Northeast Monsoon, the retreating monsoon, is the lifeline of Tamil Nadu with the coastal districts getting 60% of the annual rainfall during these three months of October – December. Interior districts get more than 40% of the rainfall during Northeast Monsoon. The Normal onset date of Northeast Monsoon is around October 20th with +/- 7 days depending on the withdrawal of Southwest Monsoon from the rest of the country.
Through this post we aim to understand how Northeast Monsoon has been over the past years. This study has been done for the period 1958 – 2009 through correlation of IOD & ENSO. The period of study has been restricted to this period in order to use the standardized DMI Index . ENSO conditions have been taken into account through ONI Index We have taken monthly rainfall data for the Met. Divisions of TN & Coastal AP the two important regions which get possibly the highest rainfall during NEM. Also Coastal AP & TN also allows us to understand if there are any spatial pattern to the precipitation during this study. We have also included Chennai monthly rainfall as well so considering most of us are from Chennai.
We start with the years that have seen Above Average rainfall for all the three places in consideration. The idea is to understand the background behind those years that have seen fairly wide spread rainfall. Though in order to understand the exact impact possibly this study has to be done with individual district rainfall data for these regions. As one would observe 12 years have seen above average rainfall over Chennai, Tamil Nadu & Coastal AP, critically 7 out of these years have seen El Nino conditions during the NEM Period. Another important point is all those 7 years have seen Positive IOD coupled with El Nino indicating the potential importance of Positive IOD & El Nino Coupling for wide spread rainfall over Southern Peninsula.
Now let us look at what could be the statistical probability of an above average rainfall NEM season for the respective places. While La Nina with Positive IOD appears the highest probability for Chennai & TN, in the past years only once this event has happened during the year 1983 when a devolving positive IOD until October happened under La Nina conditions though this also subsequently changed to neutral for the rest of the season. Setting this aside if one sees the probability it is quite clear coupled El Nino & Positive IOD are the best bets for Above Rainfall. Chenani & Coastal AP possibly enjoys a good NEM under Enso & IOD Neutral conditions. Critically if one sees the pattern of IOD influence it is clear the statistical probability of above average years during Positive IOD years is higher than Negative IOD years under all the phases of ENSO. Only in the case of Coastal AP & Chennai under Neutral Phase of Enso Positive IOD possibly does not enhance precipitation. 
In all probability 2015 Northeast Monsoon is going to be under the impact of a strong El Nino & possibly a devolving Positive IOD, now let us look at how the past years have been under similar conditions. While this is no certainty the ensuing monsoon would follow similar pattern it would give an idea of what to expect. Coastal AP & Tamil Nadu on a whole has got average & above average rainfall all of the years under these conditions. In the case of Chennai if one sees the pattern it is either a very good year, 1976 & 1977 or has been fairly less than average so we could call this as possibly a 50:50 situation evolving as far as Chennai is concerned while TN overall & Coastal AP could be in line for a normal Northeast Monsoon.
To sum up we are putting up the years that has seen the best rainfall years & the worst rainfall years for these places. The positive impact of the El Nino & Positive IOD coupling is fairly clear in this case as well. Except for 2005 which was an exception for Chennai almost all years have been under the coupled influence of El Nino & Positive IOD. The important point which one should not ignore is the best 5 years for Coastal AP has come under Positive IOD & El Nino coupling effectively confirming Positive IOD helping only South TN is a myth.
A look at the worst years also indicates why Chennai should be worried under La Nina Conditions. As one would observe from the table La Nina conditions have been responsible for the worst years across Coastal AP & Tamil Nadu and most of the years La Nina is coupled with Negative IOD.
In our next series we would be putting out a study on how the cyclogenesis and landfall of cyclones are impacted under various ENSO & IOD conditions. Watch out for that in the days to come.