Basically, the South West Monsoon advances and covers the entire Sub Continent region in June and completes its advance around early July.
Now, these advancing parameters and criteria are not linked or affected by the El Nino or the La Nina. vagaries
The advancing parameters are initiated from the South Indian Ocean, and are largely influenced by the Mascrene Highs between Madagascar and Australia.
Other parameters, which largely influence the advance is the seasonal low over the sub continent and the pressure gradient thus created. Many of the initial parameters are discussed in the Vagaries' Monsoon Watch Series, which are found in the Archives Page of this blog.
The ENSO factors affect the subsequent performance of the Monsoon rains.
In an El -Nino year, the waters of the Pacific Ocean off the Peru Coast in South American Continent heats up by 2/4c. Now, as a result, the Central Pacific Ocean , roughly where the "tongue" of the heated waters end, rise, and form rain bearing clouds, and precipitation in the Central Pacific Region.
El- Nino induced warm zones in the Pacific cause the warm air over them to rise and initiate circulation cells. Such cells along northern Australia, Indonesia and the eastern edge of the Indian Ocean could have their down draft sides over a nascent monsoon circulation cell in the Indian Ocean, which would disrupt Monsoon Depression formation, causing poor monsoon rains over the subcontinent.
Thus, there is a marked reduction in Typhoons and Depressions forming. Consequently, in an El Nino year, we would expect lesser depressions and lows forming in the Bay. This sequence implies that El Nino years should coincide with deficient monsoon rains.
Normally, when in a neutral or La Nina year, the waters normally heat up towards the far West of the Pacific Ocean, rain bearing clouds are formed much to the West and near the Philippines.
Now, Typhoons and Lows forming from these regions travel West, and send Low pulses into the Bay. These are the "life giving" depressions in the Bay which criss- cross across the Indian regions.
But, that does not mean lower rainfall all over the Sub Continent. Absence of Lows from the Bay will keep the WDs active, and may increase the number of WDs crossing the Northern Regions even till July/August. This has happened, when the Lows from the Bay do not "interfere" and WDs are given a "free hand".
But, the above sequence may not necessarily happen, and sometimes, even in an El Nino year, the Central Regions of India get good rains from Line of Wind Discontinuity forming from localised heat developments. Deeps troughs in the jet streams could bring or create UAC in the Gujarat regions, even in an El Nino year.
So, all El -Nino years are not bad for Monsoons.This Graphical Plotting from IITM, shows the inconsistency of 130 years..