The Year's Weather – 2023

Globally, 2023 is likely to be the warmest year on record according to the World Meteorological Organization’s preliminary assessment. Global mean sea level continued to rise, reaching a new record high in 2023. Over the Arctic, sea-ice extent remained well below average in the year and the minimum sea-ice extent in September was the sixth lowest in the satellite record. In 2023, different parts of the world were ravaged by various extreme weather events, including heatwaves in many parts of the world, including Europe, North Africa, southern China, Japan and much of Southeast Asia; severe drought in northwestern Africa, the Greater Horn of Africa region, parts of the Iberian Peninsula, parts of central and southwest Asia, many parts of Central America, northern South America, parts of Canada and the United States; extreme rainfall triggered severe flooding in China, India, Pakistan, Nepal, Greece, southern Bulgaria, parts of Türkiye, northeastern Libya, Spain, and the North Island of New Zealand; and severe damages and heavy casualties brought by tropical cyclones in Mozambique, Malawi, Madagascar, Zimbabwe, Sri Lanka, Myanmar, India, Bangladesh, the Philippines and China. Rampant wildfires also wreaked great havoc in Greece, Hawaii and Canada.

The La Niña event of 2022 lasted till January 2023. Sea surface temperatures of the central and eastern equatorial Pacific continued to increase in the following months. An El Niño event was established in August 2023 and persisted through to the end of the year.

In Hong Kong, with all twelve months warmer than usual, 2023 was one of the second warmest years on record with the annual mean temperature reaching 24.5 degrees, 1.0 degree above the 1991-2020 normal^[1]. The annual mean minimum temperature of 22.6 degrees and annual mean maximum temperature of 27.2 degrees were respectively one of the highest and one of the second highest since records began in 1884. In particular, the mean temperature for summer (June to August) reached 29.7 degrees, the highest on record. The highest temperature recorded at the Hong Kong Observatory in the year was 36.1 degrees on 27 July, one of the third highest on record. There were 54 Very Hot Days^[2], 56 Hot Nights^[3] and 4 Extremely Hot Days^[4] in Hong Kong in 2023, respectively ranking one of the highest, the second highest and one of the fifth highest on record.

The lowest temperature recorded at the Observatory in the year was 8.1 degrees on 23 December. The number of Cold Days^[5] in the year was 14 days, 1.2 days less than the 1991-2020 normal.

Fig. 1 Monthly mean temperature anomalies (against the 1991-2020 normal) in Hong Kong in 2023

Fig. 2 Long-term time series of number of Hot Nights in Hong Kong 1884-2023

Fig. 3 Long-term time series of number of Very Hot Days in Hong Kong 1884-2023

Fig. 4 Long-term time series of number of Extremely Hot Days in Hong Kong 1884-2023

Fig. 5 Long-term time series of number of Cold Days in Hong Kong 1884-2023

Fig. 6 Long-term time series of annual mean temperature in Hong Kong 1885-2023

2023 was characterised by a dry spring and summer, and very wet autumn. Mainly attributing to the record-breaking rainfall in autumn, the annual total rainfall in 2023 was 2774.5 millimetres, about 14 percent above the 1991-2020 normal of 2431.2 millimetres. Eight red rainstorm warnings and two black rainstorm warnings were issued by the Observatory in the year. The number of days with thunderstorms reported in Hong Kong was 40 days in 2023, about 2 days less than the 1991-2020 normal.

Fig. 7 Monthly rainfall anomalies (against the 1991-2020 normal) in Hong Kong in 2023

Fig. 8 Annual rainfall distribution in Hong Kong in 2023 (in mm)

A total of 19 tropical cyclones occurred over the western North Pacific and the South China Sea in 2023, less than the long-term (1961-2020) average of about 30. There were 11 tropical cyclones reaching typhoon intensity^[6] or above during the year, less than the long-term average of about 15, and five of them reached super typhoon intensity (with maximum 10-minute wind speed of 185 km/h or above near the centre). In Hong Kong, five tropical cyclones necessitated the issuance of tropical cyclone warning signals, slightly less than the long-term average of about six in a year. The Hurricane Signal No. 10 was issued during the passage of Saola in September, the Increasing Gale or Storm Signal No. 9 was issued during the passage of Koinu in October and the Gale or Storm Signal No. 8 was issued during the passage of Talim in July.

Mainly attributing to the warmer than normal sea surface temperature over the northern part of the South China Sea and a stronger than usual southwesterly flow in the lower atmosphere over the south China coast, August 2023 was much hotter than usual in Hong Kong. The monthly mean temperature of 29.7 degrees and monthly mean minimum temperature of 27.8 degrees were respectively 1.0 degree and 1.1 degrees above their normal and both were the highest on record for August. Moreover, the monthly mean maximum temperature of 32.4 degrees was 1.1 degrees above normal and one of the second highest on record for August. There were 15 hot nights in August 2023, one of the highest on record for August.

Hong Kong experienced the hottest summer on record from June to August 2023 with a record-breaking high mean temperature of 29.7 degrees. The mean maximum temperature of 32.4 degrees and mean minimum temperature of 27.6 degrees were both the second highest on record for the same period.

In terms of extreme weather, September 2023 was an eventful month in Hong Kong with the ferocious strike by Super Typhoon Saola on 1 – 2 September and the phenomenal rainstorm on 7 – 8 September. Mainly attributing to the heavy rain associated with Saola and troughs of low pressure in the first half of the month, the Observatory recorded an all-time high September rainfall of 1067.1 millimetres, more than three times of the September normal of 321.4 millimetres and easily breaking the previous record of 844.2 millimetres set way back in September 1952.

With a maximum sustained wind of 230 km/h near its centre, Saola was the second most intense tropical cyclone affecting the South China Sea since 1950. The Hurricane Signal No. 10 was issued in Hong Kong during the passage of Saola, again since Super Typhoon Mangkhut hit Hong Kong in September 2018. Super Typhoon Saola moved generally westwards across the coastal waters of Guangdong on 1 September and skirted past about 40 kilometres to the south-southeast of Hong Kong that night. The storm to hurricane force winds of Saola impacted many places in Hong Kong on 1 – 2 September. The maximum 60-minute mean wind speeds recorded at Waglan Island and Cheung Chau were 154 km/h and 116 km/h respectively. The storm surge induced by Saola also resulted in flooding in some low-lying coastal areas of Hong Kong, including Sha Tin, Tai Po, and Tai O. The water level at Sai Kung rose to about 4.5 mCD at midnight on 1 September.

Saola brought squally heavy showers to Hong Kong on 1 – 2 September. More than 150 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 250 millimetres over Central and Western, Wan Chai and Tsuen Wan Districts on these two days. According to preliminary reports, there were over 3,000 reports of fallen trees, 21 reports of flooding and 7 reports of landslides in Hong Kong. There were also about 40 reports of damaged scaffolding, signboards and windows. Power supply was temporarily interrupted in some places. 460 flights were cancelled at the Hong Kong International Airport. While more than 80 people were injured, there was no fatality in Hong Kong during the passage of Saola.

Fig. 9 The approach of Saola resulted in fallen trees in many parts of the territory (Courtesy of Dr. T. C. Lee and Dr. Martin Williams (bottom left))

Fig. 10 Flooding of Shing Mun River due to storm surge induced by Saola (Courtesy of Poon Chi Ming)

Under the influence of a trough of low pressure associated with the remnant of tropical cyclone Haikui over the coast of Guangdong, the weather of Hong Kong started to deteriorate with heavy rain and squally thunderstorms on the night of 7 September. The incessant downpour continued to affect the territory till the next day. During the torrential rain, the Hong Kong Observatory Headquarters registered a record-breaking hourly rainfall of 158.1 millimetres from 11 p.m. to midnight on 7 September, the highest since records began in 1884. The 2-hour total rainfall of 201.0 millimetres and 12-hour total rainfall of 605.8 millimetres recorded at the Observatory Headquarters during this phenomenal rainstorm also broke their respective records. Moreover, the 24-hour rainfall from 4 p.m. on 7 September to 4 p.m. next day reached 638.5 millimetres, about a quarter of the normal annual total rainfall of Hong Kong and just next to the highest records kept by the historical rainstorm on 30 May 1889.

The Black Rainstorm Warning was in force for 16 hours and 35 minutes, setting the longest record since the introduction of the rainstorm warning system in 1992. More than 400 millimetres of rainfall were recorded over many parts of the territory and rainfall even exceeded 800 millimetres over the Eastern District and Southern District of Hong Kong Island on 7 – 8 September. Flash floods and landslides affected many parts of the territory, causing widespread traffic disruption and damage to infrastructures. According to preliminary reports, there were over 200 reports of landslides and 60 reports of flooding. Power and water supply were temporarily interrupted in some places. At least two people were killed and more than 140 were injured during the rainstorm.

Fig. 11(a) and (b) Serious flooding in Wong Tai Sin during the rainstorm on 7 – 8 September 2023 (Courtesy of CRHK News)

Fig. 12 Landslide in Shau Kei Wan during the rainstorm on 7 – 8 September 2023 (Courtesy of CRHK News)

Fig. 13 24-hour rainfall distribution map (16:00 on 7 September 2023 to 16:00 on 8 September 2023)

Severe Typhoon Koinu moved slowly across the northern part of the South China Sea and edged towards the Pearl River Estuary on 6 – 7 October. Koinu weakened into a typhoon on 8 October and skirted past about 70 kilometres to the south of Hong Kong that night. With Koinu moving towards the vicinity of the Pearl River Estuary, the weather of Hong Kong deteriorated significantly on 8 October and the Gale or Storm Signal No. 8 was issued on that afternoon. Local winds strengthened significantly and reached storm force offshore and on high ground that night. The Increasing Gale or Storm Signal No. 9 was also issued that night when the compact and hurricane force wind bearing eyewall of Koinu skirted past closely to the south of Hong Kong. Koinu also brought squally heavy showers to Hong Kong on 8 – 9 October and necessitated the issuance of the Black Rainstorm Warning on the morning of 9 October. The rainfall recorded at the Observatory on 9 October reached 369.7 millimetres, more than three times of October’s monthly total normal figure of 120.3 millimetres and the highest daily rainfall on record for October. Moreover, the 24-hour rainfall from 3 p.m. on 8 October to 3 p.m. next day reached 439.8 millimetres, breaking the highest record for October.

Fig. 14 Radar imagery of Koinu skirting to the south of Hong Kong at 8 p.m. on 8 October 2023

Mainly attributing to the record-breaking rainstorms in September and October, the autumn of 2023 from September to November was exceptionally wet with a total rainfall of 1616.4 millimetres, the highest on record for the same period.

Table 1a Summary of record-breaking high temperature events in 2023