jet stream

Jet streams are concentrated ribbons of high-velocity wind, typically flowing west-to-east between 9 and 12 kilometres altitude near the tropopause, with speeds commonly between 120 and 250 km/h and occasionally exceeding 400 km/h. They arise from the thermal wind relationship: where strong horizontal temperature gradients exist between adjacent air masses, the resulting pressure-gradient force, balanced by the Coriolis force, generates intense geostrophic flow aloft. The two principal mid-latitude jets are the Polar Front Jet Stream, located around 40°–60° latitude along the polar front separating polar and tropical air, and the Subtropical Jet Stream, located near 30° latitude at the poleward limit of the Hadley cell, associated with the subsidence of tropical air. The phenomenon was systematically documented by Japanese meteorologist Wasaburo Ooishi in the 1920s and brought to wider scientific attention during the Second World War when high-altitude bombers encountered these winds.

The jet streams are not continuous straight bands but meander in large north–south oscillations called Rossby waves, whose crests and troughs steer surface cyclones and anticyclones. The jet flows fastest in segments called jet streaks, where divergence aloft promotes surface low-pressure development and convergence promotes high pressure, making the jet a key control on mid-latitude weather. Beyond the mid-latitude jets, the Tropical Easterly Jet (TEJ) flows east-to-west at around 100–150 hPa over the Indian Ocean and Africa during the northern summer, intensified by the upper-tropospheric anticyclone over the heated Tibetan Plateau, while the low-level Somali Jet (Findlater Jet) funnels moisture toward the Indian subcontinent. The seasonal migration of these jets governs much of monsoon dynamics.

For the Indian monsoon, the jet streams are decisive. The Subtropical Westerly Jet lies south of the Himalayas in winter and steers western disturbances that bring winter rain to north-west India; its withdrawal northward and the establishment of the Tropical Easterly Jet over peninsular India signals the burst of the south-west monsoon around early June. The Eastern Hemisphere position of the TEJ correlates with monsoon strength, and a weak or displaced jet is linked to deficient rainfall. Globally, jet streams also shape aviation, with eastbound transatlantic and transpacific flights riding the jet to save fuel and time, while the same winds disperse volcanic ash and pollutants over hemispheric distances.

For UPSC aspirants, the jet stream is a high-yield topic in GS Paper I (Geography) and Prelims, frequently tested at the intersection of climatology and the Indian monsoon mechanism. Typical question angles include matching jets to their latitudes and seasons, the role of the Tibetan Plateau in generating the TEJ, the Subtropical Westerly Jet's link to western disturbances, and the cause–effect chain connecting jet migration to monsoon onset and withdrawal. Candidates should master the thermal-wind basis, distinguish the four named jets (polar front, subtropical, tropical easterly, Somali), and connect Rossby-wave meandering to surface weather and to ENSO-related rainfall anomalies.