Jet streams in the upper atmosphere are essential to weather forecasting. You will understand the movement of mid-latitude cyclones and fronts together with the meandering of the westerlies.
Basic Knowledge
A jet stream is a belt of strong winds blowing through the upper troposphere to the lower stratosphere (altitude 10–12 km), sometimes reaching a maximum speed of 100–200 m/s. There are two: the subtropical jet (near latitude 30°) and the polar-front jet (near latitude 60°). Near Japan, they shift southward and strengthen in winter, and shift northward and weaken in summer.
Jet streams meander north and south while forming large-scale waves called Rossby waves (planetary waves). When the meandering intensifies, split westerlies and blocking anticyclones form, causing weather to remain fixed for long periods.
A mid-latitude cyclone forms at the boundary between cold and warm air (the frontal zone). It is accompanied by a cold front (cold air wedges beneath warm air) and a warm front (warm air rises over cold air), and passes through the stages of development, maturity, occlusion, and dissipation.
Jet stream (belt of strong winds in the upper troposphere; two types: subtropical and polar-front jets)
Rossby wave (large-scale meandering wave in the westerly belt; planetary-scale wave)
Mid-latitude cyclone (mid-latitude low-pressure system accompanied by fronts; moves southwest → northeast)
Cold front (surface where cold air wedges beneath warm air; cumulonimbus clouds, heavy rain, and a sharp temperature drop)
Warm front (surface where warm air rises over cold air; nimbostratus clouds, prolonged rain, and a gradual temperature rise)
Occluded front (state in which a cold front catches up with a warm front; declining stage of a cyclone)
Further Study (Background and Significance)
The position of a jet stream is determined by the north–south temperature difference. In winter, the temperature difference between continents and oceans is large, strengthening the jet near Japan and making mid-latitude cyclones more likely to develop.
When the westerlies meander extremely, the paths of mid-latitude cyclones change greatly. When the meander becomes cut off, blocking occurs, and unusually high and low temperatures occur simultaneously around a stationary anticyclone. The 2021 North American heat wave (+40℃ or higher) is a typical example of a blocking anticyclone.
Japan's rainy-season front is a stationary front formed at the boundary between the cold high-pressure system to the north (the Okhotsk high) and the warm, humid Pacific high to the south. The rainy season ends in June–July, when the jet stream shifts northward.
- Jet stream: altitude 10–12 km; maximum 200 m/s
- Subtropical jet (30°) / polar-front jet (60°)
- Shifts southward and strengthens in winter; shifts northward and weakens in summer
- Mid-latitude cyclone: cold front (cumulonimbus and heavy rain) + warm front (nimbostratus and prolonged rain)
- After a cold front passes: sharp temperature drop and northwest wind
- After a warm front passes: rising temperature and southwest wind
- Blocking → prolonged extreme weather
Points of Caution (Often Confused)
① Distinguish the cold front (sudden changes and cumulonimbus clouds) from the warm front (slow changes and nimbostratus clouds). ② There are two types of jet streams: the subtropical and polar-front jets. ③ An occluded front is the declining stage of a cyclone. ④ Because the westerlies blow from west to east, cyclones move from west to east.
Practice
- In which season are jet streams strongest near Japan?
- State two weather changes after a cold front passes.
- What is the large-scale meandering of the westerlies called?