. The next day, it merged with a frontal system and made landfall in Hokkaido. completed its extratropical transition. As an extratrop- As Tina was developing in the eastern part of the ical system south of the Kamchatka Peninsula, the rem- basin, the cloud system that became Typhoon Victor nants of Peter became more intense than the system had (13W) was consolidating west of Luzon in the South been as a TC, with peak winds of 70 kt (36 m sϪ1). China Sea. The system moved on a northward track and Moving eastward, it weakened and dissipated as it intensi®ed slowly in an environment of northerly upper- crossed the 180Њ meridian on 4 July. level shear (®rst JTWC warning valid at 1800 UTC 30 July). It ®nally reached minimal typhoon intensity just prior to making landfall near Waglin Island, Hong Kong, 7) JULY on 2 August. After the extratropical remnant of Typhoon Peter (09W) dissipated on 4 July, there was not another named 8) AUGUST TC in the WNP basin until 19 July when Supertyphoon Rosie (10W) was upgraded from a tropical depression August was extremely busy with a total of 10 TCs (®rst JTWC warning valid at 1800 UTC 18 July) to a spending some part of their life in the month. As Tina tropical storm. Rosie became the third supertyphoon of (12W) and Victor (13W) were maturing in the western 1997 (Fig. 10c). It originated in the monsoon trough as portion of the WNP basin, yet another monsoon de- a tropical disturbance in the western Caroline Islands. pression was developing in the Marshall Islands. This

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wall, Typhoon Yule (15W) and Tropical Depression 16W were organizing in a monsoon trough that extended from the Caroline Islands eastward beyond 180Њ. The disturbance that became Yule consolidated at the ex- tremely low latitude of 3ЊN at the same time as the system that became TD 16W developed east of 180Њ and a bit farther north (near 10ЊN) (®rst JTWC warning for Yule was valid at 1800 UTC 16 August, and for TD 16W at 0000 UTC 18 August). The two TCs engaged in a direct binary interaction culminating in merger (Carr and Elsberry 1994; Lander 1995) with Yule mov- ing to the north-northeast and TD 16W moving to the west. Yule became the dominant circulation and sub- sumed TD 16W. Yule brie¯y attained typhoon intensity FIG. 11. Ginger and its extensive pattern of primary and peripheral cloud bands are isolated in an otherwise relatively cloud-free Tropics when it was near Wake Island. Later, while moving on (0033 UTC 27 Sep visible GMS imagery). a long northward track, it reacquired typhoon force winds as a system exhibiting some characteristics of a subtropical cyclone (Hebert and Poteat 1975). While monsoon depression intensi®ed into Supertyphoon Win- slowly weakening, the system ®nally recurved at almost nie (14W), the fourth of 1997's 11 supertyphoons (Fig. 50ЊN. 10d). Forming at low latitudes in the Marshall Islands, Typhoon Zita (17W) was one of three TCs to reach it was one of several of the 1997 TCs that originated typhoon intensity in the South China Sea during 1997. in the El NinÄo box of Fig. 6b (®rst JTWC warning valid Developing in the monsoon trough about 300 n mi (560 at 0600 UTC 8 August). Winnie became a typhoon at km) to the west of Luzon (®rst JTWC warning valid at 0000 UTC on 10 August, and reached its peak of 140 0000 UTC 21 August), it moved in a northward direc- kt (72 m sϪ1) at 0000 UTC on 12 August. Winnie main- tion for a while before it turned to the west. Despite its tained a relatively straight west-northwest to northwest proximity to China's south coast, it intensi®ed substan- track across the Paci®c from the Marshall Islands to the tially, reaching a peak intensity of 75 kt (39 m sϪ1)as coast of China. It passed over Okinawa, moved across it tracked across the Luichow Peninsula. Zita maintained the East China Sea, and made landfall on the eastern this intensity while crossing the Gulf of Tonkin to its coast of China approximately 140 n mi (260 km) south landfall in Vietnam on the morning of 23 August. of Shanghai shortly before 1200 UTC on 18 August. The pre±Typhoon Amber (18W) tropical disturbance While passing over Okinawa, Winnie possessed con- formed southwest of Guam and moved on a slow west- centric eyewalls that were readily apparent in conven- ward, then northwestward, track toward Taiwan as it tional visible and infrared satellite imagery, microwave became a TC (®rst JTWC warning valid at 0600 UTC satellite imagery, and in imagery from the Next Gen- 21 August) and intensi®ed. After becoming a tropical eration Weather Surveillance Radar-1988 Doppler (NE- storm on 21 August, Amber intensi®ed slightly faster XRAD WSR-88D) radar located at Kadena Air Base. than the normal Dvorak rate of one T-number per day The diameter of Winnie's outer eyewall during passage (Dvorak 1975, 1984), and reached 100 kt (52 m sϪ1) over Okinawa was one of the largest ever observed in by the morning of 25 August. After Amber reached its a TC (Lander 1999). The radar at Kadena Air Base, peak intensity, Tropical Storm Cass (20W) began to Okinawa, indicated 100-kt (51 m sϪ1) winds in the large form southwest of Amber in the South China Sea, about outer eyewall in a layer from 3000 ft (0.9 km) to 6000 160 n mi (295 km) south of Hong Kong (®rst JTWC ft (1.8 km). warning valid at 0000 UTC 28 August). Strong upper- As Winnie passed through the northern Mariana Is- level northeast winds that appeared to be part of the lands, the populated islands of Guam, Rota, , and out¯ow from Typhoon Amber likely inhibited Cass's Saipan (well to the south of Winnie's track, but within intensi®cation. On 28 August, these two TCs underwent its gale area) reported damage to crops and vegetation a binary interaction that accelerated Amber toward Tai- from high winds and sea salt spray. In parts of Taiwan, wan and caused Cass to move slowly to the east. After Winnie dropped 28 in. (711 mm) of rain, and 27 people some ¯uctuations of intensity, Amber reached its peak were reported killed when an apartment building col- of 110 kt (57 m sϪ1) just prior to landfall on Taiwan lapsed. Another 12 people were reported killed from on 29 August. Amber weakened over the mountainous mudslides, ¯ooding, and high wind. In mainland China, island, and later made landfall on mainland China. Once torrential rains and winds caused at least 75 deaths. Amber moved over Taiwan, Cass moved to the north Damage from wind and ¯ooding was extensive. Mon- and intensi®ed to its peak of 45 kt (23 m sϪ1). Cass soon winds associated with Winnie overturned a ferry also made landfall in mainland China, 150 n mi (280 in the Philippines killing 26 people. km) west of Taiwan, and later dissipated over the moun- As Winnie was forming its large outer concentric eye- tains of southern China.

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While Amber (18W) was developing southwest of h. According to Dvorak (1975, 1984), the average rate Guam, the disturbance that became Supertyphoon Bing of TC intensi®cation is one T number per day, or a 24- (19W) developed near the eastern end of the monsoon h pressure drop of approximately 15 mb for tropical trough in the Marshall Islands. The system was up- storms, 20±25 mb for mid-range typhoons, and 30 mb graded to TD 19W on 27 August (®rst JTWC warning for intense typhoons. Holliday and Thompson (1979) valid at 0600 UTC 27 August) and tracked toward the identi®ed rapid deepening of a TC as a decrease in its Mariana Islands. On the afternoon of 29 August, Trop- minimum sea level pressure of Ն42 mb for 24 h. Many ical Storm Bing, possessing 40-kt (21 m sϪ1) sustained typhoons that reach high peak intensities [i.e., more than winds, passed through the 20-n mi (40-km) channel that 100 kt (51 m sϪ1)] undergo a period of rapid intensi- separates Guam and Rota. After passing Guam, Bing ®cation, but few undergo explosive deepening. During began to rapidly intensify, and 54 h later, it reached its 1997, only two TCs underwent explosive deepeningÐ peak intensity of 135 kt (70 m sϪ1), becoming the ®fth Oliwa and Ginger. It is important to repeat here that supertyphoon of the year (Fig. 10e). After Bing reached the majority of the TC intensity estimates for the WNP its peak intensity of 135 kt (69 m sϪ1), satellite and are made using the Dvorak techniques (Dvorak 1975, microwave imagery indicated the development of con- 1984) and that minimum sea level pressure is derived centric eyewalls. Over a 48-h period, Bing underwent from an empirical TC wind±pressure relationship (i.e., a complete eyewall replacement cycle (Willoughby et Atkinson and Holliday 1977). al. 1982). Near 143ЊE, Bing slowed its forward motion Oliwa made landfall in southwestern Japan where it and turned to the north. It moved northward for three was responsible for widespread damage and for loss of days, until it recurved to the northeast about 300 n mi life. On Japan's southern island of Kyushu, seven people (555 km) south of eastern Japan. Bing's forward speed were reported killed. One thousand homes were ¯ooded then accelerated to 30 kt (56 km hrϪ1) as it transitioned and dozens of homes were destroyed. Along Korea's into a 55-kt (29 m sϪ1) extratropical cyclone on 5 Sep- southern coast, 28 ships sank or were wrecked in strong tember. The more than 5 in. (ϳ125 mm) of rain on Guam winds and high waves. A crabbing ship with a crew of associated with Bing caused considerable local ¯ooding 10 aboard was reported missing. and contributed to Guam's record August rainfall of The disturbance that became Typhoon David (21W) 38.49 in. (978 mm). formed east of the 180Њ meridian in the El NinÄo±ex- tended monsoon trough. After becoming a tropical de- pression (®rst JTWC warning valid at 1800 UTC 12 9) SEPTEMBER Sep), it moved to the northwest, while gradually inten- September was also a busy month with ®ve TCs. At sifying at a normal rate of one T number per day. The the end of August, Supertyphoon Oliwa (02C) became TC was large, and a track more northward than indicated the sixth supertyphoon of 1997 (Fig. 10f). It formed by statistical and dynamic guidance was attributed to from a tropical disturbance to the southwest of Hawaii the ``beta effect'' (e.g., Holland 1983) incurred by the in a portion of the WNP monsoon trough that had ex- TC's large size. David attained its peak intensity of 95 tended abnormally far to the east. As it intensi®ed, this kt (49 m sϪ1) on the morning of 15 September, then tropical disturbance was upgraded by the Central Paci®c recurved, passed south of Japan, and became extratrop- Hurricane Center (located in Honolulu, HI) ®rst to Trop- ical on 21 September en route to the Gulf of Alaska. ical Depression 02C, and later to Tropical Storm Oliwa. While David (21W) was recurving southeast of Japan, On 4 September, Oliwa crossed the 180Њ meridian and the disturbance that became Supertyphoon Ginger entered JTWC's area of responsibility. The ®rst warning (24W), the seventh supertyphoon of 1997 (Fig. 10g), issued by the JTWC was valid at 0600 UTC 4 Septem- was consolidating near the 180Њ meridian and was one ber. After it entered the WNP basin, Oliwa moved on of 10 TCs that formed east of 160ЊE and south of 20ЊN, a steady west-northwestward track and intensi®ed. At within the El NinÄo box shown on Fig. 5b (®rst JTWC ®rst, the rate of intensi®cation was slow; during the 102- warning valid at 1800 UTC 22 September). It moved h period from 0600 UTC on 4 September to 1200 UTC out of this box on a northwestward track in the eastern on 8 September, Oliwa's intensity increased from 35 kt portion of the WNP basin. After becoming a typhoon, (18 m sϪ1; minimal tropical storm) to 65 kt (33 m sϪ1; Ginger explosively deepened, and in the 24-h period minimal typhoon). Then, during the 24-h period from from 0000 UTC 26 September to 0000 UTC 27 Sep- 1800 UTC 8 September to 1800 UTC 9 September, tember, the intensity jumped from 75 kt (39 m sϪ1)to Oliwa's intensity climbed from 75 kt (39 m sϪ1) to its 145 kt (75 m sϪ1). The 24-h estimated pressure drop peak of 140 kt (72 m sϪ1). The 24-h pressure drop associated with this wind speed increase was 75 mb. associated with this wind speed increase was 69 mb, for This easily quali®es as a case of explosive deepening, an average of 2.9 mb hϪ1 (please see italicized cau- as de®ned by Dunnavan (1981). When Ginger reached tionary note at the end of this paragraph). This quali®es 30ЊN, it accelerated within the midlatitude westerlies as a case of explosive deepening, de®ned by Dunnavan where it transformed into a vigorous extratropical low. (1981) as a drop of minimum sea level pressure Ն2.5 The cloud system of this TC when near its peak was mb hϪ1 for at least 12 h, or Ն5mbhϪ1 for at least 6 isolated in a large-scale environment that was unusually

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The tropical disturbance that became Hank was ®rst observed on 27 September in the South China Sea, and the ®rst warning was issued on 3 October based on ship observations of 25±35 kt (12±18 m sϪ1) wind speeds. The system moved erratically, and easterly vertical wind shear prevented it from intensifying beyond 40 kt (21 msϪ1). Hank made landfall in northern Vietnam on 5 October and dissipated soon thereafter. As Hank was developing in the South China Sea, Tropical Depression 26W formed southeast of Guam (®rst JTWC warning valid at 0000 UTC 4 October). The disturbance initially moved northward, then turned to the west and passed north of Guam on 3 October, where it attained its peak intensity of 30 kt (15 m sϪ1). TD 26W maintained this intensity for 3 more days, but west- erly vertical wind shear appeared to be a limiting factor FIG. 12. Objective Dvorak T numbers for (a) Ivan and (b) Joan. on its intensity. As an exposed low-level circulation, Objective Dvorak T numbers (small black dots) were derived from TD 26W merged with a frontal cloud band over the an algorithm installed on the image-processing equipment of the Philippine Sea. JTWC. Warning intensity is shown by open circles. The 4.5 days of supertyphoon intensity for Joan [Ն130 kt (67 m sϪ1)] is a record. The eighth and ninth supertyphoons of 1997, Super- Note also the near-simultaneous peak of these two typhoons. typhoon Ivan (27W) and Supertyphoon Joan (28W) were two of three TCs in the WNP during 1997 to attain the extreme intensity of 160 kt (82 m sϪ1). They reached free of deep convection, other TCs, and tropical dis- their peak intensities at nearly the same time: Ivan at turbances (Fig. 11). A reduction of deep convection 1800 UTC 17 October and Joan at 0600 UTC 17 Oc- throughout Micronesia and within the low latitudes of tober. At 1200 UTC 17 October, Ivan's intensity was the WNP basin became more pronounced during the 155 kt (80 m sϪ1) while Joan's was still at 160 kt (82 latter half of 1997 as the low-level monsoon westerlies msϪ1); the ®rst observation of two TCs of such extreme and their associated deep convection moved eastward intensity existing simultaneously in the WNP (Fig. 10i). to the and beyond. The separation distance between the two TCs at this Typhoon Fritz (22W) began as an area of enhanced time was 1090 n mi (a bit over 2000 km)Ðwell over convection in the South China Sea. As this tropical dis- the threshold distance of 780 n mi (1450 km) noted by turbance moved away from the coast of Vietnam, it Brand (1970) for binary (i.e., direct) TC interactions to slowly intensi®ed (®rst JTWC warning valid at 1800 occur. Indeed, the centroid-relative motion of Ivan and UTC 20 Sep). After a few days of eastward movement, Joan (not shown) does not indicate that any form of TC Fritz turned back to the west toward Vietnam and con- interaction took place. tinued to intensify. It reached a peak intensity of 75 kt Joan remained at or above the supertyphoon threshold (39 m sϪ1) at 0000 UTC 24 September, which it main- (130 kt, 67 m sϪ1) for 4.5 daysÐa record. Ivan recurved tained until it made landfall in Vietnam on 25 Septem- in the Luzon Strait, and after weakening, became ex- ber. The system dissipated over land, but torrential rains tratropical south of Japan. Joan also recurved, and while triggered landslides that killed 25 people, many of traveling eastward along 30ЊN, became an intense ex- whom were gold prospectors. tratropical cyclone. The T-number estimates for both Tropical Storm Ella (23W) developed from a very Ivan and Joan (Fig. 12) reached T8.0 [equivalent to 170 small low-level circulation that originated east of 180Њ. kt (87 m sϪ1) intensity]. It is unlikely, however, that in By 21 September, convection associated with the low- the absence of aircraft (or a reliable ground measure- level circulation became well organized, prompting the ment) that the intensity of a WNP typhoon will ever be ®rst JTWC warning valid at 0000 UTC 21 September. reported to exceed 160 kt (82 m sϪ1) and thus tie or Ella sped to the west-northwest at 18±25 kt (33±46 overtake Supertyphoon Tip's intensity record of 870 mb km hϪ1), nearly double the climatological average speed and 165 kt (85 m sϪ1), which was derived from aircraft of TCs at low latitude in the WNP, and reached its peak measurements. Hence, Ivan and JoanÐand also PakaÐ intensity of 40 kt (21 m sϪ1) on 22 September. It re- were given peak warning intensities of 160 kt (82 m sϪ1) curved and dissipated on 24 September near 40ЊN, despite Dvorak analysis that indicated that the intensity 170ЊE. of these TCs may have been higher. Why Ivan and Joan became so intense is unknown. Early in their lives (®rst JTWC warnings on Ivan and 10) OCTOBER Joan were valid at 0600 UTC 13 October), neither ob- Tropical Storm Hank (25W) was the shortest-lived jective guidance nor human forecasters anticipated the TC of the season, with advisories issued for only 36 h. extreme intensities that Ivan and Joan would reach. The

Unauthenticated | Downloaded 10/03/21 08:34 PM UTC 3030 MONTHLY WEATHER REVIEW VOLUME 129 initial disturbances from which they developed were and the typhoon began to weaken and recurve. A few very poorly organized and were isolated in an environ- days later, it was speeding at 45 kt (83 km hϪ1) to the ment that was unusually free of deep convection. At that east-northeast and becoming extratropical. time, the monsoon trough across the WNP was relatively When it reached its peak intensity, Keith was moving weak and free of deep convection, and sea level pres- west-northwest and was just over a day away from pass- sures were near or above normal. For Ivan, nearly all ing through the Mariana Islands. During the 6-h period intensity forecasts leading up to its peak were low by 0600±1200 UTC 2 November, Keith passed between as much as 40 kt for the 12-h forecast, and 45, 50, 45, the Mariana Islands of Rota and Tinian, which are only and 50 kt for the 24-, 36-, 48-, and 72-h forecasts, re- 40 n mi (75 km) apart. Though weakened slightly from spectively. For Joan, many of the intensity forecasts its peak, it was still a powerful 140-kt (72 m sϪ1) su- were low by an even greater margin, and nearly all pertyphoon. NEXRAD WSR-88D imagery from Guam forecasts for the entire life of this TC were too low. indicated that the eyewall of Keith never touched land Leading up to its peak, the intensity forecasts for Joan as it threaded the narrow channel between these two were low by as much as 30, 55, 65, 65, and 65 kt for islands. As such, these two islandsÐand Saipan, which the 12-, 24-, 36-, 48-, and 72-h forecasts, respectively. is separated from Tinian by an ocean channel only 3 n Despite the slow passage of these two TCs across much mi wideÐwere spared the full force of Keith. Never- of the WNP basin, the monthly average wind for Oc- theless, Keith caused considerable damage on the is- tober (Fig. 4) was more easterly than normal everywhere lands of Rota, Tinian, and Saipan. Red Cross of®cials except at low latitudes east of 150ЊE. reported that at least 790 houses were destroyed or dam- Both Ivan and Joan affected the Mariana Islands. On aged on these islands. About 15 power poles were re- the night of 14 October, Ivan passed 55 n mi (100 km) ported downed on Saipan, and 20 on Tinian. A wind to the south of Guam where a peak wind gust of 41 kt gust of 95 kt (49 m sϪ1) was recorded at Saipan's In- (21 m sϪ1) was recorded at ; ternational Airport. Sea level pressure fell to 964 mb the heaviest 24-h rainfall of 5.85 in. (148.6 mm) was on Rota and to 977 mb on Saipan. On Guam, little also recorded at Andersen. Ivan also affected the Phil- damage occurred, but power was knocked out to the ippinesÐthe ®rst and only TC of 1997 to make landfall entire island for nearly a day. Wind gusts reached 67 in the Philippines while still a tropical storm or typhoon. kt (35 m sϪ1) and upward of nearly 6 in. (152 mm) of At least one person was reported drowned and another rain fell on parts of the island. As Keith approached, missing on the northeastern tip of Luzon. Ivan damaged very large surf from the east pushed rubble onto the thousands of houses and destroyed large amounts of rice coastal road on the southeast side of the island, forcing and corn in this region. More than $500,000 (U.S.) of®cials to close it. worth of ®sh stocks in ponds and cages were also de- stroyed. Joan largely spared the Mariana Islands any 11) NOVEMBER signi®cant damage when it passed between the islands of Saipan and Anatahan on 18 October. Peak wind gusts The disturbance that became Typhoon Linda (30W) of 85 kt (44 m sϪ1) were experienced on Saipan when developed at the end of October near 10ЊN about 200 Joan passed approximately 45 n mi (80 km) to the north. n mi (370 km) east of the Philippines (®rst JTWC warn- A Red Cross initial assessment indicated that Joan de- ing valid at 1800 UTC 31 October). The system moved stroyed 4 houses, caused major damage to 15 other tin- westward and reached tropical storm intensity within 24 and-wood structures, and caused minor damage to 17 h of moving into the South China Sea. The system con- homes on Saipan. On Guam, winds gusted to only 33 tinued to intensify as it approached the Ca Mau province kt (17 m sϪ1) at the commercial port on the west side of Vietnam on 2 November, and reached typhoon in- of the island. tensity in the Gulf of Thailand. The typhoon weakened As Ivan and Joan began to recurve, yet another dis- while crossing the Malay Peninsula, but reintensi®ed in turbance was developing in the monsoon trough in the the Bay of Bengal. Linda was the ®rst TC since Typhoon eastern Caroline and Marshall Islands. This disturbance Forrest (30W) in 1992 to cross the Malay Peninsula, would become the 10th of the 11 supertyphoons of retain at least TS intensity, and reintensify to a typhoon 1997ÐSupertyphoon Keith (29W). After several days again in the Bay of Bengal. After attaining typhoon of westward movement and dif®culty in organizing, the intensity in the Bay of Bengal on 6 November, Linda system ®nally consolidated and began to intensify (®rst began to steadily weaken, as a result of unfavorable JTWC warning valid at 1800 UTC 27 October). Upon vertical wind shear. Four days later, it dissipated over reaching 105 kt (55 m sϪ1), the typhoon began to rapidly the bay. Linda caused considerable damage and loss of intensify, peaking at 155 kt (81 m sϪ1) 24 h later. The life in Vietnam and Thailand, with over 330 people small eye and narrow but intense eyewall of Keith reported killed and 2250 missing. passed between the islands of Saipan and Rota in the Typhoon Mort (31W) was the last TC of November Mariana Islands. Keith remained a supertyphoon for 3.5 and the last TC of 1997 to form in the WNP basin. Mort days as it moved to the northwest at the end of the formed in a weak monsoon trough south of Guam (®rst month. On 4 November, Keith's forward motion slowed, JTWC warning valid at 1800 UTC 10 November).

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While moving to the west, the system reached its peak intensity of 65 kt (34 m sϪ1) in the Philippine Sea on 12 November. After some ¯uctuation of intensity, ver- tical wind shear ®nally pushed the deep convection to the south and away from the low-level circulation center, and Mort began to dissipate. On 16 November, Mort made landfall on the east coast of Luzon as a tropical depression.

12) DECEMBER While no TCs originated in the WNP during Decem- ber 1997, one of the most destructive of the year, Su- pertyphoon Paka (05C), moved into the basin from the central North Paci®c as a tropical storm. Although trade winds dominated the Tropics of the WNP for most of December, low-level westerly wind ¯ow persisted at eastern longitudes near the international date line. Not since the strong 1982/83 El NinÄo had monsoonal west- erlies pushed so far to the east. Twin near-equatorial troughs extended along 8ЊN and 5ЊS from near 160ЊE FIG. 13. A view of Paka from Guam's NEXRAD WSR-88D at to about 160ЊW. This led to the formation of twin TCs: 0712 UTC 16 Dec 1997. The image shows Paka with concentric eyewalls. The outer eyewall is making its way across the island of Paka (05C), in the , and Pam Guam while the inner eyewall is in the process of collapsing as it (07P98) in the Southern Hemisphere. Pam moved south, moves over water between Guam and Rota. Data are NEXRAD WSR- became a hurricane, and passed through French Poly- 88D base re¯ectivity. Yellow indicates re¯ectivity values of 40 dBZ nesia on its way to the higher latitudes of the South or higher. Guam is approximately 30 n mi (50 km) in length, and the Paci®c. Meanwhile, Paka moved west and did not in- distance between Guam and Rota is approximately 40 n mi (70 km). tensify much until it crossed 180Њ on 7 December, and became a typhoon in the Marshall Islands. Continuing moving west at 6 kt (11 km hϪ1), was centered 15 n mi on a west-northwestward track, it intensi®ed and became (28 km) north of GuamÐits closest point of approach the last of the record 11 super typhoons in the WNP to the island. The outer eyewall crossed the northern during 1997. During the night of 16 December, Paka two-thirds of the island, and only the northern half of passed over Guam causing over $600 million (U.S.) in the island experienced a period of relatively calm wind damages. The intense typhoon continued to move west- while in the moat between the outer eyewall and the northwestward into the Philippine Sea where it attained inner. Microwave imagery shows that Paka underwent its extreme estimated intensity of 160 kt (82 m sϪ1)Ð an eyewall replacement cycle during the period between the third typhoon to do so during 1997. The TC then its ®rst and second peaks. While over Guam the typhoon slowed as it encountered an approaching shear line; and, was approximately midway through the cycle. After it by ingestion of stable air, increasing vertical wind shear, passed Guam, the inner eyewall collapsed, the outer and possible adverse changes to the upper-ocean thermal eyewall contracted, and Paka reintensi®ed to its absolute structure, Paka gradually dissipated over water. peak of 160 kt (82 m sϪ1) brie¯y on 18 DecemberÐ Majuro and Kwajalein atolls both received peak wind the third typhoon of 1997 to attain this extreme intensity. gusts of over 40 kt (20 m sϪ1) as Paka passed near on On Guam, high winds were the cause of most of the 10 and 11 December, respectively. Alinglapalap and Ja- damage. The Commercial Port National Weather Ser- luit atolls received the brunt of 90-kt (47 m sϪ1) winds, vice (NWS) Handar wind instrument at and experienced considerable damage to structures and reported sustained (peak) gust of 100 (149) kt [51 (77) vegetation, and signi®cant coastal erosion from waves. msϪ1] which is plausible; the Andersen Air Force Base The typhoon moved west-northwest and reached its ®rst (AFB) anemometer (an FMQ-13 hot plate instrument) relative peak of 140 kt (72 m sϪ1) on 15 December. recorded 96 (205) kt [49 (105) m sϪ1] which was not Paka was then a very serious threat to the southern considered representative ( Marianas. 1998). Had it stood, the Andersen wind gust would have A day away from Guam, Paka slowed, and there were greatly exceeded the highest wind gust ever recorded signs of weakening. Now within NEXRAD WSR-88D in a typhoonÐ166 kt (85.3 m sϪ1) (Joint Typhoon Warn- radar range, the inner structure of Paka was revealed. ing Center 1997); and would have set a new world re- There were concentric eyewallsÐa primary wall cloud cord for the highest surface wind speed ever measured; approximately 40 n mi (75 km) in diameter and a sec- the 231-mph (103 m sϪ1) wind gust recorded atop ondary fragmented inner wall cloud 10 n mi (19 km) Mount Washington, , still retains that in diameter (Fig. 13). At 1200 UTC 16 December, Paka, title. The Guam Commercial Port sensor failed after

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TABLE 2. North Indian Ocean 1997 tropical cyclone statistics. Tropical Maximum Minimum cyclone 1-min wind SLP no.a Name Classb Datesc (m sϪ1) (mb) 01B Ð H 14±20 May 59 927 02B Ð H 24±27 Sep 33 976 03A Ð TS 8±9 Nov 18 997 04A Ð TS 10±14 Nov 28 984

a The numbers are according to the JTWC: A ϭ Arabian Sea, B ϭ Bay of Bengal. The TCs in the north Indian Ocean were not named by the New Delhi RSMC. b TDÐtropical depression, wind speed less than 17 m sϪ1; TSÐtropical storm, wind speed 17±32 m sϪ1; HÐhurricane wind speed 33 msϪ1 or higher. c Dates begin at 0000 UTC and include only the period of warning. recording4hof135±149 kt (69±77 m sϪ1) gusts in the storm surge. One of the world's worst disasters occurred outer eyewall. The Andersen AFB sensor lost power in 1970 when 300 000 lives were lost when a powerful during passage of the western side of the outer eyewall. TC made landfall there. A similar TC struck the coastal Additionally, the NWS sensor at Tiyan lost power dur- regions of Bangladesh during April 1991 and devastated ing the onset of the outer eyewall, the JTWC anemom- the coastal city of Chittagong with winds Ͼ130 kt (67 eter at Nimitz Hill failed at 103 kt (53 m sϪ1) before msϪ1) and a 20-ft (6 m) storm surge. The of®cial death the eyewall arrived, the anemometer at the Apra Harbor toll in 1991 was estimated at 138 000 and the damage tide gauge failed in the outer eyewall, and the NWS at $1.5 billion (U.S.). Handar instrument at the University of Guam, Mangilao, weathered the entire storm to report a peak gust to 123 kt (63 m sϪ1). In the ®nal analysis, the Handar instru- a. Annual statistics and the large-scale circulation ment at Apra Harbor became the benchmark. It faith- During 1997, four numbered TCs occurred in the fully recorded peak gusts up to 149 kt (77 m sϪ1) until north Indian Ocean. Two of these were in the Bay of the winds began backing to the southwest, at which Bengal and two in the Arabian Sea (Table 2). Spring point it too failed. and fall in the north Indian Ocean are periods of tran- In general, the damage assessment of northern Guam sition for the monsoons, and are the most favorable indicated a mixture of tropical cyclone scale categories seasons for TC activity because the monsoon trough axis 3 and 4 [Saf®r±Simpson hurricane scale as modi®ed by at these times is over water and the strong vertical wind Guard and Lander 1999] depending upon the exposure shear associated with the summer monsoon is greatly at the sites. This provides a wide range of maximum reduced. This year was no exception. The total number, sustained wind speeds from 96±115 kt (49±59 m sϪ1) four, was one under the JTWC 23-yr average of ®ve. for category 3 to 116±135 kt (59±69 m sϪ1) for category Only twoÐTC 01B and TC 02BÐreached hurricane 4. The $600 million (U.S.) level of destruction on Guam intensity. resulted in a presidential declaration of the island as a In this major El NinÄo year, one of the largest anom- disaster area. With a combined population (160 000) on alies of the large-scale atmospheric circulation over the Guam and Rota, there was no loss of life as a direct north Indian Ocean was a persistent easterly anomaly result of Paka's passage. of the low-latitude surface wind (Climate Prediction With the dissipation of Paka, ``the year of the super- Center 1997). This anomaly, present in the spring, in- typhoon'' in the WNP came to a close. By the end of tensi®ed in the fall. During the months of peak TC ac- December, high pressure, persistent easterly winds, and tivity, an easterly anomaly in low latitudes represents a reduced amounts of deep convection prevailed in the weaker westerly ¯ow south of the monsoon trough. This Tropics of the basin. ENSO-related drought conditions is consistent with a reduced number of TCs in the north worsened and would reach record proportions in the ®rst Indian Ocean in 1997, and only two TCs of hurricane half of 1998. Tropical cyclone activity shifted into the intensity. Southern Hemisphere. b. Noteworthy tropical cyclones 3. North Indian Ocean annual summary: January±December 1997 1) TROPICAL CYCLONE 01B Tropical cyclones in the north Indian Ocean (espe- Tropical Cyclone 01B emerged from a poorly orga- cially in the Bay of Bengal) have been some of the nized area of convection embedded within the near- deadliest in history. The Bay of Bengal, particularly the equatorial trough, and became the ®rst and most intense low-lying Ganges River delta region of Bangladesh, is TC of 1997 in the north Indian Ocean. The system slow- the most dangerous TC basin in the world in terms of ly developed as it drifted in a generally northward di-

Unauthenticated | Downloaded 10/03/21 08:34 PM UTC DECEMBER 2001 ANNUAL SUMMARY 3033 rection within the Bay of Bengal. At 0600 UTC on 19 the World Meteorological Organization±designated re- May, it peaked at 115 kt (59 m sϪ1). This intensity was gional specialized meteorological centers (RSMCs) re- maintained until landfall occurred in Bangladesh shortly sponsible for TC advisories in the Southern Hemisphere. after 1200 UTC on 19 May. Tropical Cyclone 01B Some of these discrepancies may be related to JTWC's caused signi®cant damage and several hundred casu- use of a 1-min sustained wind versus the 10-min sus- alties in Bangladesh. tained wind used by the RSMCs. Small differences in interpretation of satellite imagery, especially for the weaker TCs, may also play a role in whether a TC 2) TROPICAL CYCLONE 02B obtains both a number from the JTWC and a name from Tropical Cyclone 02B began as an area of disturbed the responsible RSMC. weather in the western Bay of Bengal. The disturbance The annual total of JTWC-numbered TCs during the continued to improve in organization through 21 Sep- 1997 Southern Hemisphere season (by JTWC conven- tember, remaining quasi-stationary in the monsoon tion: 1 July 1996±30 June 1997) was 38. This was 11 trough. After 21 September, the system began to move more than the overall mean for the previous 16 yr. Of slowly northwestward, then, on 24 September, a devel- these 38 TCs there were 23 of hurricane intensity and oping midlatitude trough northwest of the TC shifted 15 tropical storms (Table 3). The regional distribution the steering ¯ow to southwesterly and by 26 September, of TCs in the Southern Hemisphere during 1997 was the forward motion had increased from 6 kt (3 m sϪ1) increased everywhere over the regional distribution of to 14 kt (7 m sϪ1). Tropical Cyclone 02B increased in TCs during 1995 and 1996. In retrospect, the latter half intensity as it tracked along the eastern coast of India, of 1996 was an onset to the major El NinÄo of 1997. and reached a peak of 65 kt (33 m sϪ1) approximately Strong equatorial westerly wind bursts occurred in No- 12 h before making landfall in Bangladesh on 27 Sep- vember and again in December 1996. The December tember. Forty-seven people were reported killed and burst was associated with a proli®c outbreak of TCs in more than 1000 injured as heavy surf, rain, and wind both the Northern and Southern Hemispheres. On 24 gusts of 80 kt (40 m sϪ1) swept the coastline. December 1996 TCs Fergus, Phil, and Ophelia were active in the Southern Hemisphere while TCs Greg and 3) TROPICAL CYCLONE 03A Fern were active in the Northern Hemisphere. In the spring of 1997, unusually strong westerly winds became The disturbance that became Tropical Cyclone 03A established along the equator between monsoon troughs was embedded within a widespread area of convection in each hemisphere. For the JTWC, there are two South- associated with a trough over the Arabian Sea, and well ern Hemisphere ocean basins for warning purposes: the offshore of the southwest coast of India. The system South Indian Ocean (west of 135ЊE) and the South Pa- tracked steadily west-northwest toward the coast of So- ci®c (east of 135ЊE). These are identi®ed by appending malia, and reached a peak of only 35 kt (18 m sϪ1)as the suf®xes S and P, respectively, to the TC number. it approached the coast. On 9 November, TC 03A made Although TC activity was enhanced in each of these TC landfall over the northeastern tip of Somalia. The par- regions, there was a tendency for the TC activity to tially exposed low-level center quickly dissipated after occur predominantly in the south Indian Ocean, S, from landfall. July 1996 to January 1997, and in the South Paci®c, P, from February to June 1997. The S (P) count was 15(7) 4) TROPICAL CYCLONE 04A from July 1996 to January 1997, and 5(11) from Feb- In early November, the tropical disturbance that be- ruary to June 1997. From March through June, seven came Tropical Cyclone 04A was located over Sri Lanka. of eight TCs formed in the South Paci®c. This eastward On 7 November, the disturbance moved west over the shift of TC activity was associated with a general and lower tip of India and on 8 November it entered the gradual eastward shift of El NinÄo±enhanced low-level Arabian Sea. The ®rst warning was issued by the JTWC westerly winds during late 1996 through 1997. Occur- valid at 0000 UTC on 10 November. Thirty hours later, ring during the second week of June 1997, the ®nal TC when it was halfway across the Arabian Sea from India of this proli®c Southern Hemisphere season, TC 38P to Africa, TC 04A reached its peak intensity of 55 kt (Keli), was the latest recorded date for a major hurricane (28 m sϪ1). It held this intensity for just over 24 h, then in the South Paci®c. began weakening under vertical wind shear. The re- maining low-level circulation weakened, lost latitude, 5. Concluding remarks and dissipated over water well east of the African coast. During 1997, the global distribution of TCs continued in the pattern of 1996 with abundant TCs in the Atlantic 4. Southern Hemisphere annual summary: (July (Pasch and Avila 1999; Rappaport 1999), fewer than 1996±June 1997) normal TCs in the eastern North Paci®c, and an above As in the north Indian Ocean, some differences exist average number of TCs in the western North Paci®c. between the JTWC TC statistics and those reported by Overall, 1997 was a very active year in the JTWC area

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TABLE 3. Southern Hemisphere 1997 (from JTWC for Jul 1996±Jun 1997) Tropical Cyclone Statistics. Tropical Maximum Minimum cyclone 1-min wind SLP no.a Name Classb Datesc (m sϪ1) (mb) 01S Lindsay TS 10±11 Jul 18 997 02S Ð TS 17±20 Aug 23 991 03S Ð TS 7±9 Sep 21 994 04S Antoinette H 17±21 Oct 33 976 05S Melanied H 29 Oct±11 Nov 64 916 06P Cyril TS 23±26 Nov 26 987 07S Chantelle H 24±29 Nov 33 976 08S Daniella H 2±10 Dec 62 922 09S Elvina TS 9±14 Dec 28 984 10P Nicholas TS 13±15 Dec 23 991 11S Ophelia TS 14±20 Dec 28 984 12P Phile H 23 Dec±2 Jan 9±12 Jan 44 958 13P Fergus H 24±30 Dec 46 954 14S Fabriola TS 2±8 Jan 31 980 15S Rachel H 2±8 Jan 41 963 16P Drena H 3±10 Jan 62 922 17P Evan H 10±16 Jan 36 972 18S Ð TS 11±13 Jan 23 991 19S Panchof H 19 Jan±6 Feb 64 916 20S Gretelle H 20±31 Jan 59 927 21S Iletta H 23±28 Jan 39 968 22P Fredae H 26±30 Jan/31 Jan±2 Feb 33 976 23S Josie H 8±16 Feb 46 954 24P Gillian TS 10-12 Feb 23 991 25S Karlette H 16±26 Feb 33 976 26P Harold H 16±26 Feb 38 964 27S Ð TS 19±25 Feb 23 991 28P Ita TS 24±24 Feb 18 997 29P Ð TS 26±27 Feb 23 991 30S Lizette H 27 Feb±2 Mar 39 968 31P Gavin H 3±12 Mar 59 927 32P Justin H 6±25 Mar 46 954 33P Hina H 13±18 Mar 39 968 34P Ian TS 17±19 Apr 28 984 35P June H 2±5 May 33 976 36S Rhonda H 10±16 May 51 944 37P Ð TS 28±30 May 18 997 38P Keli H 10±15 Jun 59 927

a The numbers are according to the JTWC: P ϭ South Paci®c, S ϭ South Indian Ocean. The names are provided by the responsible WMO- designated RSMC (e.g., La ReÂunion, Perth, Darwin, Brisbane, Fiji, Port Moresby). b TDÐtropical depression, wind speed less than 17 m sϪ1 ; TSÐtropical storm, wind speed 17±32 m sϪ1; TYÐtyphoon, wind speed 33 msϪ1 or higher; STY: supertyphoon, subset of the typhoon category with wind speed greater that 66 m sϪ1. c Dates begin at 0000 UTC and include only the period of warning. d Melanie was renamed Bellamine when it passed from Perth's area of responsibility (AOR) into the AOR of La ReÂunion. e Phil and Freda regenerated and thus have two periods of warning. f Pancho was renamed Helinda when it passed from Perth's AOR into the AOR of La ReÂunion. of responsibility with a total of 71 TCs of TS intensity along the equator to the south of Hawaii as easterly low- or higher versus a normal total of 60. By contrast, there level wind anomalies (replacing earlier westerly wind were 52 such TCs during 1995 and 69 during 1996. anomalies) became ®rmly established across Micronesia Contributing to the high number of TCs in Eastern and all low-latitude areas westward from there. The ma- Hemisphere was a well-above-normal number of TCs jor El NinÄo of 1997 was running its course with such in the Southern Hemisphere, and a slightly higher-than- certainty that the Paci®c ENSO Applications Center normal number of TCs in the western North Paci®c. El (1997) con®dently (and correctly) predicted a major NinÄo±enhanced low-latitude, low-level westerly winds drought in Micronesia during the ®rst half of 1998, and were the highlight of the large-scale circulation pattern a shift, in 1998, of the TC activity to the west of normal. of 1997. Month by month, these westerly winds pushed eastward, and dragged the TC activity in both the North- Acknowledgments. Support for the preparation and ern and Southern Hemispheres eastward with them. By publication of this paper was provided by the Of®ce of the end of 1997, westerly winds had pushed eastward Naval Research through Grant N00014-98-1-0744. The

Unauthenticated | Downloaded 10/03/21 08:34 PM UTC DECEMBER 2001 ANNUAL SUMMARY 3035 help of the personnel at the Joint Typhoon Warning ganized cyclonic wind circulation. [In Dvorak's satellite Center in allowing us access to their satellite imagery TC intensity estimation techniques, there are four basic and other meteorological data is greatly appreciated. A cloud patterns that compose the suite of ``conventional'' special thanks is owed to Rey Dalisay for his work on TCs: 1) the shear pattern, 2) the curved band pattern, the graphics. 3) the central dense overcast (CDO) pattern, and 4) the eye pattern. Additional tropical cyclone cloud-pattern APPENDIX types seen in the western North Paci®c (e.g., the mon- soon depression, and the monsoon gyre) are not ad- De®nitions Excerpted from JTWC's 1996 Annual dressed by the Dvorak techniques.] Tropical Cyclone Report Tropical depressionÐA tropical cyclone with max- imum sustained 1-min mean surface winds of 33 kt (17 Mei-yu frontÐThe term mei-yu is the Chinese ex- msϪ1) or less. pression for ``plum rains.'' The Mei-yu front is a per- Tropical disturbanceÐA discrete system of appar- sistent east±west zone of disturbed weather during ently organized convection, generally 100±300 n mi spring that is quasi-stationary and stretches from the (185±555 km) in diameter, originating in the Tropics or east China coast, across Taiwan, and eastward into the subtropics, having a nonfrontal, migratory character and Paci®c south of Japan. having maintained its identity for 12 to 24 h. The system Monsoon depressionÐA tropical cyclonic vortex may or may not be associated with a detectable pertur- characterized by 1) its large size, the outermost closed bation of the low-level wind or pressure ®eld. It is the isobar may have a diameter on the order of 600 n mi basic generic designation that, in successive stages of (1000 km); 2) a loosely organized cluster of mesoscale intensi®cation, may be classi®ed as a tropical depres- convective systems; 3) a low-level wind distribution that sion, tropical storm, typhoon, or supertyphoon. features a Ն100nmi(Ն200 km) diameter light-wind Tropical stormÐA tropical cyclone with maximum core, which may be partially surrounded by a band of 1-min mean sustained surface winds in the range of 34± gales; and, 4) a lack of persistent central convection. 63 kt (18±32 m sϪ1), inclusive. Note: most monsoon depressions that form in the west- Typhoon (hurricane)ÐA tropical cyclone with max- ern North Paci®c eventually acquire persistent central imum sustained 1-min mean surface winds of 64 kt (33 convection and accelerated core winds marking the tran- msϪ1) or more. West of 180Њ longitude they are called sition to a conventional tropical cyclone. typhoons, and east of 180Њ longitude, hurricanes. Note: Shear lineÐA shear line accompanies (or can be said in the western North Paci®c a subset of the typhoon to accompany) that band of clouds and showers that are categoryÐthe supertyphoonÐis used for typhoons of the extension into the Tropics of the cloud band asso- high intensity of 130 kt (67 m sϪ1) or greater. ciated with the cold fronts of the large extratropical storm systems that traverse the midlatitudes of the North REFERENCES Paci®c, particularly from late fall to early spring. The shear line portion of the cold front of a midlatitude storm Atkinson, G. D., and C. R. Holliday, 1977: Tropical cyclone minimum system is de®ned by the behavior of the wind shift sea level pressure maximum sustained wind relationship for the western North Paci®c. Mon. Wea. Rev., 105, 421±427. across the cloud band. Along the shear line portion of Brand, S., 1970: Interaction of binary tropical cyclones in the western the frontal cloud band the surface winds have an easterly North Paci®c Ocean. J. Appl. Meteor., 9, 433±441. component on both sides, but the wind behind the cloud Carr, L. E., and R. L. Elsberry, 1994: Systematic and integrated band is stronger and usually has a more northerly com- approach to tropical cyclone track forecasting. Part I: Description ponent than the wind ahead of it. The air behind the of the basic approach. Naval Postgraduate School Publ. NPS- MR-002, Naval Postgraduate School, Monterey, CA, 65 pp. ®g- shear line is often slightly cooler, even at low latitudes. ures and appendixes. TCs that encounter shear lines usually begin to dissipate Chen, C., W.-K. Tao, P.-L. Lin, G. S. Lai, S.-F. Tseng, and T.-C. C. from the ingestion of stable air, and effects of vertical Wang, 1998: The intensi®cation of the low-level jet during the wind shear. development of mesoscale convective systems on a mei-yu front. Mon. Wea. Rev., 126, 349±371. Subtropical cycloneÐA low pressure system that Chou, L. C., C. P. Chang, and R. T. Williams, 1990: A numerical forms over the ocean in the subtropics and has some simulation of the mei-yu front and the associated low level jet. characteristics of a tropical cyclone, but not a persistent Mon. Wea. Rev., 118, 1408±1428. central dense overcast. Although of upper cold low or Climate Prediction Center, 1997: Climate Diagnostics Bulletin. Vol. low-level baroclinic origins, these systems can transition 97. [Available from U.S. Dept. of Commerce, Washington, DC 20233.] to a tropical cyclone. Dong, K., 1988: El NinÄo and tropical cyclone frequency in the Aus- SupertyphoonÐA typhoon with maximum sustained tralian region and the northwest Paci®c. Aust. Meteor. Mag., 36, 1-min mean surface winds of 130 kt (67 m sϪ1)or 219±255. greater. Dunnavan, G. M., 1981: Forecasting intense tropical cyclones using 700 mb equivalent potential temperature and central sea-level Tropical cycloneÐA nonfrontal, migratory low pres- pressure. NOCC/JTWC Tech. Note 81-1, 12 pp. sure system, usually of synoptic scale, originating over Dvorak, V. F., 1975: Tropical cyclone intensity analysis and fore- tropical or subtropical waters, and having a de®nite or- casting from satellite imagery. Mon. Wea. Rev., 103, 420±430.

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ÐÐ, 1984: Tropical cyclone intensity analysis using satellite data. ÐÐ, 1999: A tropical cyclone with an extremely large eye. Mon. NOAA Tech. Rep. NESDIS 11, 46 pp. Wea. Rev., 127, 137±142. Guard, C. P., and M. A. Lander, 1999: A scale relating tropical cyclone National Weather Service, cited 1998: Service assessment, Super Ty- wind speed to potential damage for the tropical Paci®c Ocean phoon Paka. National Weather Service, Paci®c Region Head- region: A user's manual. Water and Environmental Research quarters. [Available online at http://www.noaa.gov/pr/hq/paka/ Institute Tech. Rep. 86, 60 pp. [Available from WERI, University htm.] of Guam, UOG Station, Mangilao, GU 96923.] Paci®c ENSO Applications Center, 1997: Paci®c ENSO Update. Vol. Harr, P. A., and R. L. Elsberry, 1996: Transformation of a large 3, Nos. 2 and 3. monsoon depression to a tropical storm during TCM-93. Mon. Pasch, R. J., and L. A. Avila, 1999: Atlantic hurricane season of Wea. Rev., 124, 2625±2643. 1996. Mon. Wea. Rev., 127, 581±610. Hebert, P. H., and K. O. Poteat, 1975: A satellite classi®cation tech- Rappaport, E. N., 1999: Atlantic hurricane season of 1997. Mon. Wea. nique for subtropical cyclones. NOAA Tech. Memo. NWS SR- Rev., 127, 2012±2026. 83, 25 pp. Sadler, J. C., 1975: Tropical cyclone initiation by the tropical upper Holland, G. J., 1983: Tropical cyclone motion: Environmental inter- tropospheric trough. Department of Meteorology Tech. Rep. actions plus a beta effect. J. Atmos. Sci., 40, 328±342. UHMET 75-02, University of Hawaii, 35 pp. [Available from Holliday, C. R., and A. H. Thompson, 1979: Climatological char- Dept. of Meteorology, University of Hawaii, 2525 Correa Rd., acteristics of rapidly intensifying typhoons. Mon. Wea. Rev., 107, Honolulu, HI 96822.] 1022±1034. ÐÐ, M. A. Lander, A. M. Hori, and L. K. Oda, 1987: Paci®c Ocean. JTWC, cited 1997: 1997 tropical cyclone statistics for the western Vol. II, Tropical Marine Climatic Atlas, Department of Mete- North Paci®c, north Indian Ocean, south Indian Ocean, and the southwest Paci®c. JTWC 1997 Annual Tropical Cyclone Report. orology, UHMET Publ. 87-02, University of Hawaii, 14 pp. [Available online at http://www.npmoc.navy.mil.] Trenberth, K. E., 1997: The de®nition of El NinÄo. Bull. Amer. Meteor. Lander, M. A., 1990: Evolution of the cloud pattern during the for- Soc., 78, 2771±2777. mation of tropical cyclone twins symmetrical with respect to the Velden, C. S., T. L. Olander, and R. M. Zehr, 1998: Development of equator. Mon. Wea. Rev., 118, 1194±1202. an objective scheme to estimate tropical cyclone intensity from ÐÐ, 1994: An exploration of the relationships between tropical digital geostationary satellite infrared imagery. Wea. Forecast- storm formation in the western North Paci®c and ENSO. Mon. ing, 13, 172±186. Wea. Rev., 122, 636±651. Willoughby, H. E., J. A. Clos, and M. G. Shoreibah, 1982: Concentric ÐÐ, 1995: The merger of two tropical cyclones. Mon. Wea. Rev., eye walls, secondary wind maxima, and the evolution of the 123, 2260±2265. hurricane vortex. J. Atmos. Sci., 39, 395±411.

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