The Subtropical Jet Stream (STJ) is a fast-moving, narrow air current located at high altitudes (typically around 12-15 kilometers above sea level) in the Earth’s atmosphere, primarily found in the subtropical latitudes (approximately 20° to 40° north and south of the equator). It plays a crucial role in the global climate system, influencing weather patterns, the distribution of heat, and the formation of other atmospheric phenomena.
Characteristics of the Subtropical Jet Stream
- Location and Formation
- The STJ forms at the boundary between the tropical air masses and the mid-latitude air masses.
- It is driven by the temperature gradient between the warm equatorial air and the cooler mid-latitude air, creating a strong pressure gradient that accelerates the wind speeds.
- Altitude and Speed
- Typically found at altitudes between 12 to 15 kilometers in the upper troposphere.
- Wind speeds can reach up to 200 to 300 kilometers per hour (120 to 190 miles per hour).
- Seasonal Variations
- The position and strength of the STJ vary with the seasons. It is generally stronger and shifts poleward during the winter months when the temperature gradients are more pronounced.
- During the summer, it weakens and shifts equatorward.
Influence on Climate and Weather Patterns
- Weather Systems
- The STJ influences the development and movement of weather systems such as cyclones, anticyclones, and jet streaks, which can enhance or suppress precipitation.
- Example: In the winter, the STJ can steer storm systems across the southern United States, bringing significant rainfall and sometimes severe weather.
- Monsoon Systems
- The STJ plays a role in the onset and intensity of monsoon systems, particularly the Indian monsoon.
- Its position and strength can affect the distribution of monsoon rainfall by influencing the upper-level wind patterns and the movement of moisture-laden air masses.
- Example: A strong STJ during the pre-monsoon period can delay the onset of the Indian monsoon by preventing the northward movement of moist air from the Indian Ocean.
Subtropical Jet Stream in Different Regions
1. Indian Subcontinent
- Winter Season: During winter, the STJ is positioned over the northern part of the Indian subcontinent. It influences the weather by bringing western disturbances, which cause winter rainfall and snowfall in the northern plains and the Himalayas.
- Example: The STJ can enhance the frequency and intensity of western disturbances, leading to significant winter precipitation in states like Punjab, Haryana, and Himachal Pradesh.
- Summer Monsoon: In the summer, the STJ shifts northward, allowing the Inter-Tropical Convergence Zone (ITCZ) to move over the Indian subcontinent, facilitating the southwest monsoon’s advance.
- Example: A well-positioned STJ during the summer helps in the smooth progression of the southwest monsoon, bringing timely and adequate rainfall to regions like Kerala and Maharashtra.
2. North America
- Winter Storms: The STJ influences the formation and trajectory of winter storms across the southern United States. It can bring heavy precipitation and severe weather, including tornado outbreaks in the southeastern states.
- Example: During the winter of 2017-2018, a strong STJ contributed to multiple storm systems that brought heavy snowfall and rainfall across the southern and eastern United States.
3. East Asia
- East Asian Monsoon: The STJ affects the East Asian monsoon by influencing the upper-level winds and the distribution of moisture. Its position can determine the onset and strength of the monsoon rains in China, Japan, and Korea.
- Example: A strong STJ can enhance the monsoon rainfall in southern China and Japan by facilitating the northward movement of moist air from the Pacific Ocean.
Comparative Analysis with Other Jet Streams
1. Polar Jet Stream
- Location: The Polar Jet Stream (PJS) is found at higher latitudes (around 50° to 70° north and south) and at slightly lower altitudes (10 to 12 kilometers).
- Characteristics: The PJS is generally stronger and more variable than the STJ, as it is influenced by the stronger temperature gradients between the polar and mid-latitude air masses.
- Comparison: While the STJ influences subtropical and mid-latitude weather patterns, the PJS has a more significant impact on the weather in higher latitudes, including the development of cold fronts and extratropical cyclones.
2. Tropical Easterly Jet Stream
- Location: The Tropical Easterly Jet (TEJ) is located near the equator, primarily in the upper troposphere over the Indian Ocean and parts of Africa during the Northern Hemisphere summer.
- Characteristics: The TEJ flows from east to west and is associated with the upper-level divergence that supports the Indian monsoon.
- Comparison: Unlike the STJ, which flows from west to east and is present year-round, the TEJ is seasonal and flows in the opposite direction. The TEJ specifically enhances monsoon rainfall in South Asia by promoting upward motion and moisture convergence.
Conclusion
The Subtropical Jet Stream is a vital component of the Earth’s atmospheric circulation, influencing weather patterns and climatic conditions in the subtropical and mid-latitude regions. Its role in driving the Indian monsoon and affecting weather systems in North America and East Asia underscores its global significance. Comparing the STJ with other jet streams, such as the Polar Jet Stream and the Tropical Easterly Jet, highlights the diverse and complex nature of atmospheric dynamics.
