There’s still about a month to go until winter officially ends, but the number of days on which weather stations in Taipei have shown temperatures falling below 10oC has already reached 13, with six days recorded last month and seven days in December.
According to the Central Weather Bureau (CWB), over the past 30 years Taipei had an average of 3.3 days of low temperatures in January — defined as less than 10oC.
The average temperature in Taipei last month was 13.8oC, 2.35oC lower than the 30-year temperature average of 16.15oC, marking the lowest average monthly temperature in 40 years and the ninth-lowest recorded in the past 100 years, it said.
“It was not only in Taipei that was cold this winter, but nationwide,” said Chia Hsin-hsing (賈新興), chief of the bureau’s long range forecast section.
Chia said that among the 25 weather stations nationwide, 21 recorded low temperatures that were among the tenth-lowest since the stations were established — the four stations in Taichung, Yushan (玉山), Hualien County and Alishan (阿里山) were exceptions.
Bureau statistics reinforced the widely held perception that this winter has been colder than average, but scientists have not yet come to a conclusion as to the cause, analysts said.
Weather Forecast Center director Cheng Ming-dean (鄭明典) attributed the low temperatures to the impact of La Nina, an abnormal cooling of the equatorial Pacific Ocean, coupled with a negative phase of Arctic Oscillation (AO), which allows frigid polar air to slide south, displacing warmer air to the north.
The AO is a natural weather pattern of differences in air pressure between the Arctic and mid-latitudes. When the AO is in a negative mode, the pressure gradient weakens, with air pressure higher than average in the Arctic and lower than average in the mid-latitudes.
“Both events bring cold to certain areas. This year the temperatures of the affected areas got cooler as La Nina coincided with the negative Arctic Oscillation,” Cheng said.
Located in one of the three paths along which arctic air masses flow southward from Siberia toward Inner Mongolia into northeastern China, Japan and the Korean Peninsula, Taiwan also receives cold arctic air when it plunges into the region, Cheng said.
Countries lying along the route when cold air from the Arctic spreads out southward into Western European countries such as Norway, Germany and the UK, also saw damage caused by unusually extensive snow, while the weather pattern allowed arctic air from northwestern Canada to flow straight into the eastern portion of the US.
It may seem contradictory that the rise in world temperature leads to extra cold winters, but Cheng said that global warming is one the reasons behind the increase in frequency of cold arctic masses moving southward, despite the link being unattested.
It’s “theoretically possible” because increased ice melting, as a result of global warming, can heat up the upper layers of the atmosphere, which will then lead to changes in wind patterns over the Arctic, causing Arctic cold air masses to move outward, Cheng said.
Peng Chi-ming (彭啟明), chief executive of the Weather Risk Management Co, said scientists did not yet know how global warming affects AO because it was not until as recently as 10 to 20 years ago that scientists gained more knowledge about the weather pattern.