High sun’s insolation due to apparent movement of sun between the equator and the Tropic of Cancer. Intense Solar Radiation in the Tropics makes the weather very hot and humid, while low values of solar energy in Polar Regions make the weather cold and dry.
These changes in Solar Energy lead to convection and create differences in air pressure at various places on earth and hence we experience wind, wind speed & direction with changes in sun’s Altitude between Tropics & Poles.
Surface Pressure and Winds
While the low-pressure belt near the surface shifts northwards, the high pressure located over the Arabian Sea and the Bay of Bengal persists.
Since, now, the system of surface pressures is uni-polar (non-cyclonic), the westerly jet stream gets greatly amplified and it arrives in India as a very pronounced and well defined phenomenon consisting of a strong upper level wind current at about 15 km altitude and a strong wind current close to the surface.
As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels.
By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, by this time, the westerly jet stream withdraws from the Indian region.
Jet Streams and Upper Air Circulation
An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour.
In August, it is confined to 15°N latitude, and in September up to 22°N latitudes. The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones
The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.
The tracks of these depressions are the areas of highest rainfall in India. The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.
