this link will help you sir, further if you want to discuss with me then I am also interested 

http://www.knmi.nl/research/global_climate/enso/effects/

http://www.ncdc.noaa.gov/paleo/ctl/images/warm.gif

El Nino and La Nina may change the amount of moisture available for the India-Pakistan monsoon, but once the moisture gets there, the duration and direction is modified by the Pakistan-Arabia Dust Cloud, and can cause either droughts or floods in the area.  Attached is today's Dust Cloud image and there are several studies linked to http://www.ecoseeds.com/contents.html

The ENSO was discovered by Sir Gilbert Walker while trying to predict the Indian monsoon. There is a biography of him and his work here, but I am not sure if it will answer your question.

http://www.stat.ualberta.ca/~wiens/stat479/walker.pdf

Hi Deepak,

there is a general association between El Nino and monsoon drought for South Asia (and La Nina with monsoon flood).  As Alastair alludes to above, one of the main mechanisms involved is via perturbations to the Walker Circulation.  So an El Nino warming, which generates anomalous precipitation and hence ascent in the central-to-east Pacific, also generates anomalous subsidence over the Indian Ocean region.  This acts to suppress convection, providing a large and seasonal-scale background on which the monsoon can operate. 

A useful graphic of this association is available in the picture on Monsoon Online at http://www.tropmet.res.in/~kolli/MOL/Monsoon/Historical/air.html .

Important points to note are:

1) Timing.  The monsoon is JJAS, but ENSO normally peaks in DJF.  So how do we choose which year to refer to an ENSO event?  The answer for South Asia is normally in the ENSO developing year, in other words the summer before the ENSO peak.  So El Ninos that developed in 2002 and 2009, for example, affected the monsoons in those years.  The large El Nino that developed over the winter of 1997/1998 was expected to affect the monsoon in 1997 summer.

2) ENSO type/position.  There is growing evidence of diversity in types of El Nino events.  Commonly referred to as canonical/traditional/East Pacific El Nino versus Modoki/central Pacific El Nino.  It is these latter events thought to have a larger impact on the Indian monsoon, see the paper by Krishna Kumar on this topic: http://dx.doi.org/doi:10.1126/science.1131152 or contact me if you need a copy.  So relatively small but central Pacific events like 2002 and 2009 have a large influence on monsoon drought, whereas the large east Pacific event of 1997 does not.

3) Intraseasonal variability: whatever the large, seasonal-scale forcing, intraseasonal variability can give much greater variance in monsoon rainfall, so this also needs to be considered.

Andy Turner.

Dear Deepak, It seems that previous responders have pointed out some valuable information and resources, but I think that it is important to add that the International Research Institute for Climate and Society has recently added new resource for ENSO events. Most notably, there are revamped sections allowing for easier access to products that are useful to monitor El Nino and La Nina events.

http://iridl.ldeo.columbia.edu/maproom/ENSO/Diagnostics.html

There is also a new section on the history of the El Nino - Southern Oscillation which explains the works of Walker, as well as other contributions of Cane, Zebiak, Ropelewski, Halpert, Wyrtki and Bjerknes. 

http://iridl.ldeo.columbia.edu/maproom/ENSO/ENSO_Info.html#tabs-2

Best regards, 

Andrew

http://iridl.ldeo.columbia.edu/maproom/ENSO/index.html

Due to El Nino episode  in any region pressure and wind pattern are changed and these two parameters are very much important for Indian monsoon.

Hi Deepak,

my apologies for this late response: You may find a very concise, qualitative discussion of the interactive nature of monsoon and ENSO in a 2-page abstract on the 'dynamic architecture' of the atmosphere that I have written for the Second GEWEX Conference (Washington 1996):

www.researchgate.net/publication/264527033

Best regards

Peter

Conference Paper Summer monsoon, MJO, annual cycle, QBO, Southern Oscillation...

 There are many theories proving the significant simultaneous co-variability of ENSO and Indian Monsoon [Rasmusson and Carpenter 1982, Walker 1924 & Webster 1998].  Its the subsiding limb of the walker cell which is shifted  over India due to shift in the SST warming regions form West to East and Central Pacific ocean. 

As Andy Turner (Sir) told that people have observed a weakening in the relationship in the recent times. Studies by Rajagopalan et al (2006) where they show how the pronounced warming over central Pacific is more likely to produce drought creating subsidence over India as compared to the El-Ninos when warming is more in the eastern Pacific regions.   Krishna Kumar et. al (1999) showed that due to the southeast ward shift in the walker circulation for the recent El-Ninos subsidence, weak subsidence was produced over Indian region and That is why ENSO-Monsoon relationship show a weakening in the trend for the recent period. 

To me, everything above put together does not give a clean insight of how El-Nino affects Indian monsoon. 

Though there are other school of thoughts as well which to me seem more promising as compared to theories given earlier. In a pioneering study done by Goswami and Xavier 2005 (GRL) it was proved that ENSO does affect the Indian monsoon by affecting the troposheric temperature gradient over the Indian monsoon region. Which ends up as the weak lower level monsoon flow and length of the long rainy season (LRS) is also altered by late onset and early withdrawal of Indian monsoon during ENSO years.

Recently (2007) Shaman and Tziperman  have shown that ENSO-Indian Monsoon tele-connection comes from the ENSO related dynamics in the upper tropospheric mid-latitude regions. Warming over the central Pacific creates upper tropospheric divergence over the region which is source of the Rossby waves there. These Rossby waves are guided by North-African-Asian jet and associated positive vorticity anomalies do get flooded over the central Asian regions. Effect of these anomalous positive vorticity is pronounced cooling over those regions which ends up as weak temperature gradients in the upper troposphere over Indian region. This disrupts the monsoon flow over Indian region and we get a bad monsoon during EL-Nino years.