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Section 2 TROPICAL CYCLONE OVERVIEW FOR 2021
2.1 Review of tropical cyclones in 2021
2.1.1 Tropical cyclones over the western North Pacific (including the South China Sea)
In 2021, a total of 27 tropical cyclones occurred over the western North Pacific (WNP) and the South China Sea (SCS) bounded by the Equator, 45°N, 100°E and 180°, less than the long-term (1961 - 2020) average figure of around 30. During the year, 10 of the tropical cyclones attained typhoon intensity or above, less than the long-term average (1961 - 2020) of about 15, with five of them reaching super typhoon intensity (maximum 10-minute wind speed of 185 km/h or above near the centre).
During the year, seven tropical cyclones made landfall over China, with one of them crossing the south China coast within 300 km of Hong Kong. Four traversed the Philippines and five made landfall over Vietnam. With an estimated maximum sustained wind speed of 240 km/h and a minimum sea-level pressure of 910 hPa near the centre (Table 4.1), Super Typhoon Surigae (2102) and Super Typhoon Chanthu (2114) in September (Figure 2.3) were the most intense tropical cyclones over the WNP and the SCS in 2021.
2.1.2 Tropical cyclones in Hong Kong’s area of responsibility
Amongst the 27 tropical cyclones in 2021, 17 of them occurred inside Hong Kong’s area of responsibility (i.e. the area bounded by 10°N, 30°N, 105°E and 125°E), slightly more than the longterm annual average (1961-2020) figure of around 16 (Table 2.1). Seven of them developed within Hong Kong’s area of responsibility. Altogether, 413 tropical cyclone warnings to ships and vessels were issued by the Hong Kong Observatory this year (Table 4.2).
2.1.3 Tropical cyclones over the South China Sea
11 tropical cyclones affected SCS bounded by 10°N, 25°N, 105°E and 120°E in 2021, slightly less than the long-term annual average (1961-2020) of around 12. Seven of them formed over the SCS.
2.1.4 Tropical cyclones affecting Hong Kong
In 2021, the typhoon season in Hong Kong started on 11 June when Tropical Depression Koguma (2104) formed over the northern part of the SCS, necessitating the issuance of the Standby Signal No. 1. It is rare to have tropical cyclones directly threatening Hong Kong in winter.
However, as Severe Tropical Storm Rai (2122) gradually edged closer to the coast of Guangdong in late December, the Observatory issued the Standby Signal No. 1 on 20 December. This is the latest tropical cyclone warning signal in a year since 1946. When Rai weakened rapidly and dissipated the next day, the typhoon season ended with the cancellation of all tropical cyclone warning signals on that day.
Eight tropical cyclones affected Hong Kong during 2021 (Figure 2.2), more than the longterm (1961-2020) average of about six in a year (Table 2.2). They were Tropical Storm Koguma (2104) in June, two Tropical Depressions and Typhoon Cempaka (2107) in July, Tropical Storm Lupit (2109) in August, Tropical Storm Lionrock (2117) and Typhoon Kompasu (2118) in October, Super Typhoon Rai (2122) in December. The No. 8 Gale or Storm Signal was issued during the passage of Lionrock and Kompasu on 9 and 12 October respectively, the highest tropical cyclone warning signal issued in 2021. The time separation between these two No. 8 signals was only 60 hours 40 minutes, the shortest record of break time between two No. 8 signals for two tropical cyclones since 1946. The No. 3 Strong Wind Signal was issued during the passage of Cempaka and Lupit. The rest of the four tropical cyclones all necessitated the issuance of the Standby Signal No. 1 in Hong Kong.
2.1.5 Tropical cyclone rainfall
Tropical cyclone rainfall for Hong Kong (total rainfall recorded at the Hong Kong Observatory Headquarters from the time when a tropical cyclone comes within 600 km of Hong Kong to 72 hours after it has dissipated or moved more than 600 km away from Hong Kong) in 2021 was 980.6 mm (Table 4.8.1). This accounted for approximately 42.5 % of the year’s total rainfall of 2307.1 mm and was about 39.3% higher than the 1961-2020 long-term average of 704.2 mm.
According to the above definition, Tropical Storm Lionrock (2117) brought 369.0 mm of rainfall to Hong Kong (Table 4.8.1) and was the wettest tropical cyclone in 2021.