Will the coming winter in Hong Kong be colder than normal?

Under the influence of the northeast monsoon, the Hong Kong Observatory recorded a minimum temperature of 22.3°C on 6 October, which is the lowest since this autumn (from 1 September 2010 to 13 October 2010). Noting that cooler conditions affected Hong Kong and parts of northern China in early October and that a La Niña event[1] is expected to develop by the end of this year, the general public is concerned about whether or not the coming winter (December 2010 - February 2011) will be colder than normal. To address the concern, the Observatory YouTube team interviewed two Senior Scientific Officers of the department, Mr. MOK Hing-yim and Dr. LEE Tsz-cheung. The interview has been uploaded to the YouTube page (http://www.youtube.com/hkweather, Cantonese only) of the Observatory's web site.

Mr. MOK indicated in the interview that air temperatures of around 22°C are not abnormal in early October, as over past ten years similar conditions occurred in 2002, 2004 and 2008. Also, cold winters in northern China do not have a definite relationship with winter temperatures in Hong Kong. For example, severely cold weather afflicted Europe, northern China and North America in the previous winter (December 2009 - February 2010) while Hong Kong experienced generally rather mild conditions.

As for La Niña, Dr. LEE told the YouTube team that the central and eastern equatorial Pacific has become cooler than normal (by 0.5°C or more) since July this year. The latest forecasts by a number of climate models around the world indicate that the sea surface temperatures would continue to decrease and develop into a La Niña event by the end of 2010. Statistically, when compared to normal condition, La Niña tends to be associated with colder winters (Dec-Feb) in Hong Kong. However, exceptions like warmer winters of 1974/75 and 1998/99 (December 1974 - February 1975 and December 1998 - February 1999) do exist. Thus, La Niña is not the sole factor affecting winter weather in Hong Kong. More information on El Niño and La Niña is available at the Observatory's web site http://www.hko.gov.hk/lrf/enso/enso-front.htm.

In respect of seasonal forecast, the views of major climate prediction centres around the world up to now are quite mixed, and there is no general consensus on the winter temperature forecast for the region around Hong Kong. The uncertainty in forecasts for the coming winter based on existing data will be rather large. The Observatory will maintain a close watch on the situation and collect the necessary data to formulate the seasonal forecast for Hong Kong for December 2010 - February 2011, which will be disseminated around 10 December at http://www.hko.gov.hk/wxinfo/season/season.htm.

Remark [1]: A La Niña event is established when the sea surface temperatures of the central and eastern equatorial Pacific remain below normal (by 0.5°C or more) for at least 5 to 6 months.