Tropical Easterly Jet Stream (Somali Jet)

The Tropical Easterly Jet (TEJ) is a strong, high-altitude band of easterly winds that forms over the Indian subcontinent and tropical Africa during the boreal summer (June–September) and is intimately bound to the dynamics of the South Asian summer monsoon. It was first identified and systematically described by the Indian meteorologist P. R. Krishnan and, most prominently, by the British scholar R. C. Sutcliffe and the Indian climatologist P. Koteswaram, whose 1958 work formally characterized the feature. The jet is a thermally driven response to the seasonal reversal of the meridional temperature gradient over Asia: as the Tibetan Plateau heats intensely in summer, it becomes a vast elevated heat source, generating an upper-tropospheric anticyclone (the Tibetan High) whose southern flank channels air from east to west. The TEJ is therefore not a permanent feature like the subtropical westerly jet but a seasonal product of differential heating between the heated Asian landmass and the cooler equatorial Indian Ocean.

Procedurally, the formation of the TEJ unfolds in a recognizable sequence. With the apparent northward migration of the sun in spring and early summer, intense insolation over the Tibetan Plateau and the Iranian highlands creates a deep, warm anticyclonic circulation in the upper troposphere, centered near 200–100 hectopascals (roughly 12–15 kilometers altitude). The reversed thermal gradient—warm to the north over the plateau and cooler to the south over the ocean—produces, through the thermal wind relationship, easterly winds aloft on the equatorward side of this high. These easterlies accelerate into a concentrated core, the Tropical Easterly Jet, which attains peak speeds of roughly 40 metres per second near its axis. The jet is positioned over peninsular India around 15°N latitude and extends westward across the Arabian Sea, the Horn of Africa, and into the Atlantic.

The TEJ exhibits a distinctive thermal-direct circulation that explains its meteorological importance. Air subsides on the right (poleward) flank of the jet over the eastern Mediterranean, North Africa, and parts of the Arabian Peninsula, reinforcing the arid, cloudless conditions of those regions during summer. Conversely, air rises on the left (equatorward) flank over the Indian Ocean and the subcontinent, promoting convection, convergence, and abundant rainfall. The strength and exact positioning of the jet axis modulate the intensity of the monsoon: a robust, well-defined TEJ is associated with vigorous monsoon rainfall over India, while a weak or southward-displaced jet correlates with weaker monsoon activity and potential drought. The jet thus operates in tandem with the low-level Somali Jet—a cross-equatorial flow at roughly 1–1.5 kilometres altitude that carries moisture-laden air from the Southern Hemisphere across the equator and into the Arabian Sea—to constitute the dynamical engine of the monsoon system.

In contemporary operational meteorology, the India Meteorological Department in Pune and New Delhi monitors the TEJ alongside the Tibetan High and the Mascarene High as part of seasonal monsoon forecasting. Research institutions including the Indian Institute of Tropical Meteorology (IITM, Pune) track interannual variations in the jet's strength against monsoon outcomes; weaker TEJ years such as those coinciding with strong El Niño events—notably 2002, 2009, and 2015—were associated with deficient Indian monsoon rainfall. Atmospheric reanalysis products from the European Centre for Medium-Range Weather Forecasts (ECMWF) and observations have refined understanding of how the jet links Tibetan heating to West African and Sahelian rainfall variability, making the TEJ a feature of interest well beyond South Asia.

The TEJ must be distinguished carefully from several adjacent features. It is not the subtropical westerly jet stream, which flows from west to east at around 25°–35°N over the Himalayan barrier in winter and whose retreat northward in summer is itself a precondition for monsoon onset. Nor is the high-level TEJ identical to the low-level Somali Jet (also called the Findlater Jet after the meteorologist who studied it), though the two are frequently conflated in introductory texts: the TEJ is an upper-tropospheric easterly, while the Somali Jet is a near-surface westerly/southwesterly moisture conveyor. The two operate at different altitudes, blow in different directions, and perform complementary rather than identical functions in the monsoon machinery.

Edge cases and ongoing debates concern the TEJ's sensitivity to a warming climate and to changes in Tibetan Plateau snow cover and heating. Reduced spring snow cover over the plateau tends to intensify summer heating and strengthen the resulting circulation, while studies have detected a long-term weakening trend in the TEJ over recent decades, raising questions about its implications for monsoon stability. The teleconnection between the TEJ and Sahel rainfall—where a stronger jet has been linked to enhanced West African precipitation—remains an active research area, complicating attribution amid competing influences such as the Atlantic Multidecadal Oscillation and aerosol loading. Scholars continue to refine the relative weighting of the TEJ versus other monsoon drivers in seasonal prediction models.

For the working practitioner—whether a UPSC aspirant preparing General Studies Paper I geography, a climate-policy analyst, or an agricultural planner—the TEJ is a compact illustration of how upper-atmospheric dynamics translate into surface livelihoods across a billion-person region. Understanding that the monsoon is not merely a surface wind reversal but a coupled system involving the Tibetan High, the TEJ aloft, and the Somali Jet below allows precise framing of monsoon variability, drought risk, and the climatic links between South Asia and Africa. Its recurrence each summer and its measurable influence on rainfall make it a durable touchstone for tropical climatology.