How do El Nino & Southern Oscillation influence the monsoon

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Blowing of strong trade wind have a frictional drag on warm surface water of ocean away from it's source environment resulting in divergence caused due to coriolis force.the removal of light warm water from the surface encourage upwelling of cold Benthos water to the surface as in for example-the Peruvian current (Humbolt current ) off-South America. Lapse rate steepening, instability & free convection over warm water is now being replaced by surface temperature inversion & stability.
                                The replacement of cold Peruvian current in the tropical pacific basin by incursion at periodic levels by a weak warm ocean current that flows south along the coast of Ecuador & Peru. This disruption is called El Nino commonly occurring during Christmas season.
Southern Oscillation is a climatologically term referring to perturbation in the inter tropical general circulation, especially in the Indian Ocean & south pacific. The term was introduced by G.Walker who noted that when pressure over pacific is high, it tends to be lower over Indian ocean resulting in movement of surface  Easterlies from the region of south pacific to over India, Fiji,& east Australia to feed the low level cyclonic convergence. This is the result of a 'normal' atmospheric-oceanic circulation mode with a distinct west ti east longitudinal cell across pacific resulting in the existence of typical trans-ocean low pressure gradient at sea level of 5mb-10mb between low pressure in west & high pressure in the east. However, it should be pointed out that El Niño normally takes 3-6 months to reach local peak intensity after onset. At about 200mb level in troposphere there is distinctive westerly counter flow feeding upper air convergence (due to low atmospheric pressure in upper troposphere due to high presure at the surface) over north-western S.America.the associated subsidence & low level cyclonic divergence over Peru & Ecuador accentuates the aridity of these desert coasts.
weak Hadley's cell activity which controls the trade wind notion results in disruption of normal El Nino-Southern Oscillation circulation. It results in replacement of cold water by warm equatorial current as El Nino which flows southwards to about 6(degrees)S along Peruvian coast which results in sudden rise of coastal water's temperature by about 4(degrees) Celsius. this periodical disturbance brings torrential precipitation & floods to coastal deserts of northern Peru & Ecuador. Thus, the intensity of surface winds, preventing warm water from flowing across the pacific eventually terminating El Niño. the high pressure subsidence in south-west Pacific introduce droughts & failure of Indian summer monsoon through a weakening of land-sea circulation system.

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