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Globally, despite the cooling influence of the La Nina conditions in early and late 2011, the year 2011 is likely to rank as the tenth warmest year on record. The current La Nina condition, which developed towards the end of 2011, is expected to persist into early 2012. For extreme weather events 2011 was an eventful year worldwide. Notably, there were drought and high temperatures in southwestern China and the southern United States, dramatic change from drought to flood conditions in east Africa and central China, severe flooding in Pakistan, eastern Australia, Thailand and southeastern Brazil, deadly tornadoes in the United States, and tropical cyclone-induced torrential rain in Japan and the Philippines.
Locally in 2011, Hong Kong experienced appreciably colder weather from January to March and in December. This is attributed to the stronger northeast monsoon over southern China usually under the effect of La Nina. However, the below normal temperatures in these four months were almost offset by the noticeably warmer August and November. The average temperature for 2011 ended up at a near-normal 23 degrees.
The year 2011 was also an exceptionally dry year in Hong Kong. With well below normal rainfall in the first nine months, the annual rainfall of 1476.7 millimetres was about 38 per cent below normal, the lowest since 1963. From January to April, under the influence of the La Nina conditions, a stronger and drier northeast monsoon prevailed over southern China and brought less rain to Hong Kong. The lack of rainfall from May to September was due to a number of factors, including weaker southerly flows over the South China coast in May and the stronger anticyclone prevailing over southern China in August and September.
During 2011, there were only two red rainstorm warnings issued by the Hong Kong Observatory, about half the average number of red rainstorm warnings in a year since the operation of the Rainstorm Warning System began in 1992. No black rainstorm warning was issued in the year.
A total of 22 tropical cyclones formed over the western North Pacific and the South China Sea in 2011, less than the normal figure of around 30. Four of them reached super typhoon intensity during the year. Super typhoon refers to a tropical cyclone 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 local tropical cyclone warning signals, slightly less than the long term average of about six in a year. Typhoon Nesat, which affected Hong Kong in September, necessitated the issuance of the No.8 Gale or Storm Signal.
Detailed descriptions of the weather for individual months are available in the Monthly Weather Summary webpage from the following URL: www.hko.gov.hk/wxinfo/pastwx/mws.htm .
A detailed version of the Year's Weather for 2011 with some significant weather events in Hong Kong is available at: www.hko.gov.hk/wxinfo/pastwx/ywx.htm .
Attachment 1, 2 and 3 show respectively the monthly total rainfall anomalies, the annual rainfall distribution and the monthly mean temperature anomalies in 2011.
Ends/Friday, January 6, 2012
Issued at HKT 20:25
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