GK:1.3.7 Seasonal Change and Weather
Geography of Korea: I. Natural Environment > 3. Climate > 7) Seasonal Change and Weather
7) Seasonal Change and Weather
Winter conditions on the Korean Peninsula are influenced by the rise and fall of the Siberian High system of high pressure. With a maximum central pressure of 1050hPa, the Siberian High is an extremely powerful high-pressure system. Looking at the Korean Peninsula, the winter atmospheric pressure is a representative of the “high west-low east” pattern, wherein its pressure is influenced by both the Siberian High and the Aleutian atmospheric pressure. When the Siberian High is being strongly developed the difference in central pressure between the two systems can reach 80hPa. During this time the cold and dry northwest seasonal monsoons blow vigorously and temperatures plummet. During these cold spells the weather is generally clear, though snow can fall on the western slopes of the Noryeongsanmaek and Sobaeksanmaek Ranges as well as on Ulleung-do Island and the upper altitudes of Jeju Island.
When the Siberian High recedes, the “high west-low east” pattern of atmospheric pressure weakens. With the retreat of the Siberian High, a migratory high-pressure system moves over the peninsula and winds become varied and temperatures rise. And when a low-pressure system that has formed behind this migratory high-pressure moves over the peninsula, weather conditions deteriorate and there can be snowstorms.
Influenced by the rise and fall of the Siberian High, Korean winter weather is popularly known to exhibit the seven-day pattern of “three cold, four warm.” In reality, the peninsula’s winter temperatures can be very irregular year by year, with some years exhibiting very low temperatures and others having long spells of warmer weather. When the Siberian High is very developed and dominant, the “west high-east low” pattern is seen and Korea experiences a cold winter. However, when this atmospheric pressure is not very developed the Siberian High tends to extend relatively more southwards and the northwest seasonal monsoons are weak, resulting in warmer winter temperatures on the peninsula.
With the arrival of spring the Siberian High gradually retreats and the northwest seasonal monsoons also weaken. As the Siberian High retreats the migratory high pressure system moves in. Because this migratory high-pressure system has a low-pressure system behind it, spring brings with it the most extreme weather changes of the year. The migratory high-pressure and low-pressure systems generally transit the peninsula every 3 or 4 days.
After the low-pressure system and its rain has passed through, weather on the peninsula becomes gradually warmer. However, if the Siberian High becomes temporarily extended, the northwest seasonal monsoons will blow and the result will be a spell of severe spring cold. Sometimes the migratory high-pressure system slows down in its transit across the peninsula resulting in a long spell of clear, dry weather and poses the threat of a serious spring drought.
In Korea, spring is the season of the “yellow dust” (hwangsa) blowing from China. This yellow dust carried in on the westerlies not brings in pollutants but worsens visibility. During periods when the hwangsa is especially severe the atmosphere can become very hazy to the extent that it can even appear foggy. What’s more, in the period from late spring to early summer there are frequent Nopsae winds caused by the Foehn phenomenon.
From late spring to early summer the Okhotsk high-pressure system extending down from the northeast has a strong influence on the peninsula’s weather. As the Okhotsk high-pressure system with its cool and humid northeast air extends towards the East Sea off the Korean peninsula and becomes dominant there it brings a cooling effect while also effectively blocking the movement of the yellow dust from the west, and thus improving visibility. These developments take place before the arrival of the summer jangma, or rainy season.
With the extension southward of the North Pacific high-pressure system the seasonal rain front moves north and peninsula’s rainy season begins. Korea’s jangma is associated with the polar front that forms between the North Pacific and Polar high-pressure systems. The strength of these two air masses will affect the timing of the rainy season’s arrival as well as the scope and impact of the rainfall it brings.
Once the rainy season commences heavy rainfalls are experienced throughout the peninsula. The rains mainly arrive over a 2 or 3 day stretch as a weak low-pressure system that forms from the rain front in the Yangtze River basin area in China moves eastwards towards Korea. However, when either a migratory low-pressure system emerging from North China moves southeast or the Okhotsk high-pressure system expands southwest, Korea’s rainy season front can be briefly pushed southward resulting in a respite from the rains. This break from the rains usually lasts 1–2 days but can stretch to 4–5 days. If the rainy season makes landfall late or passes over the peninsula too quickly the result can be a summer drought.
Around late July, the rainy season front moves north towards Manchuria and under the influence the arrival of the powerful North Pacific high-pressure system high summer begins, with temperatures on the peninsula surpassing 30˚C (86°F). In high summer the North Pacific high-pressure system dominates over a wide swath of the southern portion of the Korean Peninsula, while in the northern portion a low-pressure system forms, such that a “high south, low north” pressure pattern occurs. Under the domination of the North Pacific high, Korea’s high summer is characterized by sweltering heat and high humidity. However, if the North Pacific high retreats temporarily, then the rainy front will shift back south, resulting in worsening weather conditions but a temporary respite from the intense heat.
However, in years when the North Pacific high is exceptionally weak the result is a colder than normal summer, with cool temperatures that have a negative impact on agriculture. Conversely, in years with an exceptionally powerful North Pacific high extreme heat with abnormally high temperatures becomes a problem. The year 1980 is a representative example of a year with a cool summer. That year in Seoul there was only a single day in which the temperature reached above 30˚C, with average summertime highs that year ranging between 17˚–20˚C (62.6–68°F). The year 1994 is an example of year with abnormally high temperatures. Temperatures for July of that year were on average 2˚–5˚C higher than normal, with areas peninsula-wide attaining temperature extremes.
The period from late August to early September sees the development of the Siberian High as the high summer pressure pattern collapses. The rainy season front that had moved north now shifts again south to form the early autumn rainy season. The early autumn rainy season persists the longest on Jeju Island.
After early September one migratory high-pressure system after another emerging out of the Siberian High passes through the Korean Peninsula, bringing with them pleasant autumn conditions of lower temperatures and clear skies that continues for an extended time. However, the trough that follows these migratory high-pressure systems brings with it the autumn rains.
The autumn pressure systems are similar to their spring counterparts. After October, with the emergence of the Siberian High, a “high west, low east” pressure pattern sometimes develops, and the blowing of the northwest seasonal monsoons announces that winter is not far off.