Tropical Easterly Jet

The Tropical Easterly Jet (TEJ), also called the African Easterly Jet over its western extent, is a fast-flowing ribbon of air that develops in the upper troposphere between roughly 100 and 200 hectopascals (about 12–16 kilometres altitude) during the Northern Hemisphere summer. It was first systematically described by the Indian meteorologist P. R. Krishnan and, most influentially, by the American climatologist Herbert Riehl and the German-British scientist R. C. Sutcliffe in the 1950s, with detailed analyses following the establishment of upper-air radiosonde networks across India and Africa. Its physical basis lies in the thermal contrast generated by the intense summer heating of the Tibetan Plateau, which creates a warm anticyclone in the upper troposphere over the elevated landmass. The resulting reversal of the meridional temperature gradient—with warm air to the north and cooler air to the south—produces, through the thermal wind relationship, a strong easterly flow on the southern flank of the Tibetan high.

The jet forms only after the monsoon onset machinery is in place. As the sun migrates north of the equator from April onward, the Tibetan Plateau heats rapidly and acts as an elevated heat source, warming the overlying air column. By June a warm anticyclonic circulation establishes itself in the upper troposphere centred near Tibet. Because temperature decreases southward from this warm core, the thermal wind balance dictates an acceleration of easterly winds aloft on its equatorward side. The TEJ thus blows from east to west, running roughly from the South China Sea across the Indian subcontinent and the Arabian Sea to East and West Africa, with its core typically positioned near 15°N. Wind speeds in the jet core regularly reach 30 to 40 metres per second, occasionally exceeding 50 metres per second.

The jet exhibits a characteristic entrance–exit dynamic that links it directly to monsoon rainfall. As air enters the jet over the Bay of Bengal and India it accelerates, producing upper-level divergence on the right (poleward) side that promotes upward motion and convection over the subcontinent. As the jet exits and decelerates over Africa, the divergence pattern reverses, favouring subsidence and aridity over the eastern Sahara and Sudan. This explains why the same circulation feature that intensifies rainfall over peninsular India is associated with sinking dry air over northeast Africa. The TEJ works in tandem with the low-level Somali Jet, a cross-equatorial southwesterly current that supplies the moisture which the upper-level dynamics then lift, completing the monsoon circulation cell. The TEJ also influences the longitudinal position of monsoon depressions and the strength of the Mascarene High to the south.

In contemporary forecasting, the India Meteorological Department (IMD) in Pune and Delhi monitors the TEJ through upper-air analysis and reanalysis datasets such as those from the European Centre for Medium-Range Weather Forecasts. Operational monsoon assessments routinely reference the jet's core speed and latitude because a vigorous, well-positioned jet correlates with a robust monsoon, while a weak or displaced jet accompanies deficient seasons. Research institutions including the Indian Institute of Tropical Meteorology have used the TEJ as a diagnostic in studies of the monsoon's interannual variability and its teleconnections with the El Niño–Southern Oscillation.

The Tropical Easterly Jet must be distinguished from the subtropical westerly jet stream, with which it is frequently confused in examination contexts. The subtropical westerly jet flows from west to east at higher latitudes (around 25–30°N) and dominates the upper troposphere over India during winter; its northward retreat across the Himalaya in early summer is itself a trigger that permits monsoon onset. The TEJ, by contrast, is an easterly current confined to the summer season and to tropical latitudes. The two are seasonally exclusive over India: the westerly jet's withdrawal precedes the TEJ's establishment. The TEJ also differs from the African Easterly Jet of the mid-troposphere over West Africa, which forms at a lower altitude (around 600 hPa) from the Saharan temperature gradient and governs West African squall lines and Atlantic tropical cyclogenesis.

Edge cases and ongoing debates centre on the jet's sensitivity to climate change and to the thermal state of the Tibetan Plateau. Several studies have reported a weakening trend in the TEJ over recent decades, attributed to reduced upper-tropospheric warming over Tibet and changes in the meridional temperature gradient, with implications for monsoon rainfall distribution. The relationship between Tibetan snow cover, plateau heating, and jet intensity remains an active research question, complicating long-range seasonal prediction. Aerosol loading over South Asia and shifts in the tropopause have also been proposed as modulating influences, though attribution remains contested.

For the working practitioner—whether a UPSC aspirant preparing General Studies Paper 1, a desk officer tracking South Asian food security, or an analyst assessing monsoon-dependent agricultural economies—the Tropical Easterly Jet is the upper-air keystone of the summer monsoon system. Understanding it clarifies why the monsoon is a coupled hemispheric circulation rather than a local rainfall event, why drought over the Sahel and flooding over India can share a single dynamical origin, and why forecasting the jet's strength carries direct economic and strategic weight for a region home to nearly two billion people.