Describe the relation between Jet stream with the upper air waves of the westerlies.How is the jet stream related to the Indian summer monsoon?

jet stream is narrow belt of high altitude (>12000m.) westerly winds in the troposphere whose speed varies between 10 Km./hr(in summer) to 184Km./hr(in winter),although velocities of over 370Km./hr. has been recorded occasionally. Their location & velocities have an important effect on travel times of flying aircraft especially in mid-latitude air routes.

Westerlies are the wind system which dominates the zones between latitude 40(degrees)N/S & 70(degrees)N/S of the equator. The flows of air from the sub-tropical high pressure cells to the temperate zone of low pressure cell, thereby maintaining the fundamental meridional heat exchange mechanism.The prevailing wind direction in northern hemisphere is from south-west & in the northern hemisphere in the north-west.it is one of he strongest & most undisturbed airflows among the planetary winds. the velocity increases with height & the southern hemisphere westerlies are stronger than their northern hemespherial counterparts due to broader expanse of oceans Jet stream leads to upper air convergence leading to formation of anticyclones.

                             In the northern hemisphere the jet stream blows towards equator from the north-east & due to the seasonal variations due to pressure change due to change in the position of the sun from Tropic of Capricorn to over Tropic of cancer during the July-Sept. months. Hence, they spread westwards into central Pacific while in the Atlantic they extend to the Brazilian coast. On it's westerly path the jet stream aids the formation of Macernes high, an anticyclone formed off the Madagascar coast. Moreover, the presence of equatorial troughs at 5(degrees)N - 5(degrees)S is an additional incentive for the trade winds to take the westerly direction on the basis of the concept that any fluid moves from a region of high concentration to the region of lower concentration).The trough along 5(degrees)N weakens & it's counterpart at 5(degrees)S moves northwards & merge with the heat low & forms quasi-stationery monsoon trough. The sub-tropical Easterly jet stream which blows over central & peninsular India in eastern direction during the winter season till the month of May checks the early infiltration of S.W.monsoon winds in the sub-continent & it blows at low altitude of 300mb.this sub-tropical westerly Jet Stream encourages anitcyclonogenesis/diverging off-shore winds ,it moves north of Himalayas by end of May. If this sub-tropical westerly jet stream fails to move as far as the north as normal or when El Niño occurs, the result is failure of the south-west monsoon.

                               Upper tropospheric pressure patterns over are simple, with low pressure over polar regions (cold contracting air ) & high pressure over the tropics 9thewarm expanding air).However, due to earth's rotation, the pressure patterns are deformed into huge, long Rossby waves of alternating low-pressure troughs & high pressure ridges which wax & wane with time as has been revealed in the Index cycle.

                              Westerlies, which represent the thermal wind blows from expandng warm air (highs) to contracting cold air (lows) blowing in huge wave like oscillations following Rossby waves ,especially in the northern hemisphere during winter. Jet streams are embodied in this westerly air flow & represent zones of highly concentrated kinetic energy in the vicinity of polar front (Polar front jet stream) & sub-tropics (sub-tropical jet streams).

               As there is low pressure near the ground surface over the subcontinent due to heat low 7 due to northward movement of previously 5(degrees)S situated trough. This causes high pressure in the upper troposphere. Since, upper air surface coupling of pressure systems is necessary to maintain the essential 3-D linkage hence the onset of the tropical easterly Jet stream which moves from east to west & helps in upper tropospheric divergence or scavenging.

                      Inter Tropical Convergence Zone develops on the equatorial limb of the Hadley's cell where the ascending air above the Tibetan plateau spreads south to join a descending limb over the northern Indian ocean near the Mecernes high & the cell is completed by the south-westerly wind flow. At his limb where I.T.C.Z. exists lies the converging trade wind of both hemispheres,& is a distinct surface low pressure troughs. However, satellite imageries have revealed that I.T.C.z. is'nt a region of continuously rising air with associated deep cumuliform clouds & heavy rain. However, it must be pointed out that position of I.T.C.Z. varies seasonally in direct response to changes in location of maximum solar heating & zone of seasonal maximum temperature(i.e., thermal equator).

                                    The tropical easterly jet stream caries the heat energy of the ascending limb of the Hadley's cell to the descending limb because although low pressure exists at the surface but high pressure prevails in the upper troposphere but low pressure prevails aloft in the upper troposphere over the northern Indian ocean due to high pressure at the sea surface. The westerlies thus maintain a distance between eastern tropical jet stream & sub-easterly tropical jet stream. apart from these the major part of the low level cross equatorial penetrates East Africa in may & traverses the northern part of Arabian sea before reaching India in June .Since, it’s path at 90 (degrees)N coincide with zone of coastal upwelling, the wide variations in sea surface temperature influence monsoon winds & it's picks up the moisture & when it is forced to rise above western Ghats it brings rain.