The sub-tropical jet stream is produced by the earth’s rotation (Coriolis force) and temperature contrast between tropical and sub – tropical regions.At the equator, the rotation produces greatest velocity in the atmosphere.
As a result, the rising air which spreads out northwards and southwards, moves faster than the latitudes over which it is blowing.It is deflected to the right in the northern hemisphere and to the left in the southern hemisphere, and at about 30° latitude, it becomes concentrated as the subtropical jet streams.
During winter, the STJ is nearly continuous in both hemispheres. The STJ exists all year in the southern hemisphere. However, it is intermittent in the northern hemisphere during summer when it migrates north.The STJ can be temporarily displaced when strong mid-latitude troughs extend into subtropical latitudes. When these displacements occur, the subtropical jet can merge with the polar front jet STJ is closely connected to the Indian and African summer monsoons