South Korea's nationwide average temperature in January was minus 1.6 degrees Celsius, below the seasonal norm, but the month was marked by significant temperature fluctuations, including a brief warm spell followed by nearly ten days of severe cold, according to the Korea Meteorological Administration.
The weather agency released its analysis of January's climate characteristics and causes on Tuesday.
The nationwide average temperature of minus 1.6 degrees Celsius was 0.7 degrees lower than the seasonal average and 1.4 degrees below January last year. Compared to January temperatures over the past decade, this year's reading was similar to or higher than most years except 2018, which recorded minus 2.4 degrees.
However, cold weather began early in the month this year, and an unusually prolonged cold snap in the latter part of January pushed temperatures below normal. Strong atmospheric waves originating from the North Atlantic brought cold air into the region, causing the Han River to freeze on January 3, seven days earlier than the seasonal average.
The direct cause of the extended cold wave in late January was a weakening of the polar vortex, the agency said. During winter, a cyclonic vortex forms over the Arctic that normally traps cold air within the polar region. However, a negative Arctic Oscillation phase emerged during this period, weakening the vortex and allowing cold air to descend to mid-latitudes.
Additionally, a blocking pattern that disrupted the west-to-east flow in the upper atmosphere prevented the cold air over the Korean Peninsula from moving eastward, causing it to stagnate.
Still, a brief warm spell occurred from January 15 to 18, with temperatures rising above zero nationwide. A migratory high-pressure system brought warm southwesterly winds, smoothing atmospheric circulation.
On January 15 in particular, daytime temperatures in southern regions including Changwon and Daegu soared to around 20 degrees Celsius. Ten observation points including Changwon set new records for the highest daily maximum temperature in January.
"The dramatic temperature changes resulted from a combination of polar vortex variability and shifts in atmospheric pressure patterns," a KMA official said.