1. Introduction
This summary of 1997 western North Pacific, north Indian Ocean, and Southern Hemisphere tropical cyclones (TCs) was compiled from the archives of the Joint Typhoon Warning Center (JTWC), Guam (JTWC 1997). The JTWC is a joint U.S. Navy–Air Force activity with a forecast area of responsibility that extends from 180° westward to the coast of Africa, north and south of the equator. Seventy percent of the world's TCs develop in this area. The Naval Pacific Meteorology and Oceanography Command at Pearl Harbor, Hawaii, provided TC advisories for Southern Hemisphere TCs east of 180° that are included in this summary. In compiling annual statistics on TCs within each of the basins where the JTWC has TC advisory responsibilities, the calendar year is used for annual statistics in the Northern Hemisphere, and the yearlong period ending on 30 June of the same calendar year is used for the Southern Hemisphere.
In the majority of cases, the JTWC determines TC intensity by applying the well-known Dvorak techniques (Dvorak 1975, 1984) to visible and infrared satellite imagery. The minimum central sea level pressure is then derived from the Atkinson and Holliday (1977, hereafter AH) TC wind–pressure relationship. In some cases, the wind and pressure are reliably measured (such as during the passage of Supertyphoon Paka over Guam), and the peak wind and minimum sea level pressure may deviate from the AH relationship. To evaluate the accuracy of the operational application of Dvorak's techniques, Velden et al. (1998) compared Dvorak TC intensity estimates with those made simultaneously by aircraft. With 346 total matches of Dvorak intensity estimates (converted to minimum sea level pressure) made by operational tropical analysis centers compared with aircraft measurements in 10 recent Atlantic hurricanes, the mean bias of the Dvorak estimates was 5.91 hPa too high, and the rms error was 10.61 hPa. On Dvorak's T-number intensity scale (with a range of T1–T8), this rms error is roughly 0.5 T number.
Most of the measured winds cited for specific TCs in section 2b are the peak gusts. Many TC wind measurements in the western North Pacific (WNP) are recorded at sites on high islands or in rough terrain. The peak gusts at such sites are deemed more meaningful than the sustained winds, especially in areas where the ratio of the peak gust to sustained wind becomes very high. Overland sustained winds are difficult to relate to their corresponding overwater value unless the land roughness and exposure is well understood. The peak gust gives a better benchmark for what the potential destructive force of the wind may have been. Only at well-exposed places such as Waglin Island, Hong Kong, does the ratio between the peak gust and the sustained wind tend to be consistent and small.
Because JTWC's primary focus is on the TCs of the WNP, the summary of the TCs in this basin is more detailed than are the summaries of TCs in the other basins. An extensive summary for the WNP is found in section 2, which is subdivided into two sections: an overview of the annual statistics coupled with a discussion of the large-scale circulation, and a recap of the TC activity by month. Brief summaries for the north Indian Ocean and Southern Hemisphere are found in sections 3 and 4. Concluding remarks appear in section 5.
2. Western North Pacific tropical cyclones: January–December 1997
a. Statistics and large-scale circulation
The WNP basin is bound to the south by the equator, to the west by the Asian coastline, and to the east by the 180° meridian. It includes the South China Sea and other partly landlocked gulfs and seas such as the Yellow Sea, the Sea of Japan, and the Gulf of Thailand. Many small islands are scattered throughout this basin, and are often used as reference points in the discussions of the TCs, either to provide a reference for a TC's location, or to provide direct ground-based observations of the TCs or their environment. The islands of Micronesia are mentioned frequently; either individually (e.g., Guam, Kwajalein, and Pohnpei), or as part of their respective political or geographical group (e.g., the Mariana Islands, the Marshall Islands, and the Caroline Islands) (Fig. 1).
The distribution, character, and behavior of the TCs of the WNP during 1997 included
a very high number of supertyphoons [i.e., those typhoons with maximum sustained 1-min average surface winds ≥ 130 kt (≥67 m s−1)];
the highest annual number of typhoons since 1971;
an early start of TC activity with a higher-than-average number during the first half of the year;
a substantial eastward displacement of the mean genesis location for all TCs;
the formation east of the 180° meridian of two TCs that moved into the WNP and became supertyphoons;
the landfall in the Philippines of only one TC that was at tropical storm intensity or greater;
the simultaneous existence in the Philippine Sea of two supertyphoons, each possessing an intensity of 160 kt (82 m s−1); and
a controversial potentially record-breaking land wind speed of 236 mi h−1 (105 m s−1) measured on Guam during Typhoon Paka.
The annual number of TCs assigned a number by the JTWC in the WNP during 1997 (Table 1) was slightly above normal: 33 versus the climatological average of 31. The year of 1997 included 11 supertyphoons, 12 less intense typhoons, 8 tropical storms, and 2 tropical depressions. The calendar year total of 31 TCs of at least tropical storm intensity was three higher than the climatological average (Fig. 2). The calendar year total of 23 typhoons was five above the long-term average, and is the highest annual number of typhoons recorded in the WNP basin since 1971, when there were 24. The executive summary of JTWC's 1997 Annual Tropical Cyclone Report (JTWC 1997) describes 1997 as, “the year of the super typhoon.” A supertyphoon is a TC with a maximum sustained 1-min wind ≥ 130 kt (≥67 m s−1), which is close to the 135 kt (69 m s−1) threshold of the Saffir–Simpson category 5 hurricane. While the JTWC has long been the only agency to use the “super-” category, the India Meteorology Department recently has adopted the “supercyclone” category. The 11 supertyphoons is an unprecedented number, exceeding by 4 the previous annual high of 7 supertyphoons recorded in the years 1971, 1987, 1989, and 1991 (Fig. 3).
During the spring of 1997, El Niño (i.e., very warm SST in the central and eastern equatorial Pacific) developed rapidly, and was coupled with a large drop in the magnitude of the Southern Oscillation index (Fig. 4). Unusually persistent low-level westerly wind flow became established at low latitudes in the WNP. This westerly wind flow was also displaced eastward from its normal domain (Fig. 5). The setup of low-level, low-latitude westerly wind flow early in the year led to the early establishment of a near-equatorial trough across Micronesia from the western Caroline Islands eastward into the Marshall Islands. This trough supported the development of several TCs early in the year. The only statistic of numbers of TCs in the WNP found by Lander (1994) to be significantly correlated with an ENSO index is an increase in the number of TCs occurring in the “early season” (defined in his paper as the period 1 March–15 July). During 1997 there were eight TCs in this early season window versus an average of four.
The annual mean genesis location of TCs that form in the WNP is related to the status of ENSO: it tends to be east of normal during El Niño years and west of normal during those years characterized by large-scale climatic anomalies opposite to those of El Niño, years known as La Niña or ENSO cold phase. Consistent with the TC distribution typically associated with El Niño (or an ENSO warm phase), the annual mean genesis location for all TCs during 1997 was substantially east of normal (Fig. 6a). This was a pronounced change from the TC distributions during 1995—a La Niña year (Trenberth 1997)—and during 1996 when this position was west of normal. A breakdown of the genesis locations of all the individual WNP TCs of 1997 (Fig. 6b) shows that most formed east of 140°E, 16 formed east of 155°E, and 11 formed east of 160°E. Two of the TCs that formed east of 160°E, Oliwa and Paka, were named in the central Pacific by the Central Pacific Hurricane Center in Honolulu before moving into the WNP. By virtue of their formation in the monsoon trough, these two TCs may reasonably be regarded as typical WNP TCs of monsoonal origin, albeit under the unusual condition of an extreme eastward displacement of the monsoon trough.
During the period 1960–91, the five 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 160°E. The five 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 160°E (cf., Lander 1994). In Fig. 6b, the area east of 160°E and south of 20°N is designated as the “El Niño box.” During 1996, only 1 TC formed within the El Niño box, while 10 TCs formed there during 1997 (including Oliwa and Paka).
During 1997, there was a tendency for low-level monsoon westerly winds to persist at low latitudes and for the axis of the monsoon trough to remain near 10°N 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 located at low latitudes near and to the east of the 180° meridian (Climate Prediction Center 1997). Corresponding conditions in the upper troposphere consisted of easterly wind anomalies over most of the low latitudes of the WNP. These large-scale atmospheric flow pattern anomalies of 1997 were nearly everywhere the reverse of those persisting in the WNP for most of 1995 and 1996, and are typical of those observed during an El Niño year.
Despite the nearly continuous presence of the monsoon trough and abundant deep convection, the number of TCs of at least tropical storm intensity was near normal. The TCs of 1997 tended to emerge one by one from the eastern portion of the basin and then recurve or move north, with each subsequent development at low latitude tending to occur after the prior TC had exited the Tropics. Many of the TCs were large, very intense, and slow moving. There were relatively few cases of multiple TCs (i.e., the simultaneous occurrence of two or more) in the WNP during 1997. The most noteworthy case of multiple TCs was the simultaneous and spatially proximate formation and development of Ivan (27W)1 and Joan (28W) in October. While in the Philippine Sea, each attained an extreme intensity of 160 kt (82 m s−1); the first time in the JTWC archives that two TCs of such extreme intensity coexisted in the WNP. The westernmost of these TCs, Ivan (27W), was the only TC of at least tropical storm (TS) intensity to make landfall in the Philippine archipelago during 1997. A low number of landfalling TCs in the Philippines and along the coast of Asia (excluding Japan) has been cited as an effect of El Niño (e.g., Dong 1988). During the decade 1987–96, an annual average of approximately seven TCs of at least TS intensity made landfall in the Philippine archipelago.
Despite the low number of TCs to make landfall in eastern Asia, two that did—Winnie (10W) and Linda (31W)—caused much loss of life and great destruction at their respective landfall sites in China and Vietnam. Mainland Japan, the Ryukyu Islands, the Volcano and Bonin Islands, and the Mariana Islands and other island groups of Micronesia were each affected by several typhoons. The last TC of 1997 in the WNP, Supertyphoon Paka (05C), affected the Marshall Islands and the islands of Guam and Rota in the Mariana Islands. On Guam, a controversial potentially record-breaking land surface wind gust of 236 mi h−1 (105 m s−1) occurred during passage of Paka over that island on the night of 16 December [the extant record surface wind gust of 231 mi h−1 (103 m s−1) occurred at the observatory atop Mount Washington, NH, in April 1934]. Paka's controversial wind gust is discussed in the next section.
An illustration of the TC activity in the entire JTWC area of responsibility during 1997 is provided in Fig. 8. Table 1 lists the TCs in the western North Pacific during 1997. Composite best tracks are provided for the periods 1 January–2 August (Fig. 9a), 21 July–22 September (Fig. 9b), and 17 September–31 December (Fig. 9c).
b. Summary of monthly activity
1) January
During November of 1996, episodes of strong low-level monsoon westerlies began to occur in the low latitudes of the WNP. Most of the WNP TCs of November and December 1996 were associated with these episodes of enhanced low-level westerly flow. The simultaneous occurrence of TCs in the Southern Hemisphere, some of them twins to WNP TCs, was a notable characteristic of the TC distribution as 1996 came to a close. Twin TCs form simultaneously north and south of the equator, at low latitude, and at nearly the same longitude; they are often almost mirror images of one another (e.g., Lander 1990). Although the Southern Hemisphere became the dominant site of TC formation by January 1997, there were some episodes of enhanced westerly wind flow at near-equatorial latitudes in the WNP that enhanced the potential for the formation of an off-season TC.
During such a time of enhanced westerly flow along the equator, the tropical disturbance that became Hannah (01W) formed in the near-equatorial trough south of the Marshall Islands (first JTWC warning valid at 0600 UTC 19 January). Moving on a long westward track for over two weeks, it reached a peak intensity of 50 kt (26 m s−1) and then dissipated in the Philippine Sea. Although Tropical Storm Hannah was in most aspects a typical off-season TC, it was, in retrospect, an early manifestation of an unusual large-scale tropical circulation pattern that would see many of the TCs of 1997 form well east of normal.
2) February
In keeping with February's climatology as the month of lowest TC frequency in the WNP, there were no numbered TCs in the WNP basin during February.
3) March
The quiescent conditions of February continued into March, and there were no numbered TCs.
4) April
During most of April, a monsoon trough stretched across Micronesia, and westerly low-level winds persisted at low latitudes. During the second week of the month, sea level pressures fell across the eastern Caroline Islands, abundant deep convection increased, and a monsoon depression developed [see the appendix for JTWC definitions and descriptions of monsoon depressions, tropical depressions, and other types of TCs; see Harr and Elsberry (1996) for a detailed description of the transformation of a monsoon depression to a tropical storm]. The monsoon depression moved westward, consolidated into a tropical storm, intensified, and became Supertyphoon Isa (02W) (first JTWC warning valid at 1800 UTC 11 April). Isa was the first of the 11 supertyphoons of 1997. The abundant deep convection associated with the monsoonal cloud band from which Isa emerged dropped 20 in. (∼500 mm) of rain in 24 h on Pohnpei. These torrential rains contributed to a deadly landslide that killed 19 people on the island. On the night of 16 April, Isa passed 140 n mi (260 km) south of Guam, where a peak wind gust of 61 kt (31 m s−1) was measured and where peripheral rainbands of the typhoon produced 24-h rainfall amounts of 6–10 in. (150–250 mm) across the island. The system reached its peak intensity of 145 kt (75 m s−1) as it moved slowly northward, northwest of Guam (Fig. 10a). The spring onset of persistent westerly low-level winds in Micronesia was cited in near–real time as evidence that a strong El Niño event was under way (Pacific ENSO Applications Center 1997). The “year of the supertyphoon” was off to an early start.
As Isa was recurving, Tropical Storm Jimmy (03W) formed at a low latitude in the near-equatorial trough that extended across the southern Marshall Islands (first JTWC warning valid at 0600 UTC 22 April). This small TC moved northwest and intensified, reaching a peak of 55 kt (28 m s−1) as it made a turn to the northeast. It then encountered a shear line (see the appendix) and dissipated over water during the final week of April.
5) May
As May began, monsoon westerlies persisted across the low latitudes of the eastern half of Micronesia, and Tropical Storm Kelly (04W) formed in the southern Marshall Islands (first JTWC warning valid at 0600 UTC 7 May). It was a relatively weak TC that moved slowly to the northwest. It later turned westward, accelerated, and dissipated over water in an unfavorable environment of westerly vertical wind shear.
During mid-May, the monsoon trough weakened, amounts of large-scale deep convection lessened, and the Tropics of the WNP became inactive. The next episode of TC development commenced during the final week of May when two TCs formed at opposite ends of the basin: Tropical Storm Levi (05W) in the South China Sea (first JTWC warning valid at 1800 UTC 25 May) and Typhoon Marie (06W) at low latitudes near 160°E (first JTWC warning valid at 1800 UTC 26 May). As a tropical depression, Levi moved eastward across Luzon where it caused severe flooding in Metro-Manila. After entering the Philippine Sea, it turned to the north, intensified, and reached its peak of 45 kt (23 m s−1). It eventually recurved on 28 May, merging with the Mei-yu front (Chou et al. 1990; Chen et al. 1998) south of Japan. Marie initially moved westward, then turned to the north and maintained a northward track for several days. While intensification was initially slow, Marie eventually reached a peak intensity of 90 kt (47 m s−1). Shortly thereafter, Marie recurved and became extratropical.
6) June
Levi and Marie were still active in early June as they accelerated into midlatitudes, became extratropical, and crossed the 180° meridian to become midlatitude lows northwest of Hawaii. Meanwhile in the Tropics of the WNP, low-latitude monsoon westerlies continued to persist across Micronesia, and three TCs—Nestor (07W), Opal (08W), and Peter (09W)—formed in the monsoon trough. Supertyphoon Nestor (07W) began as a monsoon depression in the Marshall Islands and became the second supertyphoon of 1997 (Fig. 10b). Developing in the southern Marshall Islands, Nestor was the farthest east a TC has become a typhoon in June since JTWC records began in 1959. The monthly total of three typhoons in June 1997 was also an extreme event for a June, equaled before only in 1963 and again in 1965.
The tropical disturbance that became Supertyphoon Nestor was first identified on 1 June as a persistent area of convection located about 70 n mi (130 km) south-southwest of Majuro (first JTWC warning valid at 0000 UTC 6 June). On the morning of 8 June, the TC veered away from the Mariana Islands on a north-northwestward track and continued intensifying for four days, reaching peak intensity of 140 kt (72 m s−1) at 1200 UTC on 10 June, approximately 200 n mi (370 km) northeast of Saipan. The system later turned to the north and recurved. Iwo Jima recorded a wind gust to 102 kt (51 m s−1) at 1500 UTC 12 June when the typhoon passed about 20 n mi (37 km) to the east. A few hours later, at 0200 UTC 13 June, the typhoon passed about 30 n mi (56 km) west of Chichijima, where a wind gust to 96 kt (50 m s−1) was recorded.
As Nestor (07W) was recurving, another monsoon depression, originating in the eastern Caroline Islands, consolidated and ultimately became Typhoon Opal (08W) (first JTWC warning valid at 0000 UTC 15 June). Opal moved northward and reached its peak intensity of 90 kt (47 m s−1) at 1200 UTC 17 June when located in the Philippine Sea about midway between Guam and Taiwan. It later became the first TC of the season to hit Japan, when it made landfall in southern Honshu. Opal then accelerated north of Tokyo, entered the Pacific Ocean, and became extratropical.
After Opal (08W) recurved, yet another monsoon depression, also originating in the eastern Caroline Islands, consolidated, intensified, and ultimately became Typhoon Peter (09W) (first JTWC warning valid at 0600 UTC 23 June). During the last week of June, Peter approached Luzon, but abruptly turned north and became a minimal typhoon of 65 kt (34 m s−1) as it neared the Ryukyu Islands. After Peter reached 30°N, it turned to the northeast, made landfall in Kyushu, and traversed nearly the entire length of Honshu. Exiting Honshu on 28 June, the weakened tropical storm reentered the Pacific. The next day, it merged with a frontal system and completed its extratropical transition. As an extratropical system south of the Kamchatka Peninsula, the remnants of Peter became more intense than the system had been as a TC, with peak winds of 70 kt (36 m s−1). Moving eastward, it weakened and dissipated as it crossed the 180° meridian on 4 July.
7) July
After the extratropical remnant of Typhoon Peter (09W) dissipated on 4 July, there was not another named TC in the WNP basin until 19 July when Supertyphoon Rosie (10W) was upgraded from a tropical depression (first JTWC warning valid at 1800 UTC 18 July) to a tropical storm. Rosie became the third supertyphoon of 1997 (Fig. 10c). It originated in the monsoon trough as a tropical disturbance in the western Caroline Islands. After becoming a tropical storm, the northward-moving TC intensified to a typhoon by 0000 UTC 21 July and reached its peak intensity of 140 kt (72 m s−1) at 1200 UTC on July 22. Twelve hours later, Rosie began to weaken and slowly accelerated toward the north-northeast. The system made landfall near Okayama on the Japanese island of Shikoku around 0800 UTC on 26 July as a minimal typhoon with 65 kt (33 m s−1) winds. Crossing over land, Rosie rapidly weakened as the main convection sheared away from the low-level circulation center. It continued to weaken in the Sea of Japan as the exposed low-level circulation center and remnant rain clouds were tracked southeastward back over Japan and into the Pacific where it dissipated. Rosie left two dead in Japan, and its passage resulted in power failures, landslides, and widespread damage to buildings in the southern and central parts of the country.
While most of WNP TCs of 1997 developed in the monsoon trough at low latitudes, Tropical Storm Scott (11W) formed north of 20°N in direct association with a cyclonic circulation in the tropical upper tropospheric trough (Sadler 1975). As a tropical depression (first JTWC warning valid at 0600 UTC 24 July), the system was sheared from the outflow of Supertyphoon Rosie (10W) and failed to intensify as it moved on an unusual southeast track. After a few days, it reversed direction for 24 h. It then made a right turn and reached its peak intensity of 55 kt (29 m s−1) while moving to the northeast. On 2 August, it approached the 180° meridian, merged with a frontal system, and dissipated.
The tropical disturbance that became Typhoon Tina (12W) originated in the monsoon trough in the eastern Caroline Islands. For over a week, organization was very slow as the disturbance moved to the northwest. On 29 July, the system became Tropical Depression (TD) 12W (first JTWC warning valid at 1800 UTC 29 July), and on 5 August it reached its 90-kt (47 m s−1) peak intensity. Tina then turned to the north, passed between Taiwan and Okinawa, made landfall in southern Korea, crossed the Sea of Japan, and dissipated as it made landfall in Hokkaido.
As Tina was developing in the eastern part of the basin, the cloud system that became Typhoon Victor (13W) was consolidating west of Luzon in the South China Sea. The system moved on a northward track and intensified slowly in an environment of northerly upper-level shear (first JTWC warning valid at 1800 UTC 30 July). It finally reached minimal typhoon intensity just prior to making landfall near Waglin Island, Hong Kong, on 2 August.
8) August
August was extremely busy with a total of 10 TCs spending some part of their life in the month. As Tina (12W) and Victor (13W) were maturing in the western portion of the WNP basin, yet another monsoon depression was developing in the Marshall Islands. This monsoon depression intensified into Supertyphoon Winnie (14W), the fourth of 1997's 11 supertyphoons (Fig. 10d). Forming at low latitudes in the Marshall Islands, it was one of several of the 1997 TCs that originated in the El Niño box of Fig. 6b (first JTWC warning valid at 0600 UTC 8 August). Winnie became a typhoon at 0000 UTC on 10 August, and reached its peak of 140 kt (72 m s−1) at 0000 UTC on 12 August. Winnie maintained a relatively straight west-northwest to northwest track across the Pacific from the Marshall Islands to the coast of China. It passed over Okinawa, moved across the East China Sea, and made landfall on the eastern coast of China approximately 140 n mi (260 km) south of Shanghai shortly before 1200 UTC on 18 August. While passing over Okinawa, Winnie possessed concentric eyewalls that were readily apparent in conventional visible and infrared satellite imagery, microwave satellite imagery, and in imagery from the Next Generation Weather Surveillance Radar-1988 Doppler (NEXRAD WSR-88D) radar located at Kadena Air Base. The diameter of Winnie's outer eyewall during passage over Okinawa was one of the largest ever observed in a TC (Lander 1999). The radar at Kadena Air Base, Okinawa, indicated 100-kt (51 m s−1) winds in the large outer eyewall in a layer from 3000 ft (0.9 km) to 6000 ft (1.8 km).
As Winnie passed through the northern Mariana Islands, the populated islands of Guam, Rota, Tinian, and Saipan (well to the south of Winnie's track, but within its gale area) reported damage to crops and vegetation from high winds and sea salt spray. In parts of Taiwan, Winnie dropped 28 in. (711 mm) of rain, and 27 people were reported killed when an apartment building collapsed. Another 12 people were reported killed from mudslides, flooding, and high wind. In mainland China, torrential rains and winds caused at least 75 deaths. Damage from wind and flooding was extensive. Monsoon winds associated with Winnie overturned a ferry in the Philippines killing 26 people.
As Winnie was forming its large outer concentric eyewall, Typhoon Yule (15W) and Tropical Depression 16W were organizing in a monsoon trough that extended from the Caroline Islands eastward beyond 180°. The disturbance that became Yule consolidated at the extremely low latitude of 3°N at the same time as the system that became TD 16W developed east of 180° and a bit farther north (near 10°N) (first 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 moving to the north-northeast and TD 16W moving to the west. Yule became the dominant circulation and subsumed TD 16W. Yule briefly attained typhoon intensity when it was near Wake Island. Later, while moving on a long northward track, it reacquired typhoon force winds as a system exhibiting some characteristics of a subtropical cyclone (Hebert and Poteat 1975). While slowly weakening, the system finally recurved at almost 50°N.
Typhoon Zita (17W) was one of three TCs to reach typhoon intensity in the South China Sea during 1997. Developing in the monsoon trough about 300 n mi (560 km) to the west of Luzon (first JTWC warning valid at 0000 UTC 21 August), it moved in a northward direction for a while before it turned to the west. Despite its proximity to China's south coast, it intensified substantially, reaching a peak intensity of 75 kt (39 m s−1) as it tracked across the Luichow Peninsula. Zita maintained this intensity while crossing the Gulf of Tonkin to its landfall in Vietnam on the morning of 23 August.
The pre–Typhoon Amber (18W) tropical disturbance formed southwest of Guam and moved on a slow westward, then northwestward, track toward Taiwan as it became a TC (first JTWC warning valid at 0600 UTC 21 August) and intensified. After becoming a tropical storm on 21 August, Amber intensified slightly faster than the normal Dvorak rate of one T-number per day (Dvorak 1975, 1984), and reached 100 kt (52 m s−1) by the morning of 25 August. After Amber reached its peak intensity, Tropical Storm Cass (20W) began to form southwest of Amber in the South China Sea, about 160 n mi (295 km) south of Hong Kong (first JTWC warning valid at 0000 UTC 28 August). Strong upper-level northeast winds that appeared to be part of the outflow from Typhoon Amber likely inhibited Cass's intensification. On 28 August, these two TCs underwent a binary interaction that accelerated Amber toward Taiwan and caused Cass to move slowly to the east. After some fluctuations of intensity, Amber reached its peak of 110 kt (57 m s−1) just prior to landfall on Taiwan on 29 August. Amber weakened over the mountainous island, and later made landfall on mainland China. Once Amber moved over Taiwan, Cass moved to the north and intensified to its peak of 45 kt (23 m s−1). Cass also made landfall in mainland China, 150 n mi (280 km) west of Taiwan, and later dissipated over the mountains of southern China.
While Amber (18W) was developing southwest of Guam, the disturbance that became Supertyphoon Bing (19W) developed near the eastern end of the monsoon trough in the Marshall Islands. The system was upgraded to TD 19W on 27 August (first JTWC warning valid at 0600 UTC 27 August) and tracked toward the Mariana Islands. On the afternoon of 29 August, Tropical Storm Bing, possessing 40-kt (21 m s−1) sustained winds, passed through the 20-n mi (40-km) channel that separates Guam and Rota. After passing Guam, Bing began to rapidly intensify, and 54 h later, it reached its peak intensity of 135 kt (70 m s−1), becoming the fifth supertyphoon of the year (Fig. 10e). After Bing reached its peak intensity of 135 kt (69 m s−1), satellite and microwave imagery indicated the development of concentric eyewalls. Over a 48-h period, Bing underwent a complete eyewall replacement cycle (Willoughby et al. 1982). Near 143°E, Bing slowed its forward motion and turned to the north. It moved northward for three days, until it recurved to the northeast about 300 n mi (555 km) south of eastern Japan. Bing's forward speed then accelerated to 30 kt (56 km hr−1) as it transitioned into a 55-kt (29 m s−1) extratropical cyclone on 5 September. The more than 5 in. (∼125 mm) of rain on Guam associated with Bing caused considerable local flooding and contributed to Guam's record August rainfall of 38.49 in. (978 mm).
9) September
September was also a busy month with five TCs. At the end of August, Supertyphoon Oliwa (02C) became the sixth supertyphoon of 1997 (Fig. 10f). It formed from a tropical disturbance to the southwest of Hawaii in a portion of the WNP monsoon trough that had extended abnormally far to the east. As it intensified, this tropical disturbance was upgraded by the Central Pacific Hurricane Center (located in Honolulu, HI) first to Tropical Depression 02C, and later to Tropical Storm Oliwa. On 4 September, Oliwa crossed the 180° meridian and entered JTWC's area of responsibility. The first warning issued by the JTWC was valid at 0600 UTC 4 September. After it entered the WNP basin, Oliwa moved on a steady west-northwestward track and intensified. At first, the rate of intensification was slow; during the 102-h period from 0600 UTC on 4 September to 1200 UTC on 8 September, Oliwa's intensity increased from 35 kt (18 m s−1; minimal tropical storm) to 65 kt (33 m s−1; minimal typhoon). Then, during the 24-h period from 1800 UTC 8 September to 1800 UTC 9 September, Oliwa's intensity climbed from 75 kt (39 m s−1) to its peak of 140 kt (72 m s−1). The 24-h pressure drop associated with this wind speed increase was 69 mb, for an average of 2.9 mb h−1 (please see italicized cautionary note at the end of this paragraph). This qualifies as a case of explosive deepening, defined by Dunnavan (1981) as a drop of minimum sea level pressure ≥2.5 mb h−1 for at least 12 h, or ≥5 mb h−1 for at least 6 h. According to Dvorak (1975, 1984), the average rate of TC intensification is one T number per day, or a 24-h pressure drop of approximately 15 mb for tropical storms, 20–25 mb for mid-range typhoons, and 30 mb for intense typhoons. Holliday and Thompson (1979) identified rapid deepening of a TC as a decrease in its minimum sea level pressure of ≥42 mb for 24 h. Many typhoons that reach high peak intensities [i.e., more than 100 kt (51 m s−1)] undergo a period of rapid intensification, but few undergo explosive deepening. During 1997, only two TCs underwent explosive deepening—Oliwa and Ginger. It is important to repeat here that the majority of the TC intensity estimates for the WNP are made using the Dvorak techniques (Dvorak 1975, 1984) and that minimum sea level pressure is derived from an empirical TC wind–pressure relationship (i.e., Atkinson and Holliday 1977).
Oliwa made landfall in southwestern Japan where it was responsible for widespread damage and for loss of life. On Japan's southern island of Kyushu, seven people were reported killed. One thousand homes were flooded and dozens of homes were destroyed. Along Korea's southern coast, 28 ships sank or were wrecked in strong winds and high waves. A crabbing ship with a crew of 10 aboard was reported missing.
The disturbance that became Typhoon David (21W) formed east of the 180° meridian in the El Niño–extended monsoon trough. After becoming a tropical depression (first JTWC warning valid at 1800 UTC 12 Sep), it moved to the northwest, while gradually intensifying at a normal rate of one T number per day. The TC was large, and a track more northward than indicated by statistical and dynamic guidance was attributed to the “beta effect” (e.g., Holland 1983) incurred by the TC's large size. David attained its peak intensity of 95 kt (49 m s−1) on the morning of 15 September, then recurved, passed south of Japan, and became extratropical on 21 September en route to the Gulf of Alaska.
While David (21W) was recurving southeast of Japan, the disturbance that became Supertyphoon Ginger (24W), the seventh supertyphoon of 1997 (Fig. 10g), was consolidating near the 180° meridian and was one of 10 TCs that formed east of 160°E and south of 20°N, within the El Niño box shown on Fig. 5b (first JTWC warning valid at 1800 UTC 22 September). It moved out of this box on a northwestward track in the eastern portion of the WNP basin. After becoming a typhoon, Ginger explosively deepened, and in the 24-h period from 0000 UTC 26 September to 0000 UTC 27 September, the intensity jumped from 75 kt (39 m s−1) to 145 kt (75 m s−1). The 24-h estimated pressure drop associated with this wind speed increase was 75 mb. This easily qualifies as a case of explosive deepening, as defined by Dunnavan (1981). When Ginger reached 30°N, it accelerated within the midlatitude westerlies where it transformed into a vigorous extratropical low. The cloud system of this TC when near its peak was isolated in a large-scale environment that was unusually free of deep convection, other TCs, and tropical disturbances (Fig. 11). A reduction of deep convection throughout Micronesia and within the low latitudes of the WNP basin became more pronounced during the latter half of 1997 as the low-level monsoon westerlies and their associated deep convection moved eastward to the international date line and beyond.
Typhoon Fritz (22W) began as an area of enhanced convection in the South China Sea. As this tropical disturbance moved away from the coast of Vietnam, it slowly intensified (first JTWC warning valid at 1800 UTC 20 Sep). After a few days of eastward movement, Fritz turned back to the west toward Vietnam and continued to intensify. It reached a peak intensity of 75 kt (39 m s−1) at 0000 UTC 24 September, which it maintained until it made landfall in Vietnam on 25 September. The system dissipated over land, but torrential rains triggered landslides that killed 25 people, many of whom were gold prospectors.
Tropical Storm Ella (23W) developed from a very small low-level circulation that originated east of 180°. By 21 September, convection associated with the low-level circulation became well organized, prompting the first JTWC warning valid at 0000 UTC 21 September. Ella sped to the west-northwest at 18–25 kt (33–46 km h−1), nearly double the climatological average speed of TCs at low latitude in the WNP, and reached its peak intensity of 40 kt (21 m s−1) on 22 September. It recurved and dissipated on 24 September near 40°N, 170°E.
10) October
Tropical Storm Hank (25W) was the shortest-lived TC of the season, with advisories issued for only 36 h. The tropical disturbance that became Hank was first observed on 27 September in the South China Sea, and the first warning was issued on 3 October based on ship observations of 25–35 kt (12–18 m s−1) wind speeds. The system moved erratically, and easterly vertical wind shear prevented it from intensifying beyond 40 kt (21 m s−1). 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 (first 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 s−1). TD 26W maintained this intensity for 3 more days, but westerly vertical wind shear appeared to be a limiting factor on its intensity. As an exposed low-level circulation, TD 26W merged with a frontal cloud band over the Philippine Sea.
The eighth and ninth supertyphoons of 1997, Supertyphoon 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 s−1). They reached their peak intensities at nearly the same time: Ivan at 1800 UTC 17 October and Joan at 0600 UTC 17 October. At 1200 UTC 17 October, Ivan's intensity was 155 kt (80 m s−1) while Joan's was still at 160 kt (82 m s−1); the first observation of two TCs of such extreme intensity existing simultaneously in the WNP (Fig. 10i). The separation distance between the two TCs at this time was 1090 n mi (a bit over 2000 km)—well over the threshold distance of 780 n mi (1450 km) noted by Brand (1970) for binary (i.e., direct) TC interactions to occur. Indeed, the centroid-relative motion of Ivan and Joan (not shown) does not indicate that any form of TC interaction took place.
Joan remained at or above the supertyphoon threshold (130 kt, 67 m s−1) for 4.5 days—a record. Ivan recurved in the Luzon Strait, and after weakening, became extratropical south of Japan. Joan also recurved, and while traveling eastward along 30°N, became an intense extratropical cyclone. The T-number estimates for both Ivan and Joan (Fig. 12) reached T8.0 [equivalent to 170 kt (87 m s−1) intensity]. It is unlikely, however, that in the absence of aircraft (or a reliable ground measurement) that the intensity of a WNP typhoon will ever be reported to exceed 160 kt (82 m s−1) and thus tie or overtake Supertyphoon Tip's intensity record of 870 mb and 165 kt (85 m s−1), which was derived from aircraft measurements. Hence, Ivan and Joan—and also Paka—were given peak warning intensities of 160 kt (82 m s−1) despite Dvorak analysis that indicated that the intensity of these TCs may have been higher.
Why Ivan and Joan became so intense is unknown. Early in their lives (first JTWC warnings on Ivan and Joan were valid at 0600 UTC 13 October), neither objective guidance nor human forecasters anticipated the extreme intensities that Ivan and Joan would reach. The initial disturbances from which they developed were very poorly organized and were isolated in an environment that was unusually free of deep convection. At that time, the monsoon trough across the WNP was relatively weak and free of deep convection, and sea level pressures were near or above normal. For Ivan, nearly all intensity forecasts leading up to its peak were low by as much as 40 kt for the 12-h forecast, and 45, 50, 45, and 50 kt for the 24-, 36-, 48-, and 72-h forecasts, respectively. For Joan, many of the intensity forecasts were low by an even greater margin, and nearly all forecasts for the entire life of this TC were too low. Leading up to its peak, the intensity forecasts for Joan were low by as much as 30, 55, 65, 65, and 65 kt for the 12-, 24-, 36-, 48-, and 72-h forecasts, respectively. Despite the slow passage of these two TCs across much of the WNP basin, the monthly average wind for October (Fig. 4) was more easterly than normal everywhere except at low latitudes east of 150°E.
Both Ivan and Joan affected the Mariana Islands. On the night of 14 October, Ivan passed 55 n mi (100 km) to the south of Guam where a peak wind gust of 41 kt (21 m s−1) was recorded at Andersen Air Force Base; the heaviest 24-h rainfall of 5.85 in. (148.6 mm) was also recorded at Andersen. Ivan also affected the Philippines—the first and only TC of 1997 to make landfall in the Philippines while still a tropical storm or typhoon. At least one person was reported drowned and another missing on the northeastern tip of Luzon. Ivan damaged thousands of houses and destroyed large amounts of rice and corn in this region. More than $500,000 (U.S.) worth of fish stocks in ponds and cages were also destroyed. Joan largely spared the Mariana Islands any significant damage when it passed between the islands of Saipan and Anatahan on 18 October. Peak wind gusts of 85 kt (44 m s−1) were experienced on Saipan when Joan passed approximately 45 n mi (80 km) to the north. A Red Cross initial assessment indicated that Joan destroyed 4 houses, caused major damage to 15 other tin-and-wood structures, and caused minor damage to 17 homes on Saipan. On Guam, winds gusted to only 33 kt (17 m s−1) at the commercial port on the west side of the island.
As Ivan and Joan began to recurve, yet another disturbance was developing in the monsoon trough in the eastern Caroline and Marshall Islands. This disturbance would become the 10th of the 11 supertyphoons of 1997—Supertyphoon Keith (29W). After several days of westward movement and difficulty in organizing, the system finally consolidated and began to intensify (first JTWC warning valid at 1800 UTC 27 October). Upon reaching 105 kt (55 m s−1), the typhoon began to rapidly intensify, peaking at 155 kt (81 m s−1) 24 h later. The small eye and narrow but intense eyewall of Keith passed between the islands of Saipan and Rota in the Mariana Islands. Keith remained a supertyphoon for 3.5 days as it moved to the northwest at the end of the month. On 4 November, Keith's forward motion slowed, and the typhoon began to weaken and recurve. A few days later, it was speeding at 45 kt (83 km h−1) to the east-northeast and becoming extratropical.
When it reached its peak intensity, Keith was moving west-northwest and was just over a day away from passing through the Mariana Islands. During the 6-h period 0600–1200 UTC 2 November, Keith passed between the Mariana Islands of Rota and Tinian, which are only 40 n mi (75 km) apart. Though weakened slightly from its peak, it was still a powerful 140-kt (72 m s−1) supertyphoon. NEXRAD WSR-88D imagery from Guam indicated that the eyewall of Keith never touched land as it threaded the narrow channel between these two islands. As such, these two islands—and Saipan, which is separated from Tinian by an ocean channel only 3 n mi wide—were spared the full force of Keith. Nevertheless, Keith caused considerable damage on the islands of Rota, Tinian, and Saipan. Red Cross officials reported that at least 790 houses were destroyed or damaged on these islands. About 15 power poles were reported downed on Saipan, and 20 on Tinian. A wind gust of 95 kt (49 m s−1) was recorded at Saipan's International Airport. Sea level pressure fell to 964 mb on Rota and to 977 mb on Saipan. On Guam, little damage occurred, but power was knocked out to the entire island for nearly a day. Wind gusts reached 67 kt (35 m s−1) and upward of nearly 6 in. (152 mm) of rain fell on parts of the island. As Keith approached, very large surf from the east pushed rubble onto the coastal road on the southeast side of the island, forcing officials to close it.
11) November
The disturbance that became Typhoon Linda (30W) developed at the end of October near 10°N about 200 n mi (370 km) east of the Philippines (first JTWC warning valid at 1800 UTC 31 October). The system moved westward and reached tropical storm intensity within 24 h of moving into the South China Sea. The system continued to intensify as it approached the Ca Mau province of Vietnam on 2 November, and reached typhoon intensity in the Gulf of Thailand. The typhoon weakened while crossing the Malay Peninsula, but reintensified in the Bay of Bengal. Linda was the first TC since Typhoon Forrest (30W) in 1992 to cross the Malay Peninsula, retain at least TS intensity, and reintensify to a typhoon again in the Bay of Bengal. After attaining typhoon intensity in the Bay of Bengal on 6 November, Linda began to steadily weaken, as a result of unfavorable vertical wind shear. Four days later, it dissipated over the bay. Linda caused considerable damage and loss of life in Vietnam and Thailand, with over 330 people reported killed and 2250 missing.
Typhoon Mort (31W) was the last TC of November and the last TC of 1997 to form in the WNP basin. Mort formed in a weak monsoon trough south of Guam (first JTWC warning valid at 1800 UTC 10 November). While moving to the west, the system reached its peak intensity of 65 kt (34 m s−1) in the Philippine Sea on 12 November. After some fluctuation of intensity, vertical wind shear finally 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 December 1997, one of the most destructive of the year, Supertyphoon Paka (05C), moved into the basin from the central North Pacific as a tropical storm. Although trade winds dominated the Tropics of the WNP for most of December, low-level westerly wind flow persisted at eastern longitudes near the international date line. Not since the strong 1982/83 El Niño had monsoonal westerlies pushed so far to the east. Twin near-equatorial troughs extended along 8°N and 5°S from near 160°E to about 160°W. This led to the formation of twin TCs: Paka (05C), in the Northern Hemisphere, and Pam (07P98) in the Southern Hemisphere. Pam moved south, became a hurricane, and passed through French Polynesia on its way to the higher latitudes of the South Pacific. Meanwhile, Paka moved west and did not intensify much until it crossed 180° on 7 December, and became a typhoon in the Marshall Islands. Continuing on a west-northwestward track, it intensified and became the last of the record 11 super typhoons in the WNP during 1997. During the night of 16 December, Paka passed over Guam causing over $600 million (U.S.) in damages. The intense typhoon continued to move west-northwestward into the Philippine Sea where it attained its extreme estimated intensity of 160 kt (82 m s−1)—the third typhoon to do so during 1997. The TC then slowed as it encountered an approaching shear line; and, by ingestion of stable air, increasing vertical wind shear, and possible adverse changes to the upper-ocean thermal structure, Paka gradually dissipated over water.
Majuro and Kwajalein atolls both received peak wind gusts of over 40 kt (20 m s−1) as Paka passed near on 10 and 11 December, respectively. Alinglapalap and Jaluit atolls received the brunt of 90-kt (47 m s−1) winds, and experienced considerable damage to structures and vegetation, and significant coastal erosion from waves. The typhoon moved west-northwest and reached its first relative peak of 140 kt (72 m s−1) on 15 December. Paka was then a very serious threat to the southern Marianas.
A day away from Guam, Paka slowed, and there were signs of weakening. Now within NEXRAD WSR-88D radar range, the inner structure of Paka was revealed. There were concentric eyewalls—a primary wall cloud approximately 40 n mi (75 km) in diameter and a secondary fragmented inner wall cloud 10 n mi (19 km) in diameter (Fig. 13). At 1200 UTC 16 December, Paka, moving west at 6 kt (11 km h−1), was centered 15 n mi (28 km) north of Guam—its closest point of approach to the island. The outer eyewall crossed the northern two-thirds of the island, and only the northern half of the island experienced a period of relatively calm wind while in the moat between the outer eyewall and the inner. Microwave imagery shows that Paka underwent an eyewall replacement cycle during the period between its first and second peaks. While over Guam the typhoon was approximately midway through the cycle. After it passed Guam, the inner eyewall collapsed, the outer eyewall contracted, and Paka reintensified to its absolute peak of 160 kt (82 m s−1) briefly on 18 December—the third typhoon of 1997 to attain this extreme intensity.
On Guam, high winds were the cause of most of the damage. The Commercial Port National Weather Service (NWS) Handar wind instrument at Apra Harbor reported sustained (peak) gust of 100 (149) kt [51 (77) m s−1] which is plausible; the Andersen Air Force Base (AFB) anemometer (an FMQ-13 hot plate instrument) recorded 96 (205) kt [49 (105) m s−1] which was not considered representative (National Weather Service 1998). Had it stood, the Andersen wind gust would have greatly exceeded the highest wind gust ever recorded in a typhoon—166 kt (85.3 m s−1) (Joint Typhoon Warning Center 1997); and would have set a new world record for the highest surface wind speed ever measured; the 231-mph (103 m s−1) wind gust recorded atop Mount Washington, New Hampshire, still retains that title. The Guam Commercial Port sensor failed after recording 4 h of 135–149 kt (69–77 m s−1) gusts in the outer eyewall. The Andersen AFB sensor lost power during passage of the western side of the outer eyewall. Additionally, the NWS sensor at Tiyan lost power during the onset of the outer eyewall, the JTWC anemometer at Nimitz Hill failed at 103 kt (53 m s−1) before the eyewall arrived, the anemometer at the Apra Harbor tide gauge failed in the outer eyewall, and the NWS Handar instrument at the University of Guam, Mangilao, weathered the entire storm to report a peak gust to 123 kt (63 m s−1). In the final analysis, the Handar instrument at Apra Harbor became the benchmark. It faithfully recorded peak gusts up to 149 kt (77 m s−1) until the winds began backing to the southwest, at which point it too failed.
In general, the damage assessment of northern Guam indicated a mixture of tropical cyclone scale categories 3 and 4 [Saffir–Simpson hurricane scale as modified by Guard and Lander 1999] depending upon the exposure at the sites. This provides a wide range of maximum sustained wind speeds from 96–115 kt (49–59 m s−1) for category 3 to 116–135 kt (59–69 m s−1) for category 4. The $600 million (U.S.) level of destruction on Guam resulted in a presidential declaration of the island as a disaster area. With a combined population (160 000) on Guam and Rota, there was no loss of life as a direct result of Paka's passage.
With the dissipation of Paka, “the year of the supertyphoon” in the WNP came to a close. By the end of December, high pressure, persistent easterly winds, and reduced amounts of deep convection prevailed in the Tropics of the basin. ENSO-related drought conditions worsened and would reach record proportions in the first half of 1998. Tropical cyclone activity shifted into the Southern Hemisphere.
3. North Indian Ocean annual summary: January–December 1997
Tropical cyclones in the north Indian Ocean (especially in the Bay of Bengal) have been some of the deadliest in history. The Bay of Bengal, particularly the low-lying Ganges River delta region of Bangladesh, is the most dangerous TC basin in the world in terms of storm surge. One of the world's worst disasters occurred in 1970 when 300 000 lives were lost when a powerful TC made landfall there. A similar TC struck the coastal regions of Bangladesh during April 1991 and devastated the coastal city of Chittagong with winds >130 kt (67 m s−1) and a 20-ft (6 m) storm surge. The official death toll in 1991 was estimated at 138 000 and the damage at $1.5 billion (U.S.).
a. Annual statistics and the large-scale circulation
During 1997, four numbered TCs occurred in the north Indian Ocean. Two of these were in the Bay of Bengal and two in the Arabian Sea (Table 2). Spring and fall in the north Indian Ocean are periods of transition for the monsoons, and are the most favorable seasons for TC activity because the monsoon trough axis at these times is over water and the strong vertical wind shear associated with the summer monsoon is greatly reduced. This year was no exception. The total number, four, was one under the JTWC 23-yr average of five. Only two—TC 01B and TC 02B—reached hurricane intensity.
In this major El Niño year, one of the largest anomalies of the large-scale atmospheric circulation over the north Indian Ocean was a persistent easterly anomaly of the low-latitude surface wind (Climate Prediction Center 1997). This anomaly, present in the spring, intensified in the fall. During the months of peak TC activity, an easterly anomaly in low latitudes represents a weaker westerly flow south of the monsoon trough. This is consistent with a reduced number of TCs in the north Indian Ocean in 1997, and only two TCs of hurricane intensity.
b. Noteworthy tropical cyclones
1) Tropical Cyclone 01B
Tropical Cyclone 01B emerged from a poorly organized area of convection embedded within the near-equatorial trough, and became the first and most intense TC of 1997 in the north Indian Ocean. The system slowly developed as it drifted in a generally northward direction within the Bay of Bengal. At 0600 UTC on 19 May, it peaked at 115 kt (59 m s−1). This intensity was maintained until landfall occurred in Bangladesh shortly after 1200 UTC on 19 May. Tropical Cyclone 01B caused significant damage and several hundred casualties in Bangladesh.
2) Tropical Cyclone 02B
Tropical Cyclone 02B began as an area of disturbed weather in the western Bay of Bengal. The disturbance continued to improve in organization through 21 September, remaining quasi-stationary in the monsoon trough. After 21 September, the system began to move slowly northwestward, then, on 24 September, a developing midlatitude trough northwest of the TC shifted the steering flow to southwesterly and by 26 September, the forward motion had increased from 6 kt (3 m s−1) to 14 kt (7 m s−1). Tropical Cyclone 02B increased in intensity as it tracked along the eastern coast of India, and reached a peak of 65 kt (33 m s−1) approximately 12 h before making landfall in Bangladesh on 27 September. Forty-seven people were reported killed and more than 1000 injured as heavy surf, rain, and wind gusts of 80 kt (40 m s−1) swept the coastline.
3) Tropical Cyclone 03A
The disturbance that became Tropical Cyclone 03A was embedded within a widespread area of convection associated with a trough over the Arabian Sea, and well offshore of the southwest coast of India. The system tracked steadily west-northwest toward the coast of Somalia, and reached a peak of only 35 kt (18 m s−1) as it approached the coast. On 9 November, TC 03A made landfall over the northeastern tip of Somalia. The partially exposed low-level center quickly dissipated after landfall.
4) Tropical Cyclone 04A
In early November, the tropical disturbance that became Tropical Cyclone 04A was located over Sri Lanka. On 7 November, the disturbance moved west over the lower tip of India and on 8 November it entered the Arabian Sea. The first warning was issued by the JTWC valid at 0000 UTC on 10 November. Thirty hours later, when it was halfway across the Arabian Sea from India to Africa, TC 04A reached its peak intensity of 55 kt (28 m s−1). It held this intensity for just over 24 h, then began weakening under vertical wind shear. The remaining low-level circulation weakened, lost latitude, and dissipated over water well east of the African coast.
4. Southern Hemisphere annual summary: (July 1996–June 1997)
As in the north Indian Ocean, some differences exist between the JTWC TC statistics and those reported by the World Meteorological Organization–designated regional specialized meteorological centers (RSMCs) responsible for TC advisories in the Southern Hemisphere. Some of these discrepancies may be related to JTWC's use of a 1-min sustained wind versus the 10-min sustained 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 obtains both a number from the JTWC and a name from the responsible RSMC.
The annual total of JTWC-numbered TCs during the 1997 Southern Hemisphere season (by JTWC convention: 1 July 1996–30 June 1997) was 38. This was 11 more than the overall mean for the previous 16 yr. Of these 38 TCs there were 23 of hurricane intensity and 15 tropical storms (Table 3). The regional distribution of TCs in the Southern Hemisphere during 1997 was increased everywhere over the regional distribution of TCs during 1995 and 1996. In retrospect, the latter half of 1996 was an onset to the major El Niño of 1997. Strong equatorial westerly wind bursts occurred in November and again in December 1996. The December burst was associated with a prolific outbreak of TCs in both the Northern and Southern Hemispheres. On 24 December 1996 TCs Fergus, Phil, and Ophelia were active in the Southern Hemisphere while TCs Greg and Fern were active in the Northern Hemisphere. In the spring of 1997, unusually strong westerly winds became established along the equator between monsoon troughs in each hemisphere. For the JTWC, there are two Southern Hemisphere ocean basins for warning purposes: the South Indian Ocean (west of 135°E) and the South Pacific (east of 135°E). These are identified by appending the suffixes S and P, respectively, to the TC number. Although TC activity was enhanced in each of these TC regions, there was a tendency for the TC activity to occur predominantly in the south Indian Ocean, S, from July 1996 to January 1997, and in the South Pacific, P, from February to June 1997. The S (P) count was 15(7) from July 1996 to January 1997, and 5(11) from February to June 1997. From March through June, seven of eight TCs formed in the South Pacific. This eastward shift of TC activity was associated with a general and gradual eastward shift of El Niño–enhanced low-level westerly winds during late 1996 through 1997. Occurring during the second week of June 1997, the final TC of this prolific Southern Hemisphere season, TC 38P (Keli), was the latest recorded date for a major hurricane in the South Pacific.
5. Concluding remarks
During 1997, the global distribution of TCs continued in the pattern of 1996 with abundant TCs in the Atlantic (Pasch and Avila 1999; Rappaport 1999), fewer than normal TCs in the eastern North Pacific, and an above average number of TCs in the western North Pacific. Overall, 1997 was a very active year in the JTWC area of responsibility with a total of 71 TCs of TS intensity or higher versus a normal total of 60. By contrast, there were 52 such TCs during 1995 and 69 during 1996. Contributing to the high number of TCs in Eastern Hemisphere was a well-above-normal number of TCs in the Southern Hemisphere, and a slightly higher-than-normal number of TCs in the western North Pacific. El Niño–enhanced low-latitude, low-level westerly winds were the highlight of the large-scale circulation pattern of 1997. Month by month, these westerly winds pushed eastward, and dragged the TC activity in both the Northern and Southern Hemispheres eastward with them. By the end of 1997, westerly winds had pushed eastward along the equator to the south of Hawaii as easterly low-level wind anomalies (replacing earlier westerly wind anomalies) became firmly established across Micronesia and all low-latitude areas westward from there. The major El Niño of 1997 was running its course with such certainty that the Pacific ENSO Applications Center (1997) confidently (and correctly) predicted a major drought in Micronesia during the first half of 1998, and a shift, in 1998, of the TC activity to the west of normal.
Acknowledgments
Support for the preparation and publication of this paper was provided by the Office of Naval Research through Grant N00014-98-1-0744. The help of the personnel at the Joint Typhoon Warning Center in allowing us access to their satellite imagery and other meteorological data is greatly appreciated. A special thanks is owed to Rey Dalisay for his work on the graphics.
REFERENCES
Atkinson, G. D., and C. R. Holliday, 1977: Tropical cyclone minimum sea level pressure maximum sustained wind relationship for the western North Pacific. Mon. Wea. Rev, 105 , 421–427.
Brand, S., 1970: Interaction of binary tropical cyclones in the western North Pacific Ocean. J. Appl. Meteor, 9 , 433–441.
Carr, L. E., and R. L. Elsberry, 1994: Systematic and integrated approach to tropical cyclone track forecasting. Part I: Description of the basic approach. Naval Postgraduate School Publ. NPS-MR-002, Naval Postgraduate School, Monterey, CA, 65 pp. figures and appendixes.
Chen, C., W-K. Tao, P-L. Lin, G. S. Lai, S-F. Tseng, and T-C. C. Wang, 1998: The intensification of the low-level jet during the development of mesoscale convective systems on a mei-yu front. Mon. Wea. Rev, 126 , 349–371.
Chou, L. C., C. P. Chang, and R. T. Williams, 1990: A numerical simulation of the mei-yu front and the associated low level jet. Mon. Wea. Rev, 118 , 1408–1428.
Climate Prediction Center, 1997: Climate Diagnostics Bulletin. Vol. 97. [Available from U.S. Dept. of Commerce, Washington, DC 20233.].
Dong, K., 1988: El Niño and tropical cyclone frequency in the Australian region and the northwest Pacific. Aust. Meteor. Mag, 36 , 219–255.
Dvorak, V. F., 1975: Tropical cyclone intensity analysis and forecasting from satellite imagery. Mon. Wea. Rev, 103 , 420–430.
Dvorak, V. F., 1984: Tropical cyclone intensity analysis using satellite data. NOAA Tech. Rep. NESDIS 11, 46 pp.
Guard, C. P., and M. A. Lander, 1999: A scale relating tropical cyclone wind speed to potential damage for the tropical Pacific Ocean region: A user's manual. Water and Environmental Research Institute Tech. Rep. 86, 60 pp. [Available from WERI, University of Guam, UOG Station, Mangilao, GU 96923.].
Harr, P. A., and R. L. Elsberry, 1996: Transformation of a large monsoon depression to a tropical storm during TCM-93. Mon. Wea. Rev, 124 , 2625–2643.
Hebert, P. H., and K. O. Poteat, 1975: A satellite classification technique for subtropical cyclones. NOAA Tech. Memo. NWS SR-83, 25 pp.
Holland, G. J., 1983: Tropical cyclone motion: Environmental interactions plus a beta effect. J. Atmos. Sci, 40 , 328–342.
Holliday, C. R., and A. H. Thompson, 1979: Climatological characteristics of rapidly intensifying typhoons. Mon. Wea. Rev, 107 , 1022–1034.
JTWC, cited. . 1997: 1997 tropical cyclone statistics for the western North Pacific, north Indian Ocean, south Indian Ocean, and the southwest Pacific. JTWC 1997 Annual Tropical Cyclone Report. [Available online at http://www.npmoc.navy.mil.].
Lander, M. A., 1990: Evolution of the cloud pattern during the formation of tropical cyclone twins symmetrical with respect to the equator. Mon. Wea. Rev, 118 , 1194–1202.
Lander, M. A., 1994: An exploration of the relationships between tropical storm formation in the western North Pacific and ENSO. Mon. Wea. Rev, 122 , 636–651.
Lander, M. A., 1995: The merger of two tropical cyclones. Mon. Wea. Rev, 123 , 2260–2265.
Lander, M. A., 1999: A tropical cyclone with an extremely large eye. Mon. Wea. Rev, 127 , 137–142.
National Weather Service, cited. . 1998: Service assessment, Super Typhoon Paka. National Weather Service, Pacific Region Headquarters. [Available online at http://www.noaa.gov/pr/hq/paka/htm.].
Pacific ENSO Applications Center, 1997: Pacific ENSO Update. Vol. 3, Nos. 2 and 3.
Pasch, R. J., and L. A. Avila, 1999: Atlantic hurricane season of 1996. Mon. Wea. Rev, 127 , 581–610.
Rappaport, E. N., 1999: Atlantic hurricane season of 1997. Mon. Wea. Rev, 127 , 2012–2026.
Sadler, J. C., 1975: Tropical cyclone initiation by the tropical upper tropospheric trough. Department of Meteorology Tech. Rep. UHMET 75-02, University of Hawaii, 35 pp. [Available from Dept. of Meteorology, University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822.].
Sadler, J. C., M. A. Lander, A. M. Hori, and L. K. Oda, 1987: Pacific Ocean. Vol. II, Tropical Marine Climatic Atlas,. Department of Meteorology, UHMET Publ. 87-02, University of Hawaii, 14 pp.
Trenberth, K. E., 1997: The definition of El Niño. Bull. Amer. Meteor. Soc, 78 , 2771–2777.
Velden, C. S., T. L. Olander, and R. M. Zehr, 1998: Development of an objective scheme to estimate tropical cyclone intensity from digital geostationary satellite infrared imagery. Wea. Forecasting, 13 , 172–186.
Willoughby, H. E., J. A. Clos, and M. G. Shoreibah, 1982: Concentric eye walls, secondary wind maxima, and the evolution of the hurricane vortex. J. Atmos. Sci, 39 , 395–411.
APPENDIX
Definitions Excerpted from JTWC's 1996 Annual Tropical Cyclone Report
Mei-yu front—The term mei-yu is the Chinese expression for “plum rains.” The Mei-yu front is a persistent east–west zone of disturbed weather during spring that is quasi-stationary and stretches from the east China coast, across Taiwan, and eastward into the Pacific south of Japan.
Monsoon depression—A tropical cyclonic vortex characterized by 1) its large size, the outermost closed isobar may have a diameter on the order of 600 n mi (1000 km); 2) a loosely organized cluster of mesoscale convective systems; 3) a low-level wind distribution that features a ≥100 n mi (≥200 km) diameter light-wind core, which may be partially surrounded by a band of gales; and, 4) a lack of persistent central convection. Note: most monsoon depressions that form in the western North Pacific eventually acquire persistent central convection and accelerated core winds marking the transition to a conventional tropical cyclone.
Shear line—A shear line accompanies (or can be said to accompany) that band of clouds and showers that are the extension into the Tropics of the cloud band associated with the cold fronts of the large extratropical storm systems that traverse the midlatitudes of the North Pacific, particularly from late fall to early spring. The shear line portion of the cold front of a midlatitude storm system is defined by the behavior of the wind shift across the cloud band. Along the shear line portion of the frontal cloud band the surface winds have an easterly component on both sides, but the wind behind the cloud band is stronger and usually has a more northerly component than the wind ahead of it. The air behind the shear line is often slightly cooler, even at low latitudes. TCs that encounter shear lines usually begin to dissipate from the ingestion of stable air, and effects of vertical wind shear.
Subtropical cyclone—A low pressure system that forms over the ocean in the subtropics and has some characteristics of a tropical cyclone, but not a persistent central dense overcast. Although of upper cold low or low-level baroclinic origins, these systems can transition to a tropical cyclone.
Supertyphoon—A typhoon with maximum sustained 1-min mean surface winds of 130 kt (67 m s−1) or greater.
Tropical cyclone—A nonfrontal, migratory low pressure system, usually of synoptic scale, originating over tropical or subtropical waters, and having a definite organized cyclonic wind circulation. [In Dvorak's satellite TC intensity estimation techniques, there are four basic cloud patterns that compose the suite of “conventional” TCs: 1) the shear pattern, 2) the curved band pattern, 3) the central dense overcast (CDO) pattern, and 4) the eye pattern. Additional tropical cyclone cloud-pattern types seen in the western North Pacific (e.g., the monsoon depression, and the monsoon gyre) are not addressed by the Dvorak techniques.]
Tropical depression—A tropical cyclone with maximum sustained 1-min mean surface winds of 33 kt (17 m s−1) or less.
Tropical disturbance—A discrete system of apparently organized convection, generally 100–300 n mi (185–555 km) in diameter, originating in the Tropics or subtropics, having a nonfrontal, migratory character and having maintained its identity for 12 to 24 h. The system may or may not be associated with a detectable perturbation of the low-level wind or pressure field. It is the basic generic designation that, in successive stages of intensification, may be classified as a tropical depression, tropical storm, typhoon, or supertyphoon.
Tropical storm—A tropical cyclone with maximum 1-min mean sustained surface winds in the range of 34–63 kt (18–32 m s−1), inclusive.
Typhoon (hurricane)—A tropical cyclone with maximum sustained 1-min mean surface winds of 64 kt (33 m s−1) or more. West of 180° longitude they are called typhoons, and east of 180° longitude, hurricanes. Note: in the western North Pacific a subset of the typhoon category—the supertyphoon—is used for typhoons of high intensity of 130 kt (67 m s−1) or greater.
Western North Pacific area locator chart showing the positions of some of the islands (e.g., Guam), cities (e.g., Manila), and geopolitical groupings of the islands (e.g., Caroline Islands) referenced frequently in the text
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Number of tropical cyclones of tropical storm or greater intensity in the western North Pacific (1960–97)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Number of western North Pacific supertyphoons (1960–97)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Anomalies from the monthly mean for eastern equatorial Pacific Ocean sea surface temperature (SST) (hatched) in °C and the Southern Oscillation index (SOI) (shaded) for the period 1996–97. The rise in the eastern equatorial Pacific SST during 1997 is of near-record magnitude: J = Jan. [Adapted from Climate Prediction Center (1997).]
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
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 120°E–180°; bottom to top is 20°S–20°N (except for Jun–Sep when bottom to top is 0°–40°N 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)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
(a) Mean annual genesis locations for the period 1970–97. For 1997, the location is indicated by the arrow. The star lies at the intersection of the 27-yr average latitude and longitude of genesis. For statistical purposes, genesis is defined as the first 25-kt (13 m s−1) intensity on the JTWC best track. (b) Point of formation of significant tropical cyclones in 1997 as indicated by the initial intensity of 25 kt (13 m s−1) on the best track. The symbols indicate solid dots = 1 Jan–15 Jul, open triangles = 16 Jul–15 Oct; and, X = 16 Oct–31 Dec
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Schematic illustration of the low-level circulation pattern which dominated the WNP for much of 1997. The summer flow pattern is indicated, but the flow 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 = Guam, and T = Tokyo
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
The 1997 JTWC tropical cyclone tracks
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
(a) Composite best tracks for the western North Pacific Ocean tropical cyclones for the period 1 Jan–2 Aug 1997. (b) Composite best tracks for the western North Pacific Ocean tropical cyclones for the period 21 Jul–22 Sep 1997. (c) Composite best tracks for the western North Pacific Ocean tropical cyclones for the period 17 Sep–31 Dec 1997. Each track begins at the first identification of a cyclonic vortex that became the TC, and the dot on each track indicates the location of the final JTWC warning. Note that most tracks continue past the final warning to include extratropical stages of the TC
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
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 Saipan (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−1) as it approaches the northern Mariana Islands (2133 UTC 11 Aug visible GMS imagery). (e) Bing's intensity was 130 kt (67 m s−1) in this image showing a smooth eyewall with a very well defined 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−1) (2034 UTC 9 Sep visible GMS imagery). (g) possessing extensive banding features, Ginger nears its peak intensity of 145 kt (75 m s−1) (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 s−1) (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−1) (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
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
(Continued)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Ginger and its extensive pattern of primary and peripheral cloud bands are isolated in an otherwise relatively cloud-free Tropics (0033 UTC 27 Sep visible GMS imagery)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Objective Dvorak T numbers for (a) Ivan and (b) Joan. Objective Dvorak T numbers (small black dots) were derived from an algorithm installed on the image-processing equipment of the JTWC. Warning intensity is shown by open circles. The 4.5 days of supertyphoon intensity for Joan [≥130 kt (67 m s−1)] is a record. Note also the near-simultaneous peak of these two typhoons
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
A view of Paka from Guam's NEXRAD WSR-88D at 0712 UTC 16 Dec 1997. The image shows Paka with concentric eyewalls. The outer eyewall is making its way across the island of Guam while the inner eyewall is in the process of collapsing as it moves over water between Guam and Rota. Data are NEXRAD WSR-88D base reflectivity. Yellow indicates reflectivity values of 40 dBZ or higher. Guam is approximately 30 n mi (50 km) in length, and the distance between Guam and Rota is approximately 40 n mi (70 km)
Citation: Monthly Weather Review 129, 12; 10.1175/1520-0493(2001)129<3015:WNPNIO>2.0.CO;2
Western North Pacific 1997 tropical cyclone statistics
North Indian Ocean 1997 tropical cyclone statistics
Southern Hemisphere 1997 (from JTWC for Jul 1996–Jun 1997) Tropical Cyclone Statistics
The JTWC appends the following suffixes to the TC number: W = western North Pacific, A = Arabian Sea, B = Bay of Bengal, S = South Indian Ocean, P = South Pacific.
