Lot of discussions happen among weather enthusiasts on what factors provide a good Northeast Monsoon for Chennai. While there are a lot of factors that influence rains, this analysis is restricted to finding how ENSO & IOD have behaved during various rainfall years to get an understanding on what is an ideal correlation between the two as far as Chennai is concerned.
For those who want to know more about ENSO https://en.wikipedia.org/wiki/El_Ni%C3%B1o_Southern_Oscillation
And for more on IOD https://en.wikipedia.org/wiki/Indian_Ocean_Dipole
These are primarily two factors that possibly holds the highest influence on Monsoon over Peninsular India.
After doing some number crunching if one goes by historical trend the understanding is as follows. This analysis is specific for Chennai rainfall.
- IOD possibly has more impact on NEM role as compared to ENSO the reason being 46 out of 54 years in the analysis the ENSO has been neutral during OND. Similarly 43 years out of 54 have been ENSO neutral during JAS quarter. Compare this to 37 years of 54 which have been either Positive or Negative IOD phase during OND. Its a case of IOD being more active than ENSO during NEM plays a bigger role.
- Its also quite obvious if ENSO is either in La Nina or ElNino phase during OND quarter the probability of Deficit Monsoon for Chennai is lesser. Only 1 year out of 8 years with pure ENSO OND phase has been a deficit Monsoon. An ideal monsoon for us would mean an active ENSO either a positive or negative phase.
- LaNina consequent to the lower ridge position possibly is providing more excess years to Chennai.
- To a great extent Positive IOD is in favour of Chennai rainfall on account of more excess years during Positive IOD phase, similarly the probability of Deficit Monsoon is higher during pure Negative IOD phase as far as Chennai is concerned.
- IOD Neutral years tend to move towards deficit more often than not.