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Subtropical jet streams form at the poleward edge of the Hadley cell (~30° latitude), where poleward-moving aloft air encounters the Coriolis effect and is deflected strongly. These narrow, fast-moving currents (>50 m/s) are concentrated in the upper troposphere and are maintained by the thermal wind balance. Jet streams steer weather systems, separate tropical from mid-latitude air masses, and strongly influence regional precipitation and temperature patterns on subseasonal to seasonal timescales.
Examine zonal wind profiles and identify jet cores. Apply the thermal wind equation to relate jet strength to the poleward temperature gradient.
Jet streams are not fixed in position; they meander (forming ridges and troughs) and shift poleward/equatorward seasonally. Also, jets are maintained by thermal gradients, not direct heating; they weaken when meridional temperature gradients weaken.
From your study of the Hadley cell, you know that air rises near the equator, flows poleward in the upper troposphere, descends at roughly 30° latitude, and returns equatorward at the surface. The subtropical jet stream is a direct consequence of what happens to that poleward-moving upper-level air as it encounters the Coriolis effect. Air moving away from the equator conserves angular momentum: as it moves to smaller-radius latitude circles, it must speed up relative to Earth's surface. By the time the air reaches about 30° latitude, it has been deflected so strongly eastward that it forms a concentrated ribbon of fast-moving wind — the subtropical jet — typically blowing at 50 m/s or more near the tropopause.
The jet's strength is not arbitrary; it is governed by the thermal wind relationship, which links the vertical wind shear to the horizontal temperature gradient. A strong temperature contrast between the warm tropics and the cooler subtropics produces a stronger jet. This is why the subtropical jet intensifies in winter, when the equator-to-pole temperature difference is greatest, and weakens in summer, when the gradient relaxes. The thermal wind equation, which you may have encountered alongside Rossby wave dynamics, provides the quantitative link: the stronger the meridional temperature gradient at a given altitude, the faster the geostrophic wind increases with height, producing a sharper jet core.
The subtropical jet has enormous practical consequences for weather and climate. It acts as a boundary between tropical air masses and mid-latitude air, steering extratropical cyclones and influencing where precipitation falls. When the jet is strong and zonal (roughly east-west), weather systems move briskly across continents. When it weakens or develops large-amplitude meanders — the ridges and troughs you know from Rossby wave theory — weather patterns can stagnate, producing prolonged heatwaves, cold spells, or flooding. The jet's seasonal migration also determines the timing of monsoons: as the jet shifts poleward in spring and summer, it allows the intertropical convergence zone to migrate, triggering monsoon onset in South and East Asia.
It is important to distinguish the subtropical jet from its cousin, the polar front jet, which forms at higher latitudes (~50–60°) along the polar front where cold polar air meets warmer mid-latitude air. While both are upper-tropospheric wind maxima maintained by thermal gradients, they have different origins: the subtropical jet is driven by angular momentum transport in the Hadley cell, while the polar jet is driven by baroclinic instability along the polar front. In practice, the two jets can merge, split, or interact, creating complex upper-level flow patterns that are central to mid-latitude weather forecasting.