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 The northeast monsoon and its effects to the weather of southern China in autumn Written by: HO Pak­sing December 2010 The northeast monsoon in China is basically attributed to the cold air and high pressure area
residing over the Asian Continent in the latter part of a year. The air that flows from high to low
pressure regions is called the northeast monsoon. In normal circumstances, the northeast
monsoon starts to affect southern China in autumn. In Hong Kong, autumn usually begins from mid­September to late October. Since autumn this
year (2010), several surges of the northeast monsoon and their replenishments had affected
Hong Kong. Local temperatures dropped from over 30oC in summer to around 20oC. Hot
weather was replaced by mild conditions with winds turning from
southwesterlies/southeasterlies to northeasterlies/northerlies. One would find that
cumulonimbus clouds and the associated showers became rare, while stratocumulus clouds
would occasionally bring rain to the territory. All these changes are related to the northeast
monsoon. From early to mid­September, the subtropical ridge(1) shifts southwards from 29­30 to 24­25
degree North (latitude) in concert with the weakening of the summer monsoon, while the cold
high pressure system establishes itself over northern China. Northerly winds at the 850 hecto­
pascal (hpa) level (around 1500 metres above ground) spread from Changjiang region
southeastwards, signifying the establishment of the northeast monsoon. By the end of
September (around the Autumnal Equinox), cold air from the north could ride over the Nanling
mountain and intrude into Guangdong. In early October, the subtropical ridge further retreats to
the south, dry and cold air from the north could then frequently reach the coastal areas of
southern China. The daily mean temperature during this period goes below 23oC with dry and
windy weather. Apart from bringing about changes in winds and temperatures, the northeast monsoon can also
affect tropical cyclones over the South China Sea. If the northeast monsoon happens to meet
the warm and moist air associated with tropical cyclones, windy and rainy weather could follow
and is often called "Wet Cold Dew Wind" (WCDW) weather in Mainland China. One such
example is the heavy rain episode at Hainan in early October this year. The heavy rain severely
affected the harvest of late crop and vegetable. The interval between two WCDW occasions is
usually 6 to 8 days, being shorter in northern Guangdong and longer in the south. During a
WCDW occasion, the average daily rainfall is in general in the order of 30mm but it could be
over 100mm in the close proximity of a tropical cyclone. The northeast monsoon helps to
intensify the rain­bands associated with the tropical cyclone and significantly raises the
probability of daily rainfall over 100mm in the Hainan province and the coastal areas of
Guangdong. Similar weather affected Hong Kong between 10 and 11 October 2010. At that
time, the combined effect of a low depression area and the northeast monsoon brought over
50mm of rainfall to Shatin in Hong Kong (Fig.1). The Strong Monsoon Warning Signal was in
force during that period. Although a low pressure area instead of a tropical cyclone is involved
in this occasion, it is a good example of the interaction of the northeast monsoon with warm and
moist air associated with the low pressure area, bringing an intense rainband to southern China.
Fig 1: Weather chart for 1400 HKT 10 Oct 2010 Apart from enhancing the rainbands of tropical cyclones, the northeast monsoon can also affect
their movement. Tropical cyclones over the South China Sea around the autumn equinox
frequently makes landfall along the coastal areas of Guangdong and the Hainan province. Typhoon Damrey (25/9/2005), Tropical Storm Francisco (24/9/2007) (Figures 2 and 3), and
Typhoon Ketsana (28/9/2009) are a few examples. Fig 2: Weather Chart (with observation) for 0800 HKT 24 Sep 2007 Fig 3: Track of Tropical Storm FRANCISCO Tropical cyclone usually moves along the southern edge of the subtropical ridge in October and
November, crossing the Bashi Strait or Balintang Channel or the northern part of Philippines and
entering the South China Sea. They would then be steered to the southwest by the
strengthening northeast monsoon to affect the coastal areas of Guangdong. When the tropical
cyclone encounters the northeast monsoon, it usually moves on a westward track and makes
landfall to the west of the Pearl River Delta region or coastal areas of Hainan. However, there
are exceptions and Severe Typhoon Megi in 2010 was an example. Movement of tropical
cyclones in summer is usually dominated by the steering flow at the 500hpa level (around 6000
metres above ground) while the role of the 850hpa level flow becomes more prominent in
autumn. Because of the different air flow at different altitudes, the track of a tropical cyclone is
not straight forward. Between 20 and 22 October 2010, the effect of the northeast monsoon to
the movement of Megi over the South China Sea presented a challenge to weather forecasters
(Figures 4 and 5). It eventually took a northerly track and landed on the coast of Fujian, bringing
severe damages to nearby areas. Fortunately, there was no major impact to Hong Kong. Fig 4: Weather Chart (with observation) for 0800 HKT 21 Oct 2010 Fig 5: Track of Sever Typhoon MEGI References: The references in Chinese only, are available here. . Back to content